Rollup merge of #129392 - compiler-errors:raw-ref-op-doesnt-diverge-but-more, r=lcnr

Do not consider match/let/ref of place that evaluates to `!` to diverge, disallow coercions from them too

Fixes #117288.

This PR implements a heuristic which disables two things that are currently being performed on the HIR when we have **expressions that involve place-like expressions that point to `!`**. Specifically, it will (in certain cases explained below):

### (1.) Disable the `NeverToAny` coercion we implicitly insert for `!`.

Which fixes this inadvertent, sneaky unsoundness:

```
unsafe {
    let x: *const ! = &0 as *const u8 as *const !;
    let _: () = *x;
}
```

which is UB because currently rust emits an *implicit* NeverToAny coercion even though we really shouldn't be, since there's no read of the value pointed by `x`.

### (2.) Disable the logic which considers expression which evaluate to `!` to diverge, which affects the type returned by the containing block.

Which fixes this unsoundness:

```
fn make_up_a_value<T>() -> T {
    unsafe {
        let x: *const ! = &0 as *const u8 as *const !;
        let _ = *x;
    }
}
```

We disable these two operations **if** the expression is a place-like expression (locals, statics, field projections, index operations, and deref operations), and if the parent expression is either:
(1.) the LHS of an assignment
(2.) AddrOf
(3.) A match or let **unless** all of the *patterns consitute a read*, which is explained below:

And finally, a pattern currently is considered to constitute a read **unless** it is a wildcard, or an OR pattern. An OR pattern is considered to constitute a read if all of its subpatterns constitute a read, to remain as conservative as possible in cases like `_ | subpat` or `subpat | _`.

All other patterns are considered currently to constitute a read. Specifically, because `NeverToAny` is a coercion performed on a *value* and not a *place*, `Struct { .. }` on a `!` type must be a coercion currently, and we currently rely on this behavior to allow us to perform coercions like `let _: i32 = x;` where `x: !`.

This is already considered UB by [miri](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=daf3a2246433fe43fdc07d1389c276c9), but also means it does not affect the preexisting UB in this case:

```
let Struct { .. } = *never_ptr;
```

Even though it's likely up for debate since we're not actually reading any data out of the struct, it almost certainly causes inference changes which I do *NOT* want to fix in this PR.

compiler/rustc_hir_typeck/src/coercion.rs

@@ -82,6 +82,11 @@ struct Coerce<'a, 'tcx> {
     /// See #47489 and #48598
     /// See docs on the "AllowTwoPhase" type for a more detailed discussion
     allow_two_phase: AllowTwoPhase,
+    /// Whether we allow `NeverToAny` coercions. This is unsound if we're
+    /// coercing a place expression without it counting as a read in the MIR.
+    /// This is a side-effect of HIR not really having a great distinction
+    /// between places and values.
+    coerce_never: bool,
 }
 
 impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
@@ -125,8 +130,9 @@ fn new(
         fcx: &'f FnCtxt<'f, 'tcx>,
         cause: ObligationCause<'tcx>,
         allow_two_phase: AllowTwoPhase,
+        coerce_never: bool,
     ) -> Self {
-        Coerce { fcx, cause, allow_two_phase, use_lub: false }
+        Coerce { fcx, cause, allow_two_phase, use_lub: false, coerce_never }
     }
 
     fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
@@ -177,7 +183,12 @@ fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
 
         // Coercing from `!` to any type is allowed:
         if a.is_never() {
-            return success(simple(Adjust::NeverToAny)(b), b, vec![]);
+            if self.coerce_never {
+                return success(simple(Adjust::NeverToAny)(b), b, vec![]);
+            } else {
+                // Otherwise the only coercion we can do is unification.
+                return self.unify_and(a, b, identity);
+            }
         }
 
         // Coercing *from* an unresolved inference variable means that
@@ -1038,7 +1049,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// The expressions *must not* have any preexisting adjustments.
     pub(crate) fn coerce(
         &self,
-        expr: &hir::Expr<'_>,
+        expr: &'tcx hir::Expr<'tcx>,
         expr_ty: Ty<'tcx>,
         mut target: Ty<'tcx>,
         allow_two_phase: AllowTwoPhase,
@@ -1055,7 +1066,12 @@ pub(crate) fn coerce(
 
         let cause =
             cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
-        let coerce = Coerce::new(self, cause, allow_two_phase);
+        let coerce = Coerce::new(
+            self,
+            cause,
+            allow_two_phase,
+            self.expr_guaranteed_to_constitute_read_for_never(expr),
+        );
         let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
 
         let (adjustments, _) = self.register_infer_ok_obligations(ok);
@@ -1077,8 +1093,9 @@ pub(crate) fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
         debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);
 
         let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
-        // We don't ever need two-phase here since we throw out the result of the coercion
+        // We don't ever need two-phase here since we throw out the result of the coercion.
-        let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
+        // We also just always set `coerce_never` to true, since this is a heuristic.
+        let coerce = Coerce::new(self, cause, AllowTwoPhase::No, true);
         self.probe(|_| {
             let Ok(ok) = coerce.coerce(source, target) else {
                 return false;
@@ -1090,12 +1107,16 @@ pub(crate) fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
     }
 
     /// Given a type and a target type, this function will calculate and return
-    /// how many dereference steps needed to achieve `expr_ty <: target`. If
+    /// how many dereference steps needed to coerce `expr_ty` to `target`. If
     /// it's not possible, return `None`.
-    pub(crate) fn deref_steps(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> Option<usize> {
+    pub(crate) fn deref_steps_for_suggestion(
+        &self,
+        expr_ty: Ty<'tcx>,
+        target: Ty<'tcx>,
+    ) -> Option<usize> {
         let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
-        // We don't ever need two-phase here since we throw out the result of the coercion
+        // We don't ever need two-phase here since we throw out the result of the coercion.
-        let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
+        let coerce = Coerce::new(self, cause, AllowTwoPhase::No, true);
         coerce
             .autoderef(DUMMY_SP, expr_ty)
             .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
@@ -1252,7 +1273,10 @@ fn try_find_coercion_lub<E>(
         // probably aren't processing function arguments here and even if we were,
         // they're going to get autorefed again anyway and we can apply 2-phase borrows
         // at that time.
-        let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
+        //
+        // NOTE: we set `coerce_never` to `true` here because coercion LUBs only
+        // operate on values and not places, so a never coercion is valid.
+        let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No, true);
         coerce.use_lub = true;
 
         // First try to coerce the new expression to the type of the previous ones,

compiler/rustc_hir_typeck/src/expr.rs

@@ -1,3 +1,6 @@
+// ignore-tidy-filelength
+// FIXME: we should move the field error reporting code somewhere else.
+
 //! Type checking expressions.
 //!
 //! See [`rustc_hir_analysis::check`] for more context on type checking in general.
@@ -62,7 +65,7 @@ pub(crate) fn check_expr_has_type_or_error(
 
         // While we don't allow *arbitrary* coercions here, we *do* allow
         // coercions from ! to `expected`.
-        if ty.is_never() {
+        if ty.is_never() && self.expr_guaranteed_to_constitute_read_for_never(expr) {
             if let Some(_) = self.typeck_results.borrow().adjustments().get(expr.hir_id) {
                 let reported = self.dcx().span_delayed_bug(
                     expr.span,
@@ -238,8 +241,11 @@ pub(super) fn check_expr_with_expectation_and_args(
             _ => self.warn_if_unreachable(expr.hir_id, expr.span, "expression"),
         }
 
-        // Any expression that produces a value of type `!` must have diverged
+        // Any expression that produces a value of type `!` must have diverged,
-        if ty.is_never() {
+        // unless it's a place expression that isn't being read from, in which case
+        // diverging would be unsound since we may never actually read the `!`.
+        // e.g. `let _ = *never_ptr;` with `never_ptr: *const !`.
+        if ty.is_never() && self.expr_guaranteed_to_constitute_read_for_never(expr) {
             self.diverges.set(self.diverges.get() | Diverges::always(expr.span));
         }
 
@@ -257,6 +263,185 @@ pub(super) fn check_expr_with_expectation_and_args(
         ty
     }
 
+    /// Whether this expression constitutes a read of value of the type that
+    /// it evaluates to.
+    ///
+    /// This is used to determine if we should consider the block to diverge
+    /// if the expression evaluates to `!`, and if we should insert a `NeverToAny`
+    /// coercion for values of type `!`.
+    ///
+    /// This function generally returns `false` if the expression is a place
+    /// expression and the *parent* expression is the scrutinee of a match or
+    /// the pointee of an `&` addr-of expression, since both of those parent
+    /// expressions take a *place* and not a value.
+    pub(super) fn expr_guaranteed_to_constitute_read_for_never(
+        &self,
+        expr: &'tcx hir::Expr<'tcx>,
+    ) -> bool {
+        // We only care about place exprs. Anything else returns an immediate
+        // which would constitute a read. We don't care about distinguishing
+        // "syntactic" place exprs since if the base of a field projection is
+        // not a place then it would've been UB to read from it anyways since
+        // that constitutes a read.
+        if !expr.is_syntactic_place_expr() {
+            return true;
+        }
+
+        let parent_node = self.tcx.parent_hir_node(expr.hir_id);
+        match parent_node {
+            hir::Node::Expr(parent_expr) => {
+                match parent_expr.kind {
+                    // Addr-of, field projections, and LHS of assignment don't constitute reads.
+                    // Assignment does call `drop_in_place`, though, but its safety
+                    // requirements are not the same.
+                    ExprKind::AddrOf(..) | hir::ExprKind::Field(..) => false,
+                    ExprKind::Assign(lhs, _, _) => {
+                        // Only the LHS does not constitute a read
+                        expr.hir_id != lhs.hir_id
+                    }
+
+                    // See note on `PatKind::Or` below for why this is `all`.
+                    ExprKind::Match(scrutinee, arms, _) => {
+                        assert_eq!(scrutinee.hir_id, expr.hir_id);
+                        arms.iter()
+                            .all(|arm| self.pat_guaranteed_to_constitute_read_for_never(arm.pat))
+                    }
+                    ExprKind::Let(hir::LetExpr { init, pat, .. }) => {
+                        assert_eq!(init.hir_id, expr.hir_id);
+                        self.pat_guaranteed_to_constitute_read_for_never(*pat)
+                    }
+
+                    // Any expression child of these expressions constitute reads.
+                    ExprKind::Array(_)
+                    | ExprKind::Call(_, _)
+                    | ExprKind::MethodCall(_, _, _, _)
+                    | ExprKind::Tup(_)
+                    | ExprKind::Binary(_, _, _)
+                    | ExprKind::Unary(_, _)
+                    | ExprKind::Cast(_, _)
+                    | ExprKind::Type(_, _)
+                    | ExprKind::DropTemps(_)
+                    | ExprKind::If(_, _, _)
+                    | ExprKind::Closure(_)
+                    | ExprKind::Block(_, _)
+                    | ExprKind::AssignOp(_, _, _)
+                    | ExprKind::Index(_, _, _)
+                    | ExprKind::Break(_, _)
+                    | ExprKind::Ret(_)
+                    | ExprKind::Become(_)
+                    | ExprKind::InlineAsm(_)
+                    | ExprKind::Struct(_, _, _)
+                    | ExprKind::Repeat(_, _)
+                    | ExprKind::Yield(_, _) => true,
+
+                    // These expressions have no (direct) sub-exprs.
+                    ExprKind::ConstBlock(_)
+                    | ExprKind::Loop(_, _, _, _)
+                    | ExprKind::Lit(_)
+                    | ExprKind::Path(_)
+                    | ExprKind::Continue(_)
+                    | ExprKind::OffsetOf(_, _)
+                    | ExprKind::Err(_) => unreachable!("no sub-expr expected for {:?}", expr.kind),
+                }
+            }
+
+            // If we have a subpattern that performs a read, we want to consider this
+            // to diverge for compatibility to support something like `let x: () = *never_ptr;`.
+            hir::Node::LetStmt(hir::LetStmt { init: Some(target), pat, .. }) => {
+                assert_eq!(target.hir_id, expr.hir_id);
+                self.pat_guaranteed_to_constitute_read_for_never(*pat)
+            }
+
+            // These nodes (if they have a sub-expr) do constitute a read.
+            hir::Node::Block(_)
+            | hir::Node::Arm(_)
+            | hir::Node::ExprField(_)
+            | hir::Node::AnonConst(_)
+            | hir::Node::ConstBlock(_)
+            | hir::Node::ConstArg(_)
+            | hir::Node::Stmt(_)
+            | hir::Node::Item(hir::Item {
+                kind: hir::ItemKind::Const(..) | hir::ItemKind::Static(..),
+                ..
+            })
+            | hir::Node::TraitItem(hir::TraitItem {
+                kind: hir::TraitItemKind::Const(..), ..
+            })
+            | hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Const(..), .. }) => true,
+
+            // These nodes do not have direct sub-exprs.
+            hir::Node::Param(_)
+            | hir::Node::Item(_)
+            | hir::Node::ForeignItem(_)
+            | hir::Node::TraitItem(_)
+            | hir::Node::ImplItem(_)
+            | hir::Node::Variant(_)
+            | hir::Node::Field(_)
+            | hir::Node::PathSegment(_)
+            | hir::Node::Ty(_)
+            | hir::Node::AssocItemConstraint(_)
+            | hir::Node::TraitRef(_)
+            | hir::Node::Pat(_)
+            | hir::Node::PatField(_)
+            | hir::Node::LetStmt(_)
+            | hir::Node::Synthetic
+            | hir::Node::Err(_)
+            | hir::Node::Ctor(_)
+            | hir::Node::Lifetime(_)
+            | hir::Node::GenericParam(_)
+            | hir::Node::Crate(_)
+            | hir::Node::Infer(_)
+            | hir::Node::WhereBoundPredicate(_)
+            | hir::Node::ArrayLenInfer(_)
+            | hir::Node::PreciseCapturingNonLifetimeArg(_)
+            | hir::Node::OpaqueTy(_) => {
+                unreachable!("no sub-expr expected for {parent_node:?}")
+            }
+        }
+    }
+
+    /// Whether this pattern constitutes a read of value of the scrutinee that
+    /// it is matching against. This is used to determine whether we should
+    /// perform `NeverToAny` coercions.
+    ///
+    /// See above for the nuances of what happens when this returns true.
+    pub(super) fn pat_guaranteed_to_constitute_read_for_never(&self, pat: &hir::Pat<'_>) -> bool {
+        match pat.kind {
+            // Does not constitute a read.
+            hir::PatKind::Wild => false,
+
+            // This is unnecessarily restrictive when the pattern that doesn't
+            // constitute a read is unreachable.
+            //
+            // For example `match *never_ptr { value => {}, _ => {} }` or
+            // `match *never_ptr { _ if false => {}, value => {} }`.
+            //
+            // It is however fine to be restrictive here; only returning `true`
+            // can lead to unsoundness.
+            hir::PatKind::Or(subpats) => {
+                subpats.iter().all(|pat| self.pat_guaranteed_to_constitute_read_for_never(pat))
+            }
+
+            // Does constitute a read, since it is equivalent to a discriminant read.
+            hir::PatKind::Never => true,
+
+            // All of these constitute a read, or match on something that isn't `!`,
+            // which would require a `NeverToAny` coercion.
+            hir::PatKind::Binding(_, _, _, _)
+            | hir::PatKind::Struct(_, _, _)
+            | hir::PatKind::TupleStruct(_, _, _)
+            | hir::PatKind::Path(_)
+            | hir::PatKind::Tuple(_, _)
+            | hir::PatKind::Box(_)
+            | hir::PatKind::Ref(_, _)
+            | hir::PatKind::Deref(_)
+            | hir::PatKind::Lit(_)
+            | hir::PatKind::Range(_, _, _)
+            | hir::PatKind::Slice(_, _, _)
+            | hir::PatKind::Err(_) => true,
+        }
+    }
+
     #[instrument(skip(self, expr), level = "debug")]
     fn check_expr_kind(
         &self,

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -2608,7 +2608,7 @@ pub(crate) fn suggest_deref_or_ref(
                     }
 
                     if let hir::ExprKind::Unary(hir::UnOp::Deref, inner) = expr.kind
-                        && let Some(1) = self.deref_steps(expected, checked_ty)
+                        && let Some(1) = self.deref_steps_for_suggestion(expected, checked_ty)
                     {
                         // We have `*&T`, check if what was expected was `&T`.
                         // If so, we may want to suggest removing a `*`.
@@ -2738,7 +2738,7 @@ pub(crate) fn suggest_deref_or_ref(
                 }
             }
             (_, &ty::RawPtr(ty_b, mutbl_b), &ty::Ref(_, ty_a, mutbl_a)) => {
-                if let Some(steps) = self.deref_steps(ty_a, ty_b)
+                if let Some(steps) = self.deref_steps_for_suggestion(ty_a, ty_b)
                     // Only suggest valid if dereferencing needed.
                     && steps > 0
                     // The pointer type implements `Copy` trait so the suggestion is always valid.
@@ -2782,7 +2782,7 @@ pub(crate) fn suggest_deref_or_ref(
                 }
             }
             _ if sp == expr.span => {
-                if let Some(mut steps) = self.deref_steps(checked_ty, expected) {
+                if let Some(mut steps) = self.deref_steps_for_suggestion(checked_ty, expected) {
                     let mut expr = expr.peel_blocks();
                     let mut prefix_span = expr.span.shrink_to_lo();
                     let mut remove = String::new();

src/tools/miri/tests/pass/underscore_pattern.rs

@@ -1,5 +1,7 @@
 // Various tests ensuring that underscore patterns really just construct the place, but don't check its contents.
 #![feature(strict_provenance)]
+#![feature(never_type)]
+
 use std::ptr;
 
 fn main() {
@@ -9,6 +11,7 @@ fn main() {
     invalid_let();
     dangling_let_type_annotation();
     invalid_let_type_annotation();
+    never();
 }
 
 fn dangling_match() {
@@ -34,6 +37,13 @@ union Uninit<T: Copy> {
             _ => {}
         }
     }
+
+    unsafe {
+        let x: Uninit<!> = Uninit { uninit: () };
+        match x.value {
+            _ => {}
+        }
+    }
 }
 
 fn dangling_let() {
@@ -41,6 +51,11 @@ fn dangling_let() {
         let ptr = ptr::without_provenance::<bool>(0x40);
         let _ = *ptr;
     }
+
+    unsafe {
+        let ptr = ptr::without_provenance::<!>(0x40);
+        let _ = *ptr;
+    }
 }
 
 fn invalid_let() {
@@ -49,6 +64,12 @@ fn invalid_let() {
         let ptr = ptr::addr_of!(val).cast::<bool>();
         let _ = *ptr;
     }
+
+    unsafe {
+        let val = 3u8;
+        let ptr = ptr::addr_of!(val).cast::<!>();
+        let _ = *ptr;
+    }
 }
 
 // Adding a type annotation used to change how MIR is generated, make sure we cover both cases.
@@ -57,6 +78,11 @@ fn dangling_let_type_annotation() {
         let ptr = ptr::without_provenance::<bool>(0x40);
         let _: bool = *ptr;
     }
+
+    unsafe {
+        let ptr = ptr::without_provenance::<!>(0x40);
+        let _: ! = *ptr;
+    }
 }
 
 fn invalid_let_type_annotation() {
@@ -65,7 +91,28 @@ fn invalid_let_type_annotation() {
         let ptr = ptr::addr_of!(val).cast::<bool>();
         let _: bool = *ptr;
     }
+
+    unsafe {
+        let val = 3u8;
+        let ptr = ptr::addr_of!(val).cast::<!>();
+        let _: ! = *ptr;
+    }
 }
 
-// FIXME: we should also test `!`, not just `bool` -- but that s currently buggy:
+// Regression test from <https://github.com/rust-lang/rust/issues/117288>.
-// https://github.com/rust-lang/rust/issues/117288
+fn never() {
+    unsafe {
+        let x = 3u8;
+        let x: *const ! = &x as *const u8 as *const _;
+        let _: ! = *x;
+    }
+
+    // Without a type annotation, make sure we don't implicitly coerce `!` to `()`
+    // when we do the noop `*x` (as that would require a `!` *value*, creating
+    // which is UB).
+    unsafe {
+        let x = 3u8;
+        let x: *const ! = &x as *const u8 as *const _;
+        let _ = *x;
+    }
+}

tests/mir-opt/uninhabited_enum.process_never.SimplifyLocals-final.after.mir

@@ -3,9 +3,8 @@
 fn process_never(_1: *const !) -> () {
     debug input => _1;
     let mut _0: ();
-    let _2: &!;
     scope 1 {
-        debug _input => _2;
+        debug _input => const ();
     }
 
     bb0: {

tests/mir-opt/uninhabited_enum.process_void.SimplifyLocals-final.after.mir

@@ -4,7 +4,7 @@ fn process_void(_1: *const Void) -> () {
     debug input => _1;
     let mut _0: ();
     scope 1 {
-        debug _input => _1;
+        debug _input => const ZeroSized: Void;
     }
 
     bb0: {

tests/mir-opt/uninhabited_enum.rs

@@ -1,18 +1,19 @@
 // skip-filecheck
 #![feature(never_type)]
 
+#[derive(Copy, Clone)]
 pub enum Void {}
 
 // EMIT_MIR uninhabited_enum.process_never.SimplifyLocals-final.after.mir
 #[no_mangle]
 pub fn process_never(input: *const !) {
-    let _input = unsafe { &*input };
+    let _input = unsafe { *input };
 }
 
 // EMIT_MIR uninhabited_enum.process_void.SimplifyLocals-final.after.mir
 #[no_mangle]
 pub fn process_void(input: *const Void) {
-    let _input = unsafe { &*input };
+    let _input = unsafe { *input };
     // In the future, this should end with `unreachable`, but we currently only do
     // unreachability analysis for `!`.
 }

tests/mir-opt/uninhabited_not_read.main.SimplifyLocals-final.after.mir

@@ -0,0 +1,49 @@
+// MIR for `main` after SimplifyLocals-final
+
+fn main() -> () {
+    let mut _0: ();
+    let _1: u8;
+    let mut _2: *const !;
+    let mut _3: *const u8;
+    let _4: u8;
+    let mut _5: *const !;
+    let mut _6: *const u8;
+    scope 1 {
+        debug x => _1;
+        scope 2 {
+            debug x => _2;
+            scope 3 {
+            }
+        }
+    }
+    scope 4 {
+        debug x => _4;
+        scope 5 {
+            debug x => _5;
+            scope 6 {
+            }
+        }
+    }
+
+    bb0: {
+        StorageLive(_1);
+        _1 = const 3_u8;
+        StorageLive(_2);
+        StorageLive(_3);
+        _3 = &raw const _1;
+        _2 = move _3 as *const ! (PtrToPtr);
+        StorageDead(_3);
+        StorageDead(_2);
+        StorageDead(_1);
+        StorageLive(_4);
+        _4 = const 3_u8;
+        StorageLive(_5);
+        StorageLive(_6);
+        _6 = &raw const _4;
+        _5 = move _6 as *const ! (PtrToPtr);
+        StorageDead(_6);
+        StorageDead(_5);
+        StorageDead(_4);
+        return;
+    }
+}

tests/mir-opt/uninhabited_not_read.rs

@@ -0,0 +1,26 @@
+// skip-filecheck
+
+//@ edition: 2021
+// In ed 2021 and below, we don't fallback `!` to `()`.
+// This would introduce a `! -> ()` coercion which would
+// be UB if we didn't disallow this explicitly.
+
+#![feature(never_type)]
+
+// EMIT_MIR uninhabited_not_read.main.SimplifyLocals-final.after.mir
+fn main() {
+    // With a type annotation
+    unsafe {
+        let x = 3u8;
+        let x: *const ! = &x as *const u8 as *const _;
+        let _: ! = *x;
+    }
+
+    // Without a type annotation, make sure we don't implicitly coerce `!` to `()`
+    // when we do the noop `*x`.
+    unsafe {
+        let x = 3u8;
+        let x: *const ! = &x as *const u8 as *const _;
+        let _ = *x;
+    }
+}

tests/ui/closures/2229_closure_analysis/match/pattern-matching-should-fail.stderr

@@ -7,14 +7,12 @@ LL |     let c1 = || match x { };
    |                       ^ `x` used here but it isn't initialized
 
 error[E0381]: used binding `x` isn't initialized
-  --> $DIR/pattern-matching-should-fail.rs:15:14
+  --> $DIR/pattern-matching-should-fail.rs:15:23
    |
 LL |     let x: !;
    |         - binding declared here but left uninitialized
 LL |     let c2 = || match x { _ => () };
-   |              ^^       - borrow occurs due to use in closure
+   |                       ^ `x` used here but it isn't initialized
-   |              |
-   |              `x` used here but it isn't initialized
 
 error[E0381]: used binding `variant` isn't initialized
   --> $DIR/pattern-matching-should-fail.rs:27:13

tests/ui/never_type/diverging-place-match.rs

@@ -0,0 +1,74 @@
+#![feature(never_type)]
+
+fn not_a_read() -> ! {
+    unsafe {
+    //~^ ERROR mismatched types
+        let x: *const ! = 0 as _;
+        let _: ! = *x;
+        // Since `*x` "diverges" in HIR, but doesn't count as a read in MIR, this
+        // is unsound since we act as if it diverges but it doesn't.
+    }
+}
+
+fn not_a_read_implicit() -> ! {
+    unsafe {
+    //~^ ERROR mismatched types
+        let x: *const ! = 0 as _;
+        let _ = *x;
+    }
+}
+
+fn not_a_read_guide_coercion() -> ! {
+    unsafe {
+        //~^ ERROR mismatched types
+        let x: *const ! = 0 as _;
+        let _: () = *x;
+        //~^ ERROR mismatched types
+    }
+}
+
+fn empty_match() -> ! {
+    unsafe {
+    //~^ ERROR mismatched types
+        let x: *const ! = 0 as _;
+        match *x { _ => {} };
+    }
+}
+
+fn field_projection() -> ! {
+    unsafe {
+    //~^ ERROR mismatched types
+        let x: *const (!, ()) = 0 as _;
+        let _ = (*x).0;
+        // ^ I think this is still UB, but because of the inbounds projection.
+    }
+}
+
+fn covered_arm() -> ! {
+    unsafe {
+    //~^ ERROR mismatched types
+        let x: *const ! = 0 as _;
+        let (_ | 1i32) = *x;
+        //~^ ERROR mismatched types
+    }
+}
+
+// FIXME: This *could* be considered a read of `!`, but we're not that sophisticated..
+fn uncovered_arm() -> ! {
+    unsafe {
+    //~^ ERROR mismatched types
+        let x: *const ! = 0 as _;
+        let (1i32 | _) = *x;
+        //~^ ERROR mismatched types
+    }
+}
+
+fn coerce_ref_binding() -> ! {
+    unsafe {
+        let x: *const ! = 0 as _;
+        let ref _x: () = *x;
+        //~^ ERROR mismatched types
+    }
+}
+
+fn main() {}

tests/ui/never_type/diverging-place-match.stderr

@@ -0,0 +1,142 @@
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:4:5
+   |
+LL | /     unsafe {
+LL | |
+LL | |         let x: *const ! = 0 as _;
+LL | |         let _: ! = *x;
+LL | |         // Since `*x` "diverges" in HIR, but doesn't count as a read in MIR, this
+LL | |         // is unsound since we act as if it diverges but it doesn't.
+LL | |     }
+   | |_____^ expected `!`, found `()`
+   |
+   = note:   expected type `!`
+           found unit type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:14:5
+   |
+LL | /     unsafe {
+LL | |
+LL | |         let x: *const ! = 0 as _;
+LL | |         let _ = *x;
+LL | |     }
+   | |_____^ expected `!`, found `()`
+   |
+   = note:   expected type `!`
+           found unit type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:25:21
+   |
+LL |         let _: () = *x;
+   |                --   ^^ expected `()`, found `!`
+   |                |
+   |                expected due to this
+   |
+   = note: expected unit type `()`
+                   found type `!`
+
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:22:5
+   |
+LL | /     unsafe {
+LL | |
+LL | |         let x: *const ! = 0 as _;
+LL | |         let _: () = *x;
+LL | |
+LL | |     }
+   | |_____^ expected `!`, found `()`
+   |
+   = note:   expected type `!`
+           found unit type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:31:5
+   |
+LL | /     unsafe {
+LL | |
+LL | |         let x: *const ! = 0 as _;
+LL | |         match *x { _ => {} };
+LL | |     }
+   | |_____^ expected `!`, found `()`
+   |
+   = note:   expected type `!`
+           found unit type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:39:5
+   |
+LL | /     unsafe {
+LL | |
+LL | |         let x: *const (!, ()) = 0 as _;
+LL | |         let _ = (*x).0;
+LL | |         // ^ I think this is still UB, but because of the inbounds projection.
+LL | |     }
+   | |_____^ expected `!`, found `()`
+   |
+   = note:   expected type `!`
+           found unit type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:51:18
+   |
+LL |         let (_ | 1i32) = *x;
+   |                  ^^^^    -- this expression has type `!`
+   |                  |
+   |                  expected `!`, found `i32`
+   |
+   = note: expected type `!`
+              found type `i32`
+
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:48:5
+   |
+LL | /     unsafe {
+LL | |
+LL | |         let x: *const ! = 0 as _;
+LL | |         let (_ | 1i32) = *x;
+LL | |
+LL | |     }
+   | |_____^ expected `!`, found `()`
+   |
+   = note:   expected type `!`
+           found unit type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:61:14
+   |
+LL |         let (1i32 | _) = *x;
+   |              ^^^^        -- this expression has type `!`
+   |              |
+   |              expected `!`, found `i32`
+   |
+   = note: expected type `!`
+              found type `i32`
+
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:58:5
+   |
+LL | /     unsafe {
+LL | |
+LL | |         let x: *const ! = 0 as _;
+LL | |         let (1i32 | _) = *x;
+LL | |
+LL | |     }
+   | |_____^ expected `!`, found `()`
+   |
+   = note:   expected type `!`
+           found unit type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/diverging-place-match.rs:69:26
+   |
+LL |         let ref _x: () = *x;
+   |                          ^^ expected `()`, found `!`
+   |
+   = note: expected unit type `()`
+                   found type `!`
+
+error: aborting due to 11 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/raw-ref-op/never-place-isnt-diverging.rs

@@ -0,0 +1,22 @@
+#![feature(never_type)]
+
+fn make_up_a_value<T>() -> T {
+    unsafe {
+    //~^ ERROR mismatched types
+        let x: *const ! = 0 as _;
+        &raw const *x;
+        // Since `*x` is `!`, HIR typeck used to think that it diverges
+        // and allowed the block to coerce to any value, leading to UB.
+    }
+}
+
+
+fn make_up_a_pointer<T>() -> *const T {
+    unsafe {
+        let x: *const ! = 0 as _;
+        &raw const *x
+        //~^ ERROR mismatched types
+    }
+}
+
+fn main() {}

tests/ui/raw-ref-op/never-place-isnt-diverging.stderr

@@ -0,0 +1,34 @@
+error[E0308]: mismatched types
+  --> $DIR/never-place-isnt-diverging.rs:4:5
+   |
+LL |   fn make_up_a_value<T>() -> T {
+   |                      - expected this type parameter
+LL | /     unsafe {
+LL | |
+LL | |         let x: *const ! = 0 as _;
+LL | |         &raw const *x;
+LL | |         // Since `*x` is `!`, HIR typeck used to think that it diverges
+LL | |         // and allowed the block to coerce to any value, leading to UB.
+LL | |     }
+   | |_____^ expected type parameter `T`, found `()`
+   |
+   = note: expected type parameter `T`
+                   found unit type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/never-place-isnt-diverging.rs:17:9
+   |
+LL | fn make_up_a_pointer<T>() -> *const T {
+   |                      -       -------- expected `*const T` because of return type
+   |                      |
+   |                      expected this type parameter
+...
+LL |         &raw const *x
+   |         ^^^^^^^^^^^^^ expected `*const T`, found `*const !`
+   |
+   = note: expected raw pointer `*const T`
+              found raw pointer `*const !`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/reachable/expr_assign.stderr

@@ -14,12 +14,13 @@ LL | #![deny(unreachable_code)]
    |         ^^^^^^^^^^^^^^^^
 
 error: unreachable expression
-  --> $DIR/expr_assign.rs:20:14
+  --> $DIR/expr_assign.rs:20:9
    |
 LL |         *p = return;
-   |         --   ^^^^^^ unreachable expression
+   |         ^^^^^------
-   |         |
+   |         |    |
-   |         any code following this expression is unreachable
+   |         |    any code following this expression is unreachable
+   |         unreachable expression
 
 error: unreachable expression
   --> $DIR/expr_assign.rs:26:15