diff --git a/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs b/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
index 5a73097b0f6..da7279967da 100644
--- a/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
+++ b/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
@@ -69,6 +69,7 @@ pub(super) fn find_opaque_ty_constraints_for_tait(tcx: TyCtxt<'_>, def_id: Local
             Node::Item(it) => locator.visit_item(it),
             Node::ImplItem(it) => locator.visit_impl_item(it),
             Node::TraitItem(it) => locator.visit_trait_item(it),
+            Node::ForeignItem(it) => locator.visit_foreign_item(it),
             other => bug!("{:?} is not a valid scope for an opaque type item", other),
         }
     }
@@ -240,6 +241,12 @@ fn visit_trait_item(&mut self, it: &'tcx TraitItem<'tcx>) {
         self.check(it.owner_id.def_id);
         intravisit::walk_trait_item(self, it);
     }
+    fn visit_foreign_item(&mut self, it: &'tcx hir::ForeignItem<'tcx>) {
+        trace!(?it.owner_id);
+        assert_ne!(it.owner_id.def_id, self.def_id);
+        // No need to call `check`, as we do not run borrowck on foreign items.
+        intravisit::walk_foreign_item(self, it);
+    }
 }
 
 pub(super) fn find_opaque_ty_constraints_for_rpit<'tcx>(
diff --git a/compiler/rustc_middle/src/ty/instance.rs b/compiler/rustc_middle/src/ty/instance.rs
index 2ac3cddfa15..dd41cb5a61f 100644
--- a/compiler/rustc_middle/src/ty/instance.rs
+++ b/compiler/rustc_middle/src/ty/instance.rs
@@ -293,12 +293,16 @@ pub fn has_polymorphic_mir_body(&self) -> bool {
 fn fmt_instance(
     f: &mut fmt::Formatter<'_>,
     instance: &Instance<'_>,
-    type_length: rustc_session::Limit,
+    type_length: Option<rustc_session::Limit>,
 ) -> fmt::Result {
     ty::tls::with(|tcx| {
         let args = tcx.lift(instance.args).expect("could not lift for printing");
 
-        let mut cx = FmtPrinter::new_with_limit(tcx, Namespace::ValueNS, type_length);
+        let mut cx = if let Some(type_length) = type_length {
+            FmtPrinter::new_with_limit(tcx, Namespace::ValueNS, type_length)
+        } else {
+            FmtPrinter::new(tcx, Namespace::ValueNS)
+        };
         cx.print_def_path(instance.def_id(), args)?;
         let s = cx.into_buffer();
         f.write_str(&s)
@@ -324,13 +328,13 @@ fn fmt_instance(
 
 impl<'a, 'tcx> fmt::Display for ShortInstance<'a, 'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt_instance(f, self.0, rustc_session::Limit(self.1))
+        fmt_instance(f, self.0, Some(rustc_session::Limit(self.1)))
     }
 }
 
 impl<'tcx> fmt::Display for Instance<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        ty::tls::with(|tcx| fmt_instance(f, self, tcx.type_length_limit()))
+        fmt_instance(f, self, None)
     }
 }
 
diff --git a/compiler/rustc_mir_transform/src/coverage/spans.rs b/compiler/rustc_mir_transform/src/coverage/spans.rs
index ed091752187..5983189984d 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans.rs
+++ b/compiler/rustc_mir_transform/src/coverage/spans.rs
@@ -1,11 +1,9 @@
-use std::cell::OnceCell;
-
 use rustc_data_structures::graph::WithNumNodes;
 use rustc_index::IndexVec;
 use rustc_middle::mir;
-use rustc_span::{BytePos, ExpnKind, MacroKind, Span, Symbol, DUMMY_SP};
+use rustc_span::{BytePos, Span, DUMMY_SP};
 
-use super::graph::{BasicCoverageBlock, CoverageGraph, START_BCB};
+use super::graph::{BasicCoverageBlock, CoverageGraph};
 use crate::coverage::ExtractedHirInfo;
 
 mod from_mir;
@@ -70,35 +68,17 @@ pub(super) fn bcbs_with_coverage_spans(
 /// `dominates()` the `BasicBlock`s in this `CoverageSpan`.
 #[derive(Debug, Clone)]
 struct CoverageSpan {
-    pub span: Span,
-    pub expn_span: Span,
-    pub current_macro_or_none: OnceCell<Option<Symbol>>,
-    pub bcb: BasicCoverageBlock,
+    span: Span,
+    bcb: BasicCoverageBlock,
     /// List of all the original spans from MIR that have been merged into this
     /// span. Mainly used to precisely skip over gaps when truncating a span.
-    pub merged_spans: Vec<Span>,
-    pub is_closure: bool,
+    merged_spans: Vec<Span>,
+    is_closure: bool,
 }
 
 impl CoverageSpan {
-    pub fn for_fn_sig(fn_sig_span: Span) -> Self {
-        Self::new(fn_sig_span, fn_sig_span, START_BCB, false)
-    }
-
-    pub(super) fn new(
-        span: Span,
-        expn_span: Span,
-        bcb: BasicCoverageBlock,
-        is_closure: bool,
-    ) -> Self {
-        Self {
-            span,
-            expn_span,
-            current_macro_or_none: Default::default(),
-            bcb,
-            merged_spans: vec![span],
-            is_closure,
-        }
+    fn new(span: Span, bcb: BasicCoverageBlock, is_closure: bool) -> Self {
+        Self { span, bcb, merged_spans: vec![span], is_closure }
     }
 
     pub fn merge_from(&mut self, other: &Self) {
@@ -123,37 +103,6 @@ pub fn is_mergeable(&self, other: &Self) -> bool {
     pub fn is_in_same_bcb(&self, other: &Self) -> bool {
         self.bcb == other.bcb
     }
-
-    /// If the span is part of a macro, returns the macro name symbol.
-    pub fn current_macro(&self) -> Option<Symbol> {
-        self.current_macro_or_none
-            .get_or_init(|| {
-                if let ExpnKind::Macro(MacroKind::Bang, current_macro) =
-                    self.expn_span.ctxt().outer_expn_data().kind
-                {
-                    return Some(current_macro);
-                }
-                None
-            })
-            .map(|symbol| symbol)
-    }
-
-    /// If the span is part of a macro, and the macro is visible (expands directly to the given
-    /// body_span), returns the macro name symbol.
-    pub fn visible_macro(&self, body_span: Span) -> Option<Symbol> {
-        let current_macro = self.current_macro()?;
-        let parent_callsite = self.expn_span.parent_callsite()?;
-
-        // In addition to matching the context of the body span, the parent callsite
-        // must also be the source callsite, i.e. the parent must have no parent.
-        let is_visible_macro =
-            parent_callsite.parent_callsite().is_none() && parent_callsite.eq_ctxt(body_span);
-        is_visible_macro.then_some(current_macro)
-    }
-
-    pub fn is_macro_expansion(&self) -> bool {
-        self.current_macro().is_some()
-    }
 }
 
 /// Converts the initial set of `CoverageSpan`s (one per MIR `Statement` or `Terminator`) into a
@@ -164,10 +113,6 @@ pub fn is_macro_expansion(&self) -> bool {
 ///    execution
 ///  * Carve out (leave uncovered) any span that will be counted by another MIR (notably, closures)
 struct CoverageSpansGenerator<'a> {
-    /// A `Span` covering the function body of the MIR (typically from left curly brace to right
-    /// curly brace).
-    body_span: Span,
-
     /// The BasicCoverageBlock Control Flow Graph (BCB CFG).
     basic_coverage_blocks: &'a CoverageGraph,
 
@@ -244,7 +189,6 @@ pub(super) fn generate_coverage_spans(
         );
 
         let coverage_spans = Self {
-            body_span: hir_info.body_span,
             basic_coverage_blocks,
             sorted_spans_iter: sorted_spans.into_iter(),
             some_curr: None,
@@ -266,7 +210,6 @@ fn to_refined_spans(mut self) -> Vec<CoverageSpan> {
             // span-processing steps don't make sense yet.
             if self.some_prev.is_none() {
                 debug!("  initial span");
-                self.maybe_push_macro_name_span();
                 continue;
             }
 
@@ -278,7 +221,6 @@ fn to_refined_spans(mut self) -> Vec<CoverageSpan> {
                 debug!("  same bcb (and neither is a closure), merge with prev={prev:?}");
                 let prev = self.take_prev();
                 self.curr_mut().merge_from(&prev);
-                self.maybe_push_macro_name_span();
             // Note that curr.span may now differ from curr_original_span
             } else if prev.span.hi() <= curr.span.lo() {
                 debug!(
@@ -286,7 +228,6 @@ fn to_refined_spans(mut self) -> Vec<CoverageSpan> {
                 );
                 let prev = self.take_prev();
                 self.refined_spans.push(prev);
-                self.maybe_push_macro_name_span();
             } else if prev.is_closure {
                 // drop any equal or overlapping span (`curr`) and keep `prev` to test again in the
                 // next iter
@@ -297,35 +238,11 @@ fn to_refined_spans(mut self) -> Vec<CoverageSpan> {
             } else if curr.is_closure {
                 self.carve_out_span_for_closure();
             } else if self.prev_original_span == curr.span {
-                // Note that this compares the new (`curr`) span to `prev_original_span`.
-                // In this branch, the actual span byte range of `prev_original_span` is not
-                // important. What is important is knowing whether the new `curr` span was
-                // **originally** the same as the original span of `prev()`. The original spans
-                // reflect their original sort order, and for equal spans, conveys a partial
-                // ordering based on CFG dominator priority.
-                if prev.is_macro_expansion() && curr.is_macro_expansion() {
-                    // Macros that expand to include branching (such as
-                    // `assert_eq!()`, `assert_ne!()`, `info!()`, `debug!()`, or
-                    // `trace!()`) typically generate callee spans with identical
-                    // ranges (typically the full span of the macro) for all
-                    // `BasicBlocks`. This makes it impossible to distinguish
-                    // the condition (`if val1 != val2`) from the optional
-                    // branched statements (such as the call to `panic!()` on
-                    // assert failure). In this case it is better (or less
-                    // worse) to drop the optional branch bcbs and keep the
-                    // non-conditional statements, to count when reached.
-                    debug!(
-                        "  curr and prev are part of a macro expansion, and curr has the same span \
-                        as prev, but is in a different bcb. Drop curr and keep prev for next iter. \
-                        prev={prev:?}",
-                    );
-                    self.take_curr(); // Discards curr.
-                } else {
-                    self.update_pending_dups();
-                }
+                // `prev` and `curr` have the same span, or would have had the
+                // same span before `prev` was modified by other spans.
+                self.update_pending_dups();
             } else {
                 self.cutoff_prev_at_overlapping_curr();
-                self.maybe_push_macro_name_span();
             }
         }
 
@@ -360,41 +277,6 @@ fn to_refined_spans(mut self) -> Vec<CoverageSpan> {
         self.refined_spans
     }
 
-    /// If `curr` is part of a new macro expansion, carve out and push a separate
-    /// span that ends just after the macro name and its subsequent `!`.
-    fn maybe_push_macro_name_span(&mut self) {
-        let curr = self.curr();
-
-        let Some(visible_macro) = curr.visible_macro(self.body_span) else { return };
-        if let Some(prev) = &self.some_prev
-            && prev.expn_span.eq_ctxt(curr.expn_span)
-        {
-            return;
-        }
-
-        // The split point is relative to `curr_original_span`,
-        // because `curr.span` may have been merged with preceding spans.
-        let split_point_after_macro_bang = self.curr_original_span.lo()
-            + BytePos(visible_macro.as_str().len() as u32)
-            + BytePos(1); // add 1 for the `!`
-        debug_assert!(split_point_after_macro_bang <= curr.span.hi());
-        if split_point_after_macro_bang > curr.span.hi() {
-            // Something is wrong with the macro name span;
-            // return now to avoid emitting malformed mappings (e.g. #117788).
-            return;
-        }
-
-        let mut macro_name_cov = curr.clone();
-        macro_name_cov.span = macro_name_cov.span.with_hi(split_point_after_macro_bang);
-        self.curr_mut().span = curr.span.with_lo(split_point_after_macro_bang);
-
-        debug!(
-            "  and curr starts a new macro expansion, so add a new span just for \
-            the macro `{visible_macro}!`, new span={macro_name_cov:?}",
-        );
-        self.refined_spans.push(macro_name_cov);
-    }
-
     #[track_caller]
     fn curr(&self) -> &CoverageSpan {
         self.some_curr.as_ref().unwrap_or_else(|| bug!("some_curr is None (curr)"))
diff --git a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
index 8f6592afe85..1b6dfccd574 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
+++ b/compiler/rustc_mir_transform/src/coverage/spans/from_mir.rs
@@ -1,11 +1,14 @@
 use rustc_data_structures::captures::Captures;
+use rustc_data_structures::fx::FxHashSet;
 use rustc_middle::mir::{
     self, AggregateKind, FakeReadCause, Rvalue, Statement, StatementKind, Terminator,
     TerminatorKind,
 };
-use rustc_span::Span;
+use rustc_span::{ExpnKind, MacroKind, Span, Symbol};
 
-use crate::coverage::graph::{BasicCoverageBlock, BasicCoverageBlockData, CoverageGraph};
+use crate::coverage::graph::{
+    BasicCoverageBlock, BasicCoverageBlockData, CoverageGraph, START_BCB,
+};
 use crate::coverage::spans::CoverageSpan;
 use crate::coverage::ExtractedHirInfo;
 
@@ -15,26 +18,29 @@ pub(super) fn mir_to_initial_sorted_coverage_spans(
     basic_coverage_blocks: &CoverageGraph,
 ) -> Vec<CoverageSpan> {
     let &ExtractedHirInfo { is_async_fn, fn_sig_span, body_span, .. } = hir_info;
+
+    let mut initial_spans = vec![SpanFromMir::for_fn_sig(fn_sig_span)];
+
     if is_async_fn {
         // An async function desugars into a function that returns a future,
         // with the user code wrapped in a closure. Any spans in the desugared
-        // outer function will be unhelpful, so just produce a single span
-        // associating the function signature with its entry BCB.
-        return vec![CoverageSpan::for_fn_sig(fn_sig_span)];
+        // outer function will be unhelpful, so just keep the signature span
+        // and ignore all of the spans in the MIR body.
+    } else {
+        for (bcb, bcb_data) in basic_coverage_blocks.iter_enumerated() {
+            initial_spans.extend(bcb_to_initial_coverage_spans(mir_body, body_span, bcb, bcb_data));
+        }
+
+        // If no spans were extracted from the body, discard the signature span.
+        // FIXME: This preserves existing behavior; consider getting rid of it.
+        if initial_spans.len() == 1 {
+            initial_spans.clear();
+        }
     }
 
-    let mut initial_spans = Vec::with_capacity(mir_body.basic_blocks.len() * 2);
-    for (bcb, bcb_data) in basic_coverage_blocks.iter_enumerated() {
-        initial_spans.extend(bcb_to_initial_coverage_spans(mir_body, body_span, bcb, bcb_data));
-    }
-
-    if initial_spans.is_empty() {
-        // This can happen if, for example, the function is unreachable (contains only a
-        // `BasicBlock`(s) with an `Unreachable` terminator).
-        return initial_spans;
-    }
-
-    initial_spans.push(CoverageSpan::for_fn_sig(fn_sig_span));
+    initial_spans.sort_by(|a, b| basic_coverage_blocks.cmp_in_dominator_order(a.bcb, b.bcb));
+    remove_unwanted_macro_spans(&mut initial_spans);
+    split_visible_macro_spans(&mut initial_spans);
 
     initial_spans.sort_by(|a, b| {
         // First sort by span start.
@@ -53,7 +59,62 @@ pub(super) fn mir_to_initial_sorted_coverage_spans(
             .then_with(|| Ord::cmp(&a.is_closure, &b.is_closure).reverse())
     });
 
-    initial_spans
+    initial_spans.into_iter().map(SpanFromMir::into_coverage_span).collect::<Vec<_>>()
+}
+
+/// Macros that expand into branches (e.g. `assert!`, `trace!`) tend to generate
+/// multiple condition/consequent blocks that have the span of the whole macro
+/// invocation, which is unhelpful. Keeping only the first such span seems to
+/// give better mappings, so remove the others.
+///
+/// (The input spans should be sorted in BCB dominator order, so that the
+/// retained "first" span is likely to dominate the others.)
+fn remove_unwanted_macro_spans(initial_spans: &mut Vec<SpanFromMir>) {
+    let mut seen_macro_spans = FxHashSet::default();
+    initial_spans.retain(|covspan| {
+        // Ignore (retain) closure spans and non-macro-expansion spans.
+        if covspan.is_closure || covspan.visible_macro.is_none() {
+            return true;
+        }
+
+        // Retain only the first macro-expanded covspan with this span.
+        seen_macro_spans.insert(covspan.span)
+    });
+}
+
+/// When a span corresponds to a macro invocation that is visible from the
+/// function body, split it into two parts. The first part covers just the
+/// macro name plus `!`, and the second part covers the rest of the macro
+/// invocation. This seems to give better results for code that uses macros.
+fn split_visible_macro_spans(initial_spans: &mut Vec<SpanFromMir>) {
+    let mut extra_spans = vec![];
+
+    initial_spans.retain(|covspan| {
+        if covspan.is_closure {
+            return true;
+        }
+
+        let Some(visible_macro) = covspan.visible_macro else { return true };
+
+        let split_len = visible_macro.as_str().len() as u32 + 1;
+        let (before, after) = covspan.span.split_at(split_len);
+        if !covspan.span.contains(before) || !covspan.span.contains(after) {
+            // Something is unexpectedly wrong with the split point.
+            // The debug assertion in `split_at` will have already caught this,
+            // but in release builds it's safer to do nothing and maybe get a
+            // bug report for unexpected coverage, rather than risk an ICE.
+            return true;
+        }
+
+        assert!(!covspan.is_closure);
+        extra_spans.push(SpanFromMir::new(before, covspan.visible_macro, covspan.bcb, false));
+        extra_spans.push(SpanFromMir::new(after, covspan.visible_macro, covspan.bcb, false));
+        false // Discard the original covspan that we just split.
+    });
+
+    // The newly-split spans are added at the end, so any previous sorting
+    // is not preserved.
+    initial_spans.extend(extra_spans);
 }
 
 // Generate a set of `CoverageSpan`s from the filtered set of `Statement`s and `Terminator`s of
@@ -66,22 +127,24 @@ fn bcb_to_initial_coverage_spans<'a, 'tcx>(
     body_span: Span,
     bcb: BasicCoverageBlock,
     bcb_data: &'a BasicCoverageBlockData,
-) -> impl Iterator<Item = CoverageSpan> + Captures<'a> + Captures<'tcx> {
+) -> impl Iterator<Item = SpanFromMir> + Captures<'a> + Captures<'tcx> {
     bcb_data.basic_blocks.iter().flat_map(move |&bb| {
         let data = &mir_body[bb];
 
         let statement_spans = data.statements.iter().filter_map(move |statement| {
             let expn_span = filtered_statement_span(statement)?;
-            let span = unexpand_into_body_span(expn_span, body_span)?;
+            let (span, visible_macro) =
+                unexpand_into_body_span_with_visible_macro(expn_span, body_span)?;
 
-            Some(CoverageSpan::new(span, expn_span, bcb, is_closure_or_coroutine(statement)))
+            Some(SpanFromMir::new(span, visible_macro, bcb, is_closure_or_coroutine(statement)))
         });
 
         let terminator_span = Some(data.terminator()).into_iter().filter_map(move |terminator| {
             let expn_span = filtered_terminator_span(terminator)?;
-            let span = unexpand_into_body_span(expn_span, body_span)?;
+            let (span, visible_macro) =
+                unexpand_into_body_span_with_visible_macro(expn_span, body_span)?;
 
-            Some(CoverageSpan::new(span, expn_span, bcb, false))
+            Some(SpanFromMir::new(span, visible_macro, bcb, false))
         });
 
         statement_spans.chain(terminator_span)
@@ -202,7 +265,83 @@ fn filtered_terminator_span(terminator: &Terminator<'_>) -> Option<Span> {
 ///
 /// [^1]Expansions result from Rust syntax including macros, syntactic sugar,
 /// etc.).
-#[inline]
-fn unexpand_into_body_span(span: Span, body_span: Span) -> Option<Span> {
-    span.find_ancestor_inside_same_ctxt(body_span)
+fn unexpand_into_body_span_with_visible_macro(
+    original_span: Span,
+    body_span: Span,
+) -> Option<(Span, Option<Symbol>)> {
+    let (span, prev) = unexpand_into_body_span_with_prev(original_span, body_span)?;
+
+    let visible_macro = prev
+        .map(|prev| match prev.ctxt().outer_expn_data().kind {
+            ExpnKind::Macro(MacroKind::Bang, name) => Some(name),
+            _ => None,
+        })
+        .flatten();
+
+    Some((span, visible_macro))
+}
+
+/// Walks through the expansion ancestors of `original_span` to find a span that
+/// is contained in `body_span` and has the same [`SyntaxContext`] as `body_span`.
+/// The ancestor that was traversed just before the matching span (if any) is
+/// also returned.
+///
+/// For example, a return value of `Some((ancestor, Some(prev))` means that:
+/// - `ancestor == original_span.find_ancestor_inside_same_ctxt(body_span)`
+/// - `ancestor == prev.parent_callsite()`
+///
+/// [`SyntaxContext`]: rustc_span::SyntaxContext
+fn unexpand_into_body_span_with_prev(
+    original_span: Span,
+    body_span: Span,
+) -> Option<(Span, Option<Span>)> {
+    let mut prev = None;
+    let mut curr = original_span;
+
+    while !body_span.contains(curr) || !curr.eq_ctxt(body_span) {
+        prev = Some(curr);
+        curr = curr.parent_callsite()?;
+    }
+
+    debug_assert_eq!(Some(curr), original_span.find_ancestor_in_same_ctxt(body_span));
+    if let Some(prev) = prev {
+        debug_assert_eq!(Some(curr), prev.parent_callsite());
+    }
+
+    Some((curr, prev))
+}
+
+#[derive(Debug)]
+struct SpanFromMir {
+    /// A span that has been extracted from MIR and then "un-expanded" back to
+    /// within the current function's `body_span`. After various intermediate
+    /// processing steps, this span is emitted as part of the final coverage
+    /// mappings.
+    ///
+    /// With the exception of `fn_sig_span`, this should always be contained
+    /// within `body_span`.
+    span: Span,
+    visible_macro: Option<Symbol>,
+    bcb: BasicCoverageBlock,
+    is_closure: bool,
+}
+
+impl SpanFromMir {
+    fn for_fn_sig(fn_sig_span: Span) -> Self {
+        Self::new(fn_sig_span, None, START_BCB, false)
+    }
+
+    fn new(
+        span: Span,
+        visible_macro: Option<Symbol>,
+        bcb: BasicCoverageBlock,
+        is_closure: bool,
+    ) -> Self {
+        Self { span, visible_macro, bcb, is_closure }
+    }
+
+    fn into_coverage_span(self) -> CoverageSpan {
+        let Self { span, visible_macro: _, bcb, is_closure } = self;
+        CoverageSpan::new(span, bcb, is_closure)
+    }
 }
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index 0fe606cacf5..821aafe2fed 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -1755,11 +1755,8 @@ fn suggest_alternative_construction_methods(
             .filter_map(|item| {
                 // Only assoc fns that return `Self`
                 let fn_sig = self.r.tcx.fn_sig(item.def_id).skip_binder();
-                let ret_ty = fn_sig.output();
-                let ret_ty = self
-                    .r
-                    .tcx
-                    .normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), ret_ty);
+                // Don't normalize the return type, because that can cause cycle errors.
+                let ret_ty = fn_sig.output().skip_binder();
                 let ty::Adt(def, _args) = ret_ty.kind() else {
                     return None;
                 };
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
index 52f91d282f0..c9e8b30c4c4 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
@@ -521,7 +521,7 @@ fn parse(
     pub fn of_item(tcx: TyCtxt<'tcx>, item_def_id: DefId) -> Result<Option<Self>, ErrorGuaranteed> {
         if let Some(attr) = tcx.get_attr(item_def_id, sym::rustc_on_unimplemented) {
             return Self::parse_attribute(attr, false, tcx, item_def_id);
-        } else if tcx.features().diagnostic_namespace {
+        } else {
             tcx.get_attrs_by_path(item_def_id, &[sym::diagnostic, sym::on_unimplemented])
                 .filter_map(|attr| Self::parse_attribute(attr, true, tcx, item_def_id).transpose())
                 .try_fold(None, |aggr: Option<Self>, directive| {
@@ -592,8 +592,6 @@ pub fn of_item(tcx: TyCtxt<'tcx>, item_def_id: DefId) -> Result<Option<Self>, Er
                         Ok(Some(directive))
                     }
                 })
-        } else {
-            Ok(None)
         }
     }
 
diff --git a/library/core/src/future/future.rs b/library/core/src/future/future.rs
index 71b9464efd2..af2e422e8a0 100644
--- a/library/core/src/future/future.rs
+++ b/library/core/src/future/future.rs
@@ -28,7 +28,7 @@
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 #[stable(feature = "futures_api", since = "1.36.0")]
 #[lang = "future_trait"]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     label = "`{Self}` is not a future",
     message = "`{Self}` is not a future",
     note = "{Self} must be a future or must implement `IntoFuture` to be awaited"
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index 9316ec93f24..637418c6333 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -217,6 +217,7 @@
 #![feature(const_trait_impl)]
 #![feature(decl_macro)]
 #![feature(deprecated_suggestion)]
+#![feature(diagnostic_namespace)]
 #![feature(doc_cfg)]
 #![feature(doc_cfg_hide)]
 #![feature(doc_notable_trait)]
diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index 99762bccd18..69d54f06407 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -75,7 +75,7 @@
 /// [ub]: ../../reference/behavior-considered-undefined.html
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Send")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "`{Self}` cannot be sent between threads safely",
     label = "`{Self}` cannot be sent between threads safely"
 )]
@@ -134,7 +134,7 @@ unsafe impl<T: Sync + ?Sized> Send for &T {}
 #[doc(alias = "?", alias = "?Sized")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[lang = "sized"]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "the size for values of type `{Self}` cannot be known at compilation time",
     label = "doesn't have a size known at compile-time"
 )]
@@ -205,7 +205,7 @@ pub trait Unsize<T: ?Sized> {
 /// [RFC1445]: https://github.com/rust-lang/rfcs/blob/master/text/1445-restrict-constants-in-patterns.md
 /// [issue 63438]: https://github.com/rust-lang/rust/issues/63438
 #[unstable(feature = "structural_match", issue = "31434")]
-#[rustc_on_unimplemented(message = "the type `{Self}` does not `#[derive(PartialEq)]`")]
+#[diagnostic::on_unimplemented(message = "the type `{Self}` does not `#[derive(PartialEq)]`")]
 #[lang = "structural_peq"]
 pub trait StructuralPartialEq {
     // Empty.
@@ -273,7 +273,7 @@ pub trait StructuralPartialEq {
 /// of the two derives (`#[derive(PartialEq)]` and `#[derive(Eq)]`) and check
 /// that both of them are present as part of structural-match checking.
 #[unstable(feature = "structural_match", issue = "31434")]
-#[rustc_on_unimplemented(message = "the type `{Self}` does not `#[derive(Eq)]`")]
+#[diagnostic::on_unimplemented(message = "the type `{Self}` does not `#[derive(Eq)]`")]
 #[lang = "structural_teq"]
 pub trait StructuralEq {
     // Empty.
@@ -941,7 +941,7 @@ impl<T: ?Sized> !Freeze for UnsafeCell<T> {}
 /// [Pin]: crate::pin::Pin
 /// [`pin` module]: crate::pin
 #[stable(feature = "pin", since = "1.33.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     note = "consider using the `pin!` macro\nconsider using `Box::pin` if you need to access the pinned value outside of the current scope",
     message = "`{Self}` cannot be unpinned"
 )]
@@ -989,7 +989,7 @@ pub trait Destruct {}
 /// for any user type.
 #[unstable(feature = "tuple_trait", issue = "none")]
 #[lang = "tuple_trait"]
-#[rustc_on_unimplemented(message = "`{Self}` is not a tuple")]
+#[diagnostic::on_unimplemented(message = "`{Self}` is not a tuple")]
 #[rustc_deny_explicit_impl(implement_via_object = false)]
 pub trait Tuple {}
 
@@ -999,7 +999,7 @@ pub trait Tuple {}
 /// `*const ()` automatically implement this trait.
 #[unstable(feature = "pointer_like_trait", issue = "none")]
 #[lang = "pointer_like"]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "`{Self}` needs to have the same ABI as a pointer",
     label = "`{Self}` needs to be a pointer-like type"
 )]
@@ -1013,7 +1013,7 @@ pub trait PointerLike {}
 /// are `StructuralPartialEq`.
 #[lang = "const_param_ty"]
 #[unstable(feature = "adt_const_params", issue = "95174")]
-#[rustc_on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
+#[diagnostic::on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
 #[allow(multiple_supertrait_upcastable)]
 pub trait ConstParamTy: StructuralEq + StructuralPartialEq + Eq {}
 
diff --git a/library/core/src/ops/arith.rs b/library/core/src/ops/arith.rs
index 1773fdbf37c..bb3cdde66d1 100644
--- a/library/core/src/ops/arith.rs
+++ b/library/core/src/ops/arith.rs
@@ -307,7 +307,7 @@ fn sub(self, other: $t) -> $t { self - other }
 /// ```
 #[lang = "mul"]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot multiply `{Self}` by `{Rhs}`",
     label = "no implementation for `{Self} * {Rhs}`"
 )]
@@ -441,7 +441,7 @@ fn mul(self, other: $t) -> $t { self * other }
 /// ```
 #[lang = "div"]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot divide `{Self}` by `{Rhs}`",
     label = "no implementation for `{Self} / {Rhs}`"
 )]
@@ -543,7 +543,7 @@ fn div(self, other: $t) -> $t { self / other }
 /// ```
 #[lang = "rem"]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot calculate the remainder of `{Self}` divided by `{Rhs}`",
     label = "no implementation for `{Self} % {Rhs}`"
 )]
@@ -729,7 +729,7 @@ fn neg(self) -> $t { -self }
 /// ```
 #[lang = "add_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot add-assign `{Rhs}` to `{Self}`",
     label = "no implementation for `{Self} += {Rhs}`"
 )]
@@ -796,7 +796,7 @@ fn add_assign(&mut self, other: $t) { *self += other }
 /// ```
 #[lang = "sub_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot subtract-assign `{Rhs}` from `{Self}`",
     label = "no implementation for `{Self} -= {Rhs}`"
 )]
@@ -854,7 +854,7 @@ fn sub_assign(&mut self, other: $t) { *self -= other }
 /// ```
 #[lang = "mul_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot multiply-assign `{Self}` by `{Rhs}`",
     label = "no implementation for `{Self} *= {Rhs}`"
 )]
@@ -912,7 +912,7 @@ fn mul_assign(&mut self, other: $t) { *self *= other }
 /// ```
 #[lang = "div_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot divide-assign `{Self}` by `{Rhs}`",
     label = "no implementation for `{Self} /= {Rhs}`"
 )]
@@ -973,7 +973,7 @@ fn div_assign(&mut self, other: $t) { *self /= other }
 /// ```
 #[lang = "rem_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot calculate and assign the remainder of `{Self}` divided by `{Rhs}`",
     label = "no implementation for `{Self} %= {Rhs}`"
 )]
diff --git a/library/core/src/ops/bit.rs b/library/core/src/ops/bit.rs
index c70f4a3da2e..6984100e498 100644
--- a/library/core/src/ops/bit.rs
+++ b/library/core/src/ops/bit.rs
@@ -137,7 +137,7 @@ fn not(self) -> ! {
 #[lang = "bitand"]
 #[doc(alias = "&")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} & {Rhs}`",
     label = "no implementation for `{Self} & {Rhs}`"
 )]
@@ -237,7 +237,7 @@ fn bitand(self, rhs: $t) -> $t { self & rhs }
 #[lang = "bitor"]
 #[doc(alias = "|")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} | {Rhs}`",
     label = "no implementation for `{Self} | {Rhs}`"
 )]
@@ -337,7 +337,7 @@ fn bitor(self, rhs: $t) -> $t { self | rhs }
 #[lang = "bitxor"]
 #[doc(alias = "^")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} ^ {Rhs}`",
     label = "no implementation for `{Self} ^ {Rhs}`"
 )]
@@ -436,7 +436,7 @@ fn bitxor(self, other: $t) -> $t { self ^ other }
 #[lang = "shl"]
 #[doc(alias = "<<")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} << {Rhs}`",
     label = "no implementation for `{Self} << {Rhs}`"
 )]
@@ -554,7 +554,7 @@ macro_rules! shl_impl_all {
 #[lang = "shr"]
 #[doc(alias = ">>")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} >> {Rhs}`",
     label = "no implementation for `{Self} >> {Rhs}`"
 )]
@@ -681,7 +681,7 @@ macro_rules! shr_impl_all {
 #[lang = "bitand_assign"]
 #[doc(alias = "&=")]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} &= {Rhs}`",
     label = "no implementation for `{Self} &= {Rhs}`"
 )]
@@ -752,7 +752,7 @@ fn bitand_assign(&mut self, other: $t) { *self &= other }
 #[lang = "bitor_assign"]
 #[doc(alias = "|=")]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} |= {Rhs}`",
     label = "no implementation for `{Self} |= {Rhs}`"
 )]
@@ -823,7 +823,7 @@ fn bitor_assign(&mut self, other: $t) { *self |= other }
 #[lang = "bitxor_assign"]
 #[doc(alias = "^=")]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} ^= {Rhs}`",
     label = "no implementation for `{Self} ^= {Rhs}`"
 )]
@@ -892,7 +892,7 @@ fn bitxor_assign(&mut self, other: $t) { *self ^= other }
 #[lang = "shl_assign"]
 #[doc(alias = "<<=")]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} <<= {Rhs}`",
     label = "no implementation for `{Self} <<= {Rhs}`"
 )]
@@ -974,7 +974,7 @@ macro_rules! shl_assign_impl_all {
 #[lang = "shr_assign"]
 #[doc(alias = ">>=")]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} >>= {Rhs}`",
     label = "no implementation for `{Self} >>= {Rhs}`"
 )]
diff --git a/library/core/src/ops/index.rs b/library/core/src/ops/index.rs
index 6ceee463729..37d9a28fb99 100644
--- a/library/core/src/ops/index.rs
+++ b/library/core/src/ops/index.rs
@@ -47,7 +47,7 @@
 /// assert_eq!(nucleotide_count[Nucleotide::T], 12);
 /// ```
 #[lang = "index"]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "the type `{Self}` cannot be indexed by `{Idx}`",
     label = "`{Self}` cannot be indexed by `{Idx}`"
 )]
diff --git a/library/core/src/panic/unwind_safe.rs b/library/core/src/panic/unwind_safe.rs
index 6a53909a8f1..37859212c0e 100644
--- a/library/core/src/panic/unwind_safe.rs
+++ b/library/core/src/panic/unwind_safe.rs
@@ -83,7 +83,7 @@
 /// implemented for any closed over variables passed to `catch_unwind`.
 #[stable(feature = "catch_unwind", since = "1.9.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "unwind_safe_trait")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "the type `{Self}` may not be safely transferred across an unwind boundary",
     label = "`{Self}` may not be safely transferred across an unwind boundary"
 )]
@@ -99,7 +99,7 @@
 /// [`UnwindSafe`] trait, for more information see that documentation.
 #[stable(feature = "catch_unwind", since = "1.9.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "ref_unwind_safe_trait")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "the type `{Self}` may contain interior mutability and a reference may not be safely \
                transferrable across a catch_unwind boundary",
     label = "`{Self}` may contain interior mutability and a reference may not be safely \
diff --git a/library/rtstartup/rsbegin.rs b/library/rtstartup/rsbegin.rs
index 1df0c897053..14bce2bbeee 100644
--- a/library/rtstartup/rsbegin.rs
+++ b/library/rtstartup/rsbegin.rs
@@ -18,6 +18,7 @@
 #![crate_type = "rlib"]
 #![no_core]
 #![allow(non_camel_case_types)]
+#![allow(internal_features)]
 
 #[lang = "sized"]
 trait Sized {}
diff --git a/library/rtstartup/rsend.rs b/library/rtstartup/rsend.rs
index d5aca80edf9..714643c8386 100644
--- a/library/rtstartup/rsend.rs
+++ b/library/rtstartup/rsend.rs
@@ -5,6 +5,7 @@
 #![feature(auto_traits)]
 #![crate_type = "rlib"]
 #![no_core]
+#![allow(internal_features)]
 
 #[lang = "sized"]
 trait Sized {}
diff --git a/library/std/src/fs/tests.rs b/library/std/src/fs/tests.rs
index 12afdef2669..5917bf8df02 100644
--- a/library/std/src/fs/tests.rs
+++ b/library/std/src/fs/tests.rs
@@ -936,8 +936,10 @@ fn read_link() {
         }
         // Check that readlink works with non-drive paths on Windows.
         let link = tmpdir.join("link_unc");
-        check!(symlink_dir(r"\\localhost\c$\", &link));
-        assert_eq!(check!(fs::read_link(&link)), Path::new(r"\\localhost\c$\"));
+        if got_symlink_permission(&tmpdir) {
+            check!(symlink_dir(r"\\localhost\c$\", &link));
+            assert_eq!(check!(fs::read_link(&link)), Path::new(r"\\localhost\c$\"));
+        };
     }
     let link = tmpdir.join("link");
     if !got_symlink_permission(&tmpdir) {
diff --git a/src/bootstrap/src/core/build_steps/compile.rs b/src/bootstrap/src/core/build_steps/compile.rs
index d699c4fe536..33d0dffe26e 100644
--- a/src/bootstrap/src/core/build_steps/compile.rs
+++ b/src/bootstrap/src/core/build_steps/compile.rs
@@ -905,34 +905,6 @@ fn run(self, builder: &Builder<'_>) {
             ));
         }
 
-        // We currently don't support cross-crate LTO in stage0. This also isn't hugely necessary
-        // and may just be a time sink.
-        if compiler.stage != 0 {
-            match builder.config.rust_lto {
-                RustcLto::Thin | RustcLto::Fat => {
-                    // Since using LTO for optimizing dylibs is currently experimental,
-                    // we need to pass -Zdylib-lto.
-                    cargo.rustflag("-Zdylib-lto");
-                    // Cargo by default passes `-Cembed-bitcode=no` and doesn't pass `-Clto` when
-                    // compiling dylibs (and their dependencies), even when LTO is enabled for the
-                    // crate. Therefore, we need to override `-Clto` and `-Cembed-bitcode` here.
-                    let lto_type = match builder.config.rust_lto {
-                        RustcLto::Thin => "thin",
-                        RustcLto::Fat => "fat",
-                        _ => unreachable!(),
-                    };
-                    cargo.rustflag(&format!("-Clto={lto_type}"));
-                    cargo.rustflag("-Cembed-bitcode=yes");
-                }
-                RustcLto::ThinLocal => { /* Do nothing, this is the default */ }
-                RustcLto::Off => {
-                    cargo.rustflag("-Clto=off");
-                }
-            }
-        } else if builder.config.rust_lto == RustcLto::Off {
-            cargo.rustflag("-Clto=off");
-        }
-
         for krate in &*self.crates {
             cargo.arg("-p").arg(krate);
         }
@@ -989,6 +961,34 @@ pub fn rustc_cargo(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetSelec
 
     cargo.rustdocflag("-Zcrate-attr=warn(rust_2018_idioms)");
 
+    // We currently don't support cross-crate LTO in stage0. This also isn't hugely necessary
+    // and may just be a time sink.
+    if stage != 0 {
+        match builder.config.rust_lto {
+            RustcLto::Thin | RustcLto::Fat => {
+                // Since using LTO for optimizing dylibs is currently experimental,
+                // we need to pass -Zdylib-lto.
+                cargo.rustflag("-Zdylib-lto");
+                // Cargo by default passes `-Cembed-bitcode=no` and doesn't pass `-Clto` when
+                // compiling dylibs (and their dependencies), even when LTO is enabled for the
+                // crate. Therefore, we need to override `-Clto` and `-Cembed-bitcode` here.
+                let lto_type = match builder.config.rust_lto {
+                    RustcLto::Thin => "thin",
+                    RustcLto::Fat => "fat",
+                    _ => unreachable!(),
+                };
+                cargo.rustflag(&format!("-Clto={lto_type}"));
+                cargo.rustflag("-Cembed-bitcode=yes");
+            }
+            RustcLto::ThinLocal => { /* Do nothing, this is the default */ }
+            RustcLto::Off => {
+                cargo.rustflag("-Clto=off");
+            }
+        }
+    } else if builder.config.rust_lto == RustcLto::Off {
+        cargo.rustflag("-Clto=off");
+    }
+
     rustc_cargo_env(builder, cargo, target, stage);
 }
 
diff --git a/src/librustdoc/html/render/mod.rs b/src/librustdoc/html/render/mod.rs
index 118c6eeb289..bea5ccd7c86 100644
--- a/src/librustdoc/html/render/mod.rs
+++ b/src/librustdoc/html/render/mod.rs
@@ -58,7 +58,7 @@
     symbol::{sym, Symbol},
     BytePos, FileName, RealFileName,
 };
-use serde::ser::{SerializeMap, SerializeSeq};
+use serde::ser::SerializeMap;
 use serde::{Serialize, Serializer};
 
 use crate::clean::{self, ItemId, RenderedLink, SelfTy};
@@ -123,44 +123,58 @@ pub(crate) struct IndexItem {
 }
 
 /// A type used for the search index.
-#[derive(Debug)]
+#[derive(Debug, Eq, PartialEq)]
 pub(crate) struct RenderType {
     id: Option<RenderTypeId>,
     generics: Option<Vec<RenderType>>,
     bindings: Option<Vec<(RenderTypeId, Vec<RenderType>)>>,
 }
 
-impl Serialize for RenderType {
-    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
-    where
-        S: Serializer,
-    {
-        let id = match &self.id {
+impl RenderType {
+    // Types are rendered as lists of lists, because that's pretty compact.
+    // The contents of the lists are always integers in self-terminating hex
+    // form, handled by `RenderTypeId::write_to_string`, so no commas are
+    // needed to separate the items.
+    pub fn write_to_string(&self, string: &mut String) {
+        fn write_optional_id(id: Option<RenderTypeId>, string: &mut String) {
             // 0 is a sentinel, everything else is one-indexed
-            None => 0,
-            // concrete type
-            Some(RenderTypeId::Index(idx)) if *idx >= 0 => idx + 1,
-            // generic type parameter
-            Some(RenderTypeId::Index(idx)) => *idx,
-            _ => panic!("must convert render types to indexes before serializing"),
-        };
-        if self.generics.is_some() || self.bindings.is_some() {
-            let mut seq = serializer.serialize_seq(None)?;
-            seq.serialize_element(&id)?;
-            seq.serialize_element(self.generics.as_ref().map(Vec::as_slice).unwrap_or_default())?;
-            if self.bindings.is_some() {
-                seq.serialize_element(
-                    self.bindings.as_ref().map(Vec::as_slice).unwrap_or_default(),
-                )?;
+            match id {
+                Some(id) => id.write_to_string(string),
+                None => string.push('`'),
             }
-            seq.end()
+        }
+        // Either just the type id, or `{type, generics, bindings?}`
+        // where generics is a list of types,
+        // and bindings is a list of `{id, typelist}` pairs.
+        if self.generics.is_some() || self.bindings.is_some() {
+            string.push('{');
+            write_optional_id(self.id, string);
+            string.push('{');
+            for generic in &self.generics.as_ref().map(Vec::as_slice).unwrap_or_default()[..] {
+                generic.write_to_string(string);
+            }
+            string.push('}');
+            if self.bindings.is_some() {
+                string.push('{');
+                for binding in &self.bindings.as_ref().map(Vec::as_slice).unwrap_or_default()[..] {
+                    string.push('{');
+                    binding.0.write_to_string(string);
+                    string.push('{');
+                    for constraint in &binding.1[..] {
+                        constraint.write_to_string(string);
+                    }
+                    string.push_str("}}");
+                }
+                string.push('}');
+            }
+            string.push('}');
         } else {
-            id.serialize(serializer)
+            write_optional_id(self.id, string);
         }
     }
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub(crate) enum RenderTypeId {
     DefId(DefId),
     Primitive(clean::PrimitiveType),
@@ -168,70 +182,122 @@ pub(crate) enum RenderTypeId {
     Index(isize),
 }
 
-impl Serialize for RenderTypeId {
-    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
-    where
-        S: Serializer,
-    {
-        let id = match &self {
+impl RenderTypeId {
+    pub fn write_to_string(&self, string: &mut String) {
+        // (sign, value)
+        let (sign, id): (bool, u32) = match &self {
             // 0 is a sentinel, everything else is one-indexed
             // concrete type
-            RenderTypeId::Index(idx) if *idx >= 0 => idx + 1,
+            RenderTypeId::Index(idx) if *idx >= 0 => (false, (idx + 1isize).try_into().unwrap()),
             // generic type parameter
-            RenderTypeId::Index(idx) => *idx,
+            RenderTypeId::Index(idx) => (true, (-*idx).try_into().unwrap()),
             _ => panic!("must convert render types to indexes before serializing"),
         };
-        id.serialize(serializer)
+        // zig-zag encoding
+        let value: u32 = (id << 1) | (if sign { 1 } else { 0 });
+        // Self-terminating hex use capital letters for everything but the
+        // least significant digit, which is lowercase. For example, decimal 17
+        // would be `` Aa `` if zig-zag encoding weren't used.
+        //
+        // Zig-zag encoding, however, stores the sign bit as the last bit.
+        // This means, in the last hexit, 1 is actually `c`, -1 is `b`
+        // (`a` is the imaginary -0), and, because all the bits are shifted
+        // by one, `` A` `` is actually 8 and `` Aa `` is -8.
+        //
+        // https://rust-lang.github.io/rustc-dev-guide/rustdoc-internals/search.html
+        // describes the encoding in more detail.
+        let mut shift: u32 = 28;
+        let mut mask: u32 = 0xF0_00_00_00;
+        while shift < 32 {
+            let hexit = (value & mask) >> shift;
+            if hexit != 0 || shift == 0 {
+                let hex =
+                    char::try_from(if shift == 0 { '`' } else { '@' } as u32 + hexit).unwrap();
+                string.push(hex);
+            }
+            shift = shift.wrapping_sub(4);
+            mask = mask >> 4;
+        }
     }
 }
 
 /// Full type of functions/methods in the search index.
-#[derive(Debug)]
+#[derive(Debug, Eq, PartialEq)]
 pub(crate) struct IndexItemFunctionType {
     inputs: Vec<RenderType>,
     output: Vec<RenderType>,
     where_clause: Vec<Vec<RenderType>>,
 }
 
-impl Serialize for IndexItemFunctionType {
-    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
-    where
-        S: Serializer,
-    {
-        // If we couldn't figure out a type, just write `0`.
+impl IndexItemFunctionType {
+    pub fn write_to_string<'a>(
+        &'a self,
+        string: &mut String,
+        backref_queue: &mut VecDeque<&'a IndexItemFunctionType>,
+    ) {
+        assert!(backref_queue.len() <= 16);
+        // If we couldn't figure out a type, just write 0,
+        // which is encoded as `` ` `` (see RenderTypeId::write_to_string).
         let has_missing = self
             .inputs
             .iter()
             .chain(self.output.iter())
             .any(|i| i.id.is_none() && i.generics.is_none());
         if has_missing {
-            0.serialize(serializer)
+            string.push('`');
+        } else if let Some(idx) = backref_queue.iter().position(|other| *other == self) {
+            // The backref queue has 16 items, so backrefs use
+            // a single hexit, disjoint from the ones used for numbers.
+            string.push(
+                char::try_from('0' as u32 + u32::try_from(idx).unwrap())
+                    .expect("last possible value is '?'"),
+            );
         } else {
-            let mut seq = serializer.serialize_seq(None)?;
+            backref_queue.push_front(self);
+            if backref_queue.len() > 16 {
+                backref_queue.pop_back();
+            }
+            string.push('{');
             match &self.inputs[..] {
                 [one] if one.generics.is_none() && one.bindings.is_none() => {
-                    seq.serialize_element(one)?
+                    one.write_to_string(string);
+                }
+                _ => {
+                    string.push('{');
+                    for item in &self.inputs[..] {
+                        item.write_to_string(string);
+                    }
+                    string.push('}');
                 }
-                _ => seq.serialize_element(&self.inputs)?,
             }
             match &self.output[..] {
                 [] if self.where_clause.is_empty() => {}
                 [one] if one.generics.is_none() && one.bindings.is_none() => {
-                    seq.serialize_element(one)?
+                    one.write_to_string(string);
+                }
+                _ => {
+                    string.push('{');
+                    for item in &self.output[..] {
+                        item.write_to_string(string);
+                    }
+                    string.push('}');
                 }
-                _ => seq.serialize_element(&self.output)?,
             }
             for constraint in &self.where_clause {
                 if let [one] = &constraint[..]
                     && one.generics.is_none()
                     && one.bindings.is_none()
                 {
-                    seq.serialize_element(one)?;
+                    one.write_to_string(string);
                 } else {
-                    seq.serialize_element(constraint)?;
+                    string.push('{');
+                    for item in &constraint[..] {
+                        item.write_to_string(string);
+                    }
+                    string.push('}');
                 }
             }
-            seq.end()
+            string.push('}');
         }
     }
 }
diff --git a/src/librustdoc/html/render/search_index.rs b/src/librustdoc/html/render/search_index.rs
index a1029320d2d..e49df400c83 100644
--- a/src/librustdoc/html/render/search_index.rs
+++ b/src/librustdoc/html/render/search_index.rs
@@ -1,5 +1,5 @@
 use std::collections::hash_map::Entry;
-use std::collections::BTreeMap;
+use std::collections::{BTreeMap, VecDeque};
 
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
 use rustc_middle::ty::TyCtxt;
@@ -409,9 +409,11 @@ fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
             let mut full_paths = Vec::with_capacity(self.items.len());
             let mut descriptions = Vec::with_capacity(self.items.len());
             let mut parents = Vec::with_capacity(self.items.len());
-            let mut functions = Vec::with_capacity(self.items.len());
+            let mut functions = String::with_capacity(self.items.len());
             let mut deprecated = Vec::with_capacity(self.items.len());
 
+            let mut backref_queue = VecDeque::new();
+
             for (index, item) in self.items.iter().enumerate() {
                 let n = item.ty as u8;
                 let c = char::try_from(n + b'A').expect("item types must fit in ASCII");
@@ -434,27 +436,10 @@ fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
                     full_paths.push((index, &item.path));
                 }
 
-                // Fake option to get `0` out as a sentinel instead of `null`.
-                // We want to use `0` because it's three less bytes.
-                enum FunctionOption<'a> {
-                    Function(&'a IndexItemFunctionType),
-                    None,
+                match &item.search_type {
+                    Some(ty) => ty.write_to_string(&mut functions, &mut backref_queue),
+                    None => functions.push('`'),
                 }
-                impl<'a> Serialize for FunctionOption<'a> {
-                    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
-                    where
-                        S: Serializer,
-                    {
-                        match self {
-                            FunctionOption::None => 0.serialize(serializer),
-                            FunctionOption::Function(ty) => ty.serialize(serializer),
-                        }
-                    }
-                }
-                functions.push(match &item.search_type {
-                    Some(ty) => FunctionOption::Function(ty),
-                    None => FunctionOption::None,
-                });
 
                 if item.deprecation.is_some() {
                     deprecated.push(index);
diff --git a/src/librustdoc/html/static/js/externs.js b/src/librustdoc/html/static/js/externs.js
index 93709e4e830..d24148b9556 100644
--- a/src/librustdoc/html/static/js/externs.js
+++ b/src/librustdoc/html/static/js/externs.js
@@ -200,3 +200,59 @@ let FunctionSearchType;
  * }}
  */
 let FunctionType;
+
+/**
+ * The raw search data for a given crate. `n`, `t`, `d`, `i`, and `f`
+ * are arrays with the same length. `q`, `a`, and `c` use a sparse
+ * representation for compactness.
+ *
+ * `n[i]` contains the name of an item.
+ *
+ * `t[i]` contains the type of that item
+ * (as a string of characters that represent an offset in `itemTypes`).
+ *
+ * `d[i]` contains the description of that item.
+ *
+ * `q` contains the full paths of the items. For compactness, it is a set of
+ * (index, path) pairs used to create a map. If a given index `i` is
+ * not present, this indicates "same as the last index present".
+ *
+ * `i[i]` contains an item's parent, usually a module. For compactness,
+ * it is a set of indexes into the `p` array.
+ *
+ * `f` contains function signatures, or `0` if the item isn't a function.
+ * More information on how they're encoded can be found in rustc-dev-guide
+ *
+ * Functions are themselves encoded as arrays. The first item is a list of
+ * types representing the function's inputs, and the second list item is a list
+ * of types representing the function's output. Tuples are flattened.
+ * Types are also represented as arrays; the first item is an index into the `p`
+ * array, while the second is a list of types representing any generic parameters.
+ *
+ * b[i] contains an item's impl disambiguator. This is only present if an item
+ * is defined in an impl block and, the impl block's type has more than one associated
+ * item with the same name.
+ *
+ * `a` defines aliases with an Array of pairs: [name, offset], where `offset`
+ * points into the n/t/d/q/i/f arrays.
+ *
+ * `doc` contains the description of the crate.
+ *
+ * `p` is a list of path/type pairs. It is used for parents and function parameters.
+ *
+ * `c` is an array of item indices that are deprecated.
+ * @typedef {{
+ *   doc: string,
+ *   a: Object,
+ *   n: Array<string>,
+ *   t: String,
+ *   d: Array<string>,
+ *   q: Array<[Number, string]>,
+ *   i: Array<Number>,
+ *   f: string,
+ *   p: Array<Object>,
+ *   b: Array<[Number, String]>,
+ *   c: Array<Number>
+ * }}
+ */
+let RawSearchIndexCrate;
diff --git a/src/librustdoc/html/static/js/search.js b/src/librustdoc/html/static/js/search.js
index e6263db3283..e0708485fc0 100644
--- a/src/librustdoc/html/static/js/search.js
+++ b/src/librustdoc/html/static/js/search.js
@@ -2767,19 +2767,65 @@ ${item.displayPath}<span class="${type}">${name}</span>\
      * The raw function search type format is generated using serde in
      * librustdoc/html/render/mod.rs: impl Serialize for IndexItemFunctionType
      *
-     * @param {RawFunctionSearchType} functionSearchType
+     * @param {{
+     *  string: string,
+     *  offset: number,
+     *  backrefQueue: FunctionSearchType[]
+     * }} itemFunctionDecoder
      * @param {Array<{name: string, ty: number}>} lowercasePaths
      * @param {Map<string, integer>}
      *
      * @return {null|FunctionSearchType}
      */
-    function buildFunctionSearchType(functionSearchType, lowercasePaths) {
-        const INPUTS_DATA = 0;
-        const OUTPUT_DATA = 1;
-        // `0` is used as a sentinel because it's fewer bytes than `null`
-        if (functionSearchType === 0) {
+    function buildFunctionSearchType(itemFunctionDecoder, lowercasePaths) {
+        const c = itemFunctionDecoder.string.charCodeAt(itemFunctionDecoder.offset);
+        itemFunctionDecoder.offset += 1;
+        const [zero, ua, la, ob, cb] = ["0", "@", "`", "{", "}"].map(c => c.charCodeAt(0));
+        // `` ` `` is used as a sentinel because it's fewer bytes than `null`, and decodes to zero
+        // `0` is a backref
+        if (c === la) {
             return null;
         }
+        // sixteen characters after "0" are backref
+        if (c >= zero && c < ua) {
+            return itemFunctionDecoder.backrefQueue[c - zero];
+        }
+        if (c !== ob) {
+            throw ["Unexpected ", c, " in function: expected ", "{", "; this is a bug"];
+        }
+        // call after consuming `{`
+        function decodeList() {
+            let c = itemFunctionDecoder.string.charCodeAt(itemFunctionDecoder.offset);
+            const ret = [];
+            while (c !== cb) {
+                ret.push(decode());
+                c = itemFunctionDecoder.string.charCodeAt(itemFunctionDecoder.offset);
+            }
+            itemFunctionDecoder.offset += 1; // eat cb
+            return ret;
+        }
+        // consumes and returns a list or integer
+        function decode() {
+            let n = 0;
+            let c = itemFunctionDecoder.string.charCodeAt(itemFunctionDecoder.offset);
+            if (c === ob) {
+                itemFunctionDecoder.offset += 1;
+                return decodeList();
+            }
+            while (c < la) {
+                n = (n << 4) | (c & 0xF);
+                itemFunctionDecoder.offset += 1;
+                c = itemFunctionDecoder.string.charCodeAt(itemFunctionDecoder.offset);
+            }
+            // last character >= la
+            n = (n << 4) | (c & 0xF);
+            const [sign, value] = [n & 1, n >> 1];
+            itemFunctionDecoder.offset += 1;
+            return sign ? -value : value;
+        }
+        const functionSearchType = decodeList();
+        const INPUTS_DATA = 0;
+        const OUTPUT_DATA = 1;
         let inputs, output;
         if (typeof functionSearchType[INPUTS_DATA] === "number") {
             inputs = [buildItemSearchType(functionSearchType[INPUTS_DATA], lowercasePaths)];
@@ -2808,9 +2854,14 @@ ${item.displayPath}<span class="${type}">${name}</span>\
                 ? [buildItemSearchType(functionSearchType[i], lowercasePaths)]
                 : buildItemSearchTypeAll(functionSearchType[i], lowercasePaths));
         }
-        return {
+        const ret = {
             inputs, output, where_clause,
         };
+        itemFunctionDecoder.backrefQueue.unshift(ret);
+        if (itemFunctionDecoder.backrefQueue.length > 16) {
+            itemFunctionDecoder.backrefQueue.pop();
+        }
+        return ret;
     }
 
     /**
@@ -2924,6 +2975,11 @@ ${item.displayPath}<span class="${type}">${name}</span>\
         return functionTypeFingerprint[(fullId * 4) + 3];
     }
 
+    /**
+     * Convert raw search index into in-memory search index.
+     *
+     * @param {[string, RawSearchIndexCrate][]} rawSearchIndex
+     */
     function buildIndex(rawSearchIndex) {
         searchIndex = [];
         typeNameIdMap = new Map();
@@ -2950,59 +3006,7 @@ ${item.displayPath}<span class="${type}">${name}</span>\
         // This loop actually generates the search item indexes, including
         // normalized names, type signature objects and fingerprints, and aliases.
         id = 0;
-        /**
-         * The raw search data for a given crate. `n`, `t`, `d`, `i`, and `f`
-         * are arrays with the same length. `q`, `a`, and `c` use a sparse
-         * representation for compactness.
-         *
-         * `n[i]` contains the name of an item.
-         *
-         * `t[i]` contains the type of that item
-         * (as a string of characters that represent an offset in `itemTypes`).
-         *
-         * `d[i]` contains the description of that item.
-         *
-         * `q` contains the full paths of the items. For compactness, it is a set of
-         * (index, path) pairs used to create a map. If a given index `i` is
-         * not present, this indicates "same as the last index present".
-         *
-         * `i[i]` contains an item's parent, usually a module. For compactness,
-         * it is a set of indexes into the `p` array.
-         *
-         * `f[i]` contains function signatures, or `0` if the item isn't a function.
-         * Functions are themselves encoded as arrays. The first item is a list of
-         * types representing the function's inputs, and the second list item is a list
-         * of types representing the function's output. Tuples are flattened.
-         * Types are also represented as arrays; the first item is an index into the `p`
-         * array, while the second is a list of types representing any generic parameters.
-         *
-         * b[i] contains an item's impl disambiguator. This is only present if an item
-         * is defined in an impl block and, the impl block's type has more than one associated
-         * item with the same name.
-         *
-         * `a` defines aliases with an Array of pairs: [name, offset], where `offset`
-         * points into the n/t/d/q/i/f arrays.
-         *
-         * `doc` contains the description of the crate.
-         *
-         * `p` is a list of path/type pairs. It is used for parents and function parameters.
-         *
-         * `c` is an array of item indices that are deprecated.
-         *
-         * @type {{
-         *   doc: string,
-         *   a: Object,
-         *   n: Array<string>,
-         *   t: String,
-         *   d: Array<string>,
-         *   q: Array<[Number, string]>,
-         *   i: Array<Number>,
-         *   f: Array<RawFunctionSearchType>,
-         *   p: Array<Object>,
-         *   b: Array<[Number, String]>,
-         *   c: Array<Number>
-         * }}
-         */
+
         for (const [crate, crateCorpus] of rawSearchIndex) {
             // This object should have exactly the same set of fields as the "row"
             // object defined below. Your JavaScript runtime will thank you.
@@ -3039,8 +3043,12 @@ ${item.displayPath}<span class="${type}">${name}</span>\
             const itemDescs = crateCorpus.d;
             // an array of (Number) the parent path index + 1 to `paths`, or 0 if none
             const itemParentIdxs = crateCorpus.i;
-            // an array of (Object | null) the type of the function, if any
-            const itemFunctionSearchTypes = crateCorpus.f;
+            // a string representing the list of function types
+            const itemFunctionDecoder = {
+                string: crateCorpus.f,
+                offset: 0,
+                backrefQueue: [],
+            };
             // an array of (Number) indices for the deprecated items
             const deprecatedItems = new Set(crateCorpus.c);
             // an array of (Number) indices for the deprecated items
@@ -3088,12 +3096,8 @@ ${item.displayPath}<span class="${type}">${name}</span>\
                     word = itemNames[i].toLowerCase();
                 }
                 const path = itemPaths.has(i) ? itemPaths.get(i) : lastPath;
-                let type = null;
-                if (itemFunctionSearchTypes[i] !== 0) {
-                    type = buildFunctionSearchType(
-                        itemFunctionSearchTypes[i],
-                        lowercasePaths
-                    );
+                const type = buildFunctionSearchType(itemFunctionDecoder, lowercasePaths);
+                if (type !== null) {
                     if (type) {
                         const fp = functionTypeFingerprint.subarray(id * 4, (id + 1) * 4);
                         const fps = new Set();
diff --git a/tests/coverage/closure.cov-map b/tests/coverage/closure.cov-map
index 522c1e73afe..c5ac17600cd 100644
--- a/tests/coverage/closure.cov-map
+++ b/tests/coverage/closure.cov-map
@@ -81,21 +81,18 @@ Number of file 0 mappings: 1
 - Code(Zero) at (prev + 171, 13) to (start + 2, 14)
 
 Function name: closure::main::{closure#14}
-Raw bytes (36): 0x[01, 01, 03, 05, 0a, 01, 05, 01, 05, 05, 03, b2, 01, 0d, 00, 15, 01, 01, 11, 01, 1b, 05, 01, 1e, 00, 25, 0a, 00, 2f, 00, 33, 03, 01, 0d, 00, 0e]
+Raw bytes (29): 0x[01, 01, 02, 01, 05, 05, 02, 04, 01, b2, 01, 0d, 02, 1b, 05, 02, 1e, 00, 25, 02, 00, 2f, 00, 33, 07, 01, 0d, 00, 0e]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 3
-- expression 0 operands: lhs = Counter(1), rhs = Expression(2, Sub)
-- expression 1 operands: lhs = Counter(0), rhs = Counter(1)
-- expression 2 operands: lhs = Counter(0), rhs = Counter(1)
-Number of file 0 mappings: 5
-- Code(Expression(0, Add)) at (prev + 178, 13) to (start + 0, 21)
-    = (c1 + (c0 - c1))
-- Code(Counter(0)) at (prev + 1, 17) to (start + 1, 27)
-- Code(Counter(1)) at (prev + 1, 30) to (start + 0, 37)
-- Code(Expression(2, Sub)) at (prev + 0, 47) to (start + 0, 51)
+Number of expressions: 2
+- expression 0 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 178, 13) to (start + 2, 27)
+- Code(Counter(1)) at (prev + 2, 30) to (start + 0, 37)
+- Code(Expression(0, Sub)) at (prev + 0, 47) to (start + 0, 51)
     = (c0 - c1)
-- Code(Expression(0, Add)) at (prev + 1, 13) to (start + 0, 14)
+- Code(Expression(1, Add)) at (prev + 1, 13) to (start + 0, 14)
     = (c1 + (c0 - c1))
 
 Function name: closure::main::{closure#15}
@@ -118,21 +115,18 @@ Number of file 0 mappings: 6
     = (c1 + (c0 - c1))
 
 Function name: closure::main::{closure#16}
-Raw bytes (36): 0x[01, 01, 03, 05, 0a, 01, 05, 01, 05, 05, 03, c4, 01, 0d, 00, 15, 01, 01, 11, 01, 1b, 05, 01, 1e, 00, 25, 0a, 00, 2f, 00, 33, 03, 01, 0d, 00, 0e]
+Raw bytes (29): 0x[01, 01, 02, 01, 05, 05, 02, 04, 01, c4, 01, 0d, 02, 1b, 05, 02, 1e, 00, 25, 02, 00, 2f, 00, 33, 07, 01, 0d, 00, 0e]
 Number of files: 1
 - file 0 => global file 1
-Number of expressions: 3
-- expression 0 operands: lhs = Counter(1), rhs = Expression(2, Sub)
-- expression 1 operands: lhs = Counter(0), rhs = Counter(1)
-- expression 2 operands: lhs = Counter(0), rhs = Counter(1)
-Number of file 0 mappings: 5
-- Code(Expression(0, Add)) at (prev + 196, 13) to (start + 0, 21)
-    = (c1 + (c0 - c1))
-- Code(Counter(0)) at (prev + 1, 17) to (start + 1, 27)
-- Code(Counter(1)) at (prev + 1, 30) to (start + 0, 37)
-- Code(Expression(2, Sub)) at (prev + 0, 47) to (start + 0, 51)
+Number of expressions: 2
+- expression 0 operands: lhs = Counter(0), rhs = Counter(1)
+- expression 1 operands: lhs = Counter(1), rhs = Expression(0, Sub)
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 196, 13) to (start + 2, 27)
+- Code(Counter(1)) at (prev + 2, 30) to (start + 0, 37)
+- Code(Expression(0, Sub)) at (prev + 0, 47) to (start + 0, 51)
     = (c0 - c1)
-- Code(Expression(0, Add)) at (prev + 1, 13) to (start + 0, 14)
+- Code(Expression(1, Add)) at (prev + 1, 13) to (start + 0, 14)
     = (c1 + (c0 - c1))
 
 Function name: closure::main::{closure#17}
diff --git a/tests/coverage/inline-dead.cov-map b/tests/coverage/inline-dead.cov-map
index 958b423f24c..ab04e746b91 100644
--- a/tests/coverage/inline-dead.cov-map
+++ b/tests/coverage/inline-dead.cov-map
@@ -31,14 +31,14 @@ Number of file 0 mappings: 2
 - Code(Counter(0)) at (prev + 7, 6) to (start + 2, 2)
 
 Function name: inline_dead::main::{closure#0}
-Raw bytes (23): 0x[01, 01, 02, 00, 06, 01, 00, 03, 01, 07, 17, 00, 18, 00, 02, 0d, 00, 0e, 03, 02, 05, 00, 06]
+Raw bytes (23): 0x[01, 01, 02, 00, 06, 01, 00, 03, 01, 07, 17, 01, 16, 00, 02, 0d, 00, 0e, 03, 02, 05, 00, 06]
 Number of files: 1
 - file 0 => global file 1
 Number of expressions: 2
 - expression 0 operands: lhs = Zero, rhs = Expression(1, Sub)
 - expression 1 operands: lhs = Counter(0), rhs = Zero
 Number of file 0 mappings: 3
-- Code(Counter(0)) at (prev + 7, 23) to (start + 0, 24)
+- Code(Counter(0)) at (prev + 7, 23) to (start + 1, 22)
 - Code(Zero) at (prev + 2, 13) to (start + 0, 14)
 - Code(Expression(0, Add)) at (prev + 2, 5) to (start + 0, 6)
     = (Zero + (c0 - Zero))
diff --git a/tests/coverage/inline-dead.coverage b/tests/coverage/inline-dead.coverage
index de96aa17acd..7c201f482db 100644
--- a/tests/coverage/inline-dead.coverage
+++ b/tests/coverage/inline-dead.coverage
@@ -5,7 +5,7 @@
    LL|      1|    println!("{}", live::<false>());
    LL|      1|
    LL|      1|    let f = |x: bool| {
-   LL|       |        debug_assert!(
+   LL|      1|        debug_assert!(
    LL|      0|            x
    LL|       |        );
    LL|      1|    };
diff --git a/tests/rustdoc/synthetic_auto/no-redundancy.rs b/tests/rustdoc/synthetic_auto/no-redundancy.rs
index ea57d7388b8..fed9c9c7ba4 100644
--- a/tests/rustdoc/synthetic_auto/no-redundancy.rs
+++ b/tests/rustdoc/synthetic_auto/no-redundancy.rs
@@ -1,3 +1,6 @@
+// FIXME(fmease, #119216): Reenable this test!
+// ignore-test
+
 pub struct Inner<T> {
     field: T,
 }
diff --git a/tests/ui/diagnostic_namespace/on_unimplemented/auxiliary/other.rs b/tests/ui/diagnostic_namespace/on_unimplemented/auxiliary/other.rs
new file mode 100644
index 00000000000..884bab2800a
--- /dev/null
+++ b/tests/ui/diagnostic_namespace/on_unimplemented/auxiliary/other.rs
@@ -0,0 +1,8 @@
+#![feature(diagnostic_namespace)]
+
+#[diagnostic::on_unimplemented(
+    message = "Message",
+    note = "Note",
+    label = "label"
+)]
+pub trait Foo {}
diff --git a/tests/ui/diagnostic_namespace/on_unimplemented/error_is_shown_in_downstream_crates.rs b/tests/ui/diagnostic_namespace/on_unimplemented/error_is_shown_in_downstream_crates.rs
new file mode 100644
index 00000000000..b39375a09f3
--- /dev/null
+++ b/tests/ui/diagnostic_namespace/on_unimplemented/error_is_shown_in_downstream_crates.rs
@@ -0,0 +1,12 @@
+// aux-build:other.rs
+
+extern crate other;
+
+use other::Foo;
+
+fn take_foo(_: impl Foo) {}
+
+fn main() {
+    take_foo(());
+    //~^ERROR Message
+}
diff --git a/tests/ui/diagnostic_namespace/on_unimplemented/error_is_shown_in_downstream_crates.stderr b/tests/ui/diagnostic_namespace/on_unimplemented/error_is_shown_in_downstream_crates.stderr
new file mode 100644
index 00000000000..a9968538d0d
--- /dev/null
+++ b/tests/ui/diagnostic_namespace/on_unimplemented/error_is_shown_in_downstream_crates.stderr
@@ -0,0 +1,19 @@
+error[E0277]: Message
+  --> $DIR/error_is_shown_in_downstream_crates.rs:10:14
+   |
+LL |     take_foo(());
+   |     -------- ^^ label
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Foo` is not implemented for `()`
+   = note: Note
+note: required by a bound in `take_foo`
+  --> $DIR/error_is_shown_in_downstream_crates.rs:7:21
+   |
+LL | fn take_foo(_: impl Foo) {}
+   |                     ^^^ required by this bound in `take_foo`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/numbers-arithmetic/overflowing-rsh-6.rs b/tests/ui/numbers-arithmetic/overflowing-rsh-6.rs
deleted file mode 100644
index f75e779ed15..00000000000
--- a/tests/ui/numbers-arithmetic/overflowing-rsh-6.rs
+++ /dev/null
@@ -1,9 +0,0 @@
-// build-fail
-// compile-flags: -C debug-assertions
-
-#![deny(arithmetic_overflow)]
-
-fn main() {
-    let _n = 1i64 >> [64][0];
-    //~^ ERROR: this arithmetic operation will overflow
-}
diff --git a/tests/ui/numbers-arithmetic/overflowing-rsh-6.stderr b/tests/ui/numbers-arithmetic/overflowing-rsh-6.stderr
deleted file mode 100644
index 005f7378226..00000000000
--- a/tests/ui/numbers-arithmetic/overflowing-rsh-6.stderr
+++ /dev/null
@@ -1,14 +0,0 @@
-error: this arithmetic operation will overflow
-  --> $DIR/overflowing-rsh-6.rs:7:14
-   |
-LL |     let _n = 1i64 >> [64][0];
-   |              ^^^^^^^^^^^^^^^ attempt to shift right by `64_i32`, which would overflow
-   |
-note: the lint level is defined here
-  --> $DIR/overflowing-rsh-6.rs:4:9
-   |
-LL | #![deny(arithmetic_overflow)]
-   |         ^^^^^^^^^^^^^^^^^^^
-
-error: aborting due to 1 previous error
-
diff --git a/tests/ui/proc-macro/meta-macro-hygiene.stdout b/tests/ui/proc-macro/meta-macro-hygiene.stdout
index 6d10cc604c2..3672a3590fd 100644
--- a/tests/ui/proc-macro/meta-macro-hygiene.stdout
+++ b/tests/ui/proc-macro/meta-macro-hygiene.stdout
@@ -50,6 +50,9 @@ crate0::{{expn1}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt:
 crate0::{{expn2}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Bang, "produce_it")
 crate0::{{expn3}}: parent: crate0::{{expn2}}, call_site_ctxt: #3, def_site_ctxt: #0, kind: Macro(Bang, "meta_macro::print_def_site")
 crate0::{{expn4}}: parent: crate0::{{expn3}}, call_site_ctxt: #4, def_site_ctxt: #0, kind: Macro(Bang, "$crate::dummy")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "derive")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "derive")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Bang, "include")
diff --git a/tests/ui/proc-macro/nonterminal-token-hygiene.stdout b/tests/ui/proc-macro/nonterminal-token-hygiene.stdout
index 5c70e780f74..d3c2c46ac75 100644
--- a/tests/ui/proc-macro/nonterminal-token-hygiene.stdout
+++ b/tests/ui/proc-macro/nonterminal-token-hygiene.stdout
@@ -73,6 +73,9 @@ crate0::{{expn1}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt:
 crate0::{{expn2}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Bang, "outer")
 crate0::{{expn3}}: parent: crate0::{{expn2}}, call_site_ctxt: #3, def_site_ctxt: #3, kind: Macro(Bang, "inner")
 crate0::{{expn4}}: parent: crate0::{{expn3}}, call_site_ctxt: #5, def_site_ctxt: #0, kind: Macro(Bang, "print_bang")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "derive")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "derive")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Bang, "include")
diff --git a/tests/ui/resolve/auxiliary/suggest-constructor-cycle-error.rs b/tests/ui/resolve/auxiliary/suggest-constructor-cycle-error.rs
new file mode 100644
index 00000000000..8de68c38bc3
--- /dev/null
+++ b/tests/ui/resolve/auxiliary/suggest-constructor-cycle-error.rs
@@ -0,0 +1,12 @@
+mod m {
+    pub struct Uuid(());
+
+    impl Uuid {
+        pub fn encode_buffer() -> [u8; LENGTH] {
+            []
+        }
+    }
+    const LENGTH: usize = 0;
+}
+
+pub use m::Uuid;
diff --git a/tests/ui/resolve/suggest-constructor-cycle-error.rs b/tests/ui/resolve/suggest-constructor-cycle-error.rs
new file mode 100644
index 00000000000..e36fffd97d1
--- /dev/null
+++ b/tests/ui/resolve/suggest-constructor-cycle-error.rs
@@ -0,0 +1,10 @@
+// aux-build:suggest-constructor-cycle-error.rs
+
+// Regression test for https://github.com/rust-lang/rust/issues/119625
+
+extern crate suggest_constructor_cycle_error as a;
+
+const CONST_NAME: a::Uuid = a::Uuid(());
+//~^ ERROR: cannot initialize a tuple struct which contains private fields [E0423]
+
+fn main() {}
diff --git a/tests/ui/resolve/suggest-constructor-cycle-error.stderr b/tests/ui/resolve/suggest-constructor-cycle-error.stderr
new file mode 100644
index 00000000000..c6ec2465a43
--- /dev/null
+++ b/tests/ui/resolve/suggest-constructor-cycle-error.stderr
@@ -0,0 +1,15 @@
+error[E0423]: cannot initialize a tuple struct which contains private fields
+  --> $DIR/suggest-constructor-cycle-error.rs:7:29
+   |
+LL | const CONST_NAME: a::Uuid = a::Uuid(());
+   |                             ^^^^^^^
+   |
+note: constructor is not visible here due to private fields
+  --> $DIR/auxiliary/suggest-constructor-cycle-error.rs:2:21
+   |
+LL |     pub struct Uuid(());
+   |                     ^^ private field
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0423`.
diff --git a/tests/ui/type-alias-impl-trait/nested-in-anon-const.rs b/tests/ui/type-alias-impl-trait/nested-in-anon-const.rs
new file mode 100644
index 00000000000..e9d53c99d04
--- /dev/null
+++ b/tests/ui/type-alias-impl-trait/nested-in-anon-const.rs
@@ -0,0 +1,21 @@
+// Regression test for issue #119295.
+
+#![feature(type_alias_impl_trait)]
+
+type Bar<T> = T;
+type S<const A: usize> = [i32; A];
+
+extern "C" {
+    pub fn lint_me(
+        x: Bar<
+            S<
+                { //~ ERROR mismatched types
+                    type B<Z> = impl Sized;
+                    //~^ ERROR unconstrained opaque type
+                },
+            >,
+        >,
+    );
+}
+
+fn main() {}
diff --git a/tests/ui/type-alias-impl-trait/nested-in-anon-const.stderr b/tests/ui/type-alias-impl-trait/nested-in-anon-const.stderr
new file mode 100644
index 00000000000..d0fe920b35f
--- /dev/null
+++ b/tests/ui/type-alias-impl-trait/nested-in-anon-const.stderr
@@ -0,0 +1,20 @@
+error: unconstrained opaque type
+  --> $DIR/nested-in-anon-const.rs:13:33
+   |
+LL |                     type B<Z> = impl Sized;
+   |                                 ^^^^^^^^^^
+   |
+   = note: `B` must be used in combination with a concrete type within the same item
+
+error[E0308]: mismatched types
+  --> $DIR/nested-in-anon-const.rs:12:17
+   |
+LL | /                 {
+LL | |                     type B<Z> = impl Sized;
+LL | |
+LL | |                 },
+   | |_________________^ expected `usize`, found `()`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.