resolve: Unload speculatively resolved crates before freezing cstore

compiler/rustc_metadata/src/creader.rs

@@ -534,7 +534,10 @@ fn resolve_crate(
     ) -> Option<CrateNum> {
         self.used_extern_options.insert(name);
         match self.maybe_resolve_crate(name, dep_kind, None) {
-            Ok(cnum) => Some(cnum),
+            Ok(cnum) => {
+                self.cstore.set_used_recursively(cnum);
+                Some(cnum)
+            }
             Err(err) => {
                 let missing_core =
                     self.maybe_resolve_crate(sym::core, CrateDepKind::Explicit, None).is_err();
@@ -1067,6 +1070,16 @@ pub fn process_path_extern(&mut self, name: Symbol, span: Span) -> Option<CrateN
     pub fn maybe_process_path_extern(&mut self, name: Symbol) -> Option<CrateNum> {
         self.maybe_resolve_crate(name, CrateDepKind::Explicit, None).ok()
     }
+
+    pub fn unload_unused_crates(&mut self) {
+        for opt_cdata in &mut self.cstore.metas {
+            if let Some(cdata) = opt_cdata
+                && !cdata.used()
+            {
+                *opt_cdata = None;
+            }
+        }
+    }
 }
 
 fn global_allocator_spans(krate: &ast::Crate) -> Vec<Span> {

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -106,6 +106,8 @@ pub(crate) struct CrateMetadata {
     private_dep: bool,
     /// The hash for the host proc macro. Used to support `-Z dual-proc-macro`.
     host_hash: Option<Svh>,
+    /// The crate was used non-speculatively.
+    used: bool,
 
     /// Additional data used for decoding `HygieneData` (e.g. `SyntaxContext`
     /// and `ExpnId`).
@@ -1811,6 +1813,7 @@ pub(crate) fn new(
             source: Lrc::new(source),
             private_dep,
             host_hash,
+            used: false,
             extern_crate: None,
             hygiene_context: Default::default(),
             def_key_cache: Default::default(),
@@ -1860,6 +1863,10 @@ pub(crate) fn update_and_private_dep(&mut self, private_dep: bool) {
         self.private_dep &= private_dep;
     }
 
+    pub(crate) fn used(&self) -> bool {
+        self.used
+    }
+
     pub(crate) fn required_panic_strategy(&self) -> Option<PanicStrategy> {
         self.root.required_panic_strategy
     }

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -26,6 +26,7 @@
 use rustc_span::Span;
 
 use std::any::Any;
+use std::mem;
 
 use super::{Decodable, DecodeContext, DecodeIterator};
 
@@ -576,12 +577,24 @@ pub fn get_proc_macro_quoted_span_untracked(
         self.get_crate_data(cnum).get_proc_macro_quoted_span(id, sess)
     }
 
+    pub fn set_used_recursively(&mut self, cnum: CrateNum) {
+        let cmeta = self.get_crate_data_mut(cnum);
+        if !cmeta.used {
+            cmeta.used = true;
+            let dependencies = mem::take(&mut cmeta.dependencies);
+            for &dep_cnum in &dependencies {
+                self.set_used_recursively(dep_cnum);
+            }
+            self.get_crate_data_mut(cnum).dependencies = dependencies;
+        }
+    }
+
     pub(crate) fn update_extern_crate(&mut self, cnum: CrateNum, extern_crate: ExternCrate) {
         let cmeta = self.get_crate_data_mut(cnum);
         if cmeta.update_extern_crate(extern_crate) {
             // Propagate the extern crate info to dependencies if it was updated.
             let extern_crate = ExternCrate { dependency_of: cnum, ..extern_crate };
-            let dependencies = std::mem::take(&mut cmeta.dependencies);
+            let dependencies = mem::take(&mut cmeta.dependencies);
             for &dep_cnum in &dependencies {
                 self.update_extern_crate(dep_cnum, extern_crate);
             }

compiler/rustc_resolve/src/late.rs

@@ -23,6 +23,7 @@
 use rustc_hir::def::{self, CtorKind, DefKind, LifetimeRes, NonMacroAttrKind, PartialRes, PerNS};
 use rustc_hir::def_id::{DefId, LocalDefId, CRATE_DEF_ID, LOCAL_CRATE};
 use rustc_hir::{BindingAnnotation, PrimTy, TraitCandidate};
+use rustc_metadata::creader::CStore;
 use rustc_middle::middle::resolve_bound_vars::Set1;
 use rustc_middle::{bug, span_bug};
 use rustc_session::config::{CrateType, ResolveDocLinks};
@@ -4541,14 +4542,20 @@ fn resolve_and_cache_rustdoc_path(&mut self, path_str: &str, ns: Namespace) -> O
                 if let Some(res) = res
                     && let Some(def_id) = res.opt_def_id()
                     && !def_id.is_local()
-                    && self.r.tcx.crate_types().contains(&CrateType::ProcMacro)
-                    && matches!(
-                        self.r.tcx.sess.opts.resolve_doc_links,
-                        ResolveDocLinks::ExportedMetadata
-                    )
                 {
-                    // Encoding foreign def ids in proc macro crate metadata will ICE.
-                    return None;
+                    if self.r.tcx.crate_types().contains(&CrateType::ProcMacro)
+                        && matches!(
+                            self.r.tcx.sess.opts.resolve_doc_links,
+                            ResolveDocLinks::ExportedMetadata
+                        )
+                    {
+                        // Encoding foreign def ids in proc macro crate metadata will ICE.
+                        return None;
+                    }
+                    // Doc paths should be resolved speculatively and should not produce any
+                    // diagnostics, but if they are indeed resolved, then we need to keep the
+                    // corresponding crate alive.
+                    CStore::from_tcx_mut(self.r.tcx).set_used_recursively(def_id.krate);
                 }
                 res
             });

compiler/rustc_resolve/src/lib.rs

@@ -1625,6 +1625,7 @@ pub fn resolve_crate(&mut self, krate: &Crate) {
             self.tcx
                 .sess
                 .time("resolve_postprocess", || self.crate_loader(|c| c.postprocess(krate)));
+            self.crate_loader(|c| c.unload_unused_crates());
         });
 
         // Make sure we don't mutate the cstore from here on.

tests/ui/extern-flag/empty-extern-arg.stderr

@@ -1,11 +1,6 @@
 error: extern location for std does not exist: 
 
-error: `#[panic_handler]` function required, but not found
+error: requires `sized` lang_item
 
-error: unwinding panics are not supported without std
-   |
-   = help: using nightly cargo, use -Zbuild-std with panic="abort" to avoid unwinding
-   = note: since the core library is usually precompiled with panic="unwind", rebuilding your crate with panic="abort" may not be enough to fix the problem
-
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors