Fix up tests

compiler/rustc_hir_typeck/src/coercion.rs

@@ -82,6 +82,10 @@ 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,
 }
 
@@ -1083,7 +1087,8 @@ 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.
+        // 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 {
@@ -1096,11 +1101,15 @@ 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, true);
         coerce
             .autoderef(DUMMY_SP, expr_ty)

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_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 +1,27 @@
+// 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/never_type/diverging-place-match.rs

@@ -1,6 +1,6 @@
 #![feature(never_type)]
 
-fn make_up_a_value<T>() -> T {
+fn not_a_read() -> ! {
     unsafe {
     //~^ ERROR mismatched types
         let x: *const ! = 0 as _;
@@ -10,4 +10,38 @@ fn make_up_a_value<T>() -> 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 main() {}

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

@@ -1,8 +1,6 @@
 error[E0308]: mismatched types
   --> $DIR/diverging-place-match.rs:4:5
    |
-LL |   fn make_up_a_value<T>() -> T {
-   |                      - expected this type parameter
 LL | /     unsafe {
 LL | |
 LL | |         let x: *const ! = 0 as _;
@@ -10,11 +8,76 @@ 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 type parameter `T`, found `()`
+   | |_____^ expected `!`, found `()`
    |
-   = note: expected type parameter `T`
+   = note:   expected type `!`
            found unit type `()`
 
-error: aborting due to 1 previous error
+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: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0308`.