Rollup merge of rust-lang#97116 - RalfJung:ref-validity, r=oli-obk

interpret/validity: reject references to uninhabited types

According to https://doc.rust-lang.org/reference/behavior-considered-undefined.html, this is definitely UB. And we can check this without actually looking up anything in memory, we just need the reference value and its type, making this a great candidate for a validity invariant IMO and my favorite resolution of rust-lang/unsafe-code-guidelines#77.

With this PR, Miri with `-Zmiri-check-number-validity` implements all my preferred options for what the validity invariants of our types could be. :)

CTFE has been doing recursive checking anyway, so this is backwards compatible but might change the error output. I will submit a PR with the new Miri tests soon.

r? `@oli-obk`

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -412,22 +412,27 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
             self.path,
             err_ub!(AlignmentCheckFailed { required, has }) =>
                 {
-                    "an unaligned {} (required {} byte alignment but found {})",
-                    kind,
+                    "an unaligned {kind} (required {} byte alignment but found {})",
                     required.bytes(),
                     has.bytes()
                 },
             err_ub!(DanglingIntPointer(0, _)) =>
-                { "a null {}", kind },
+                { "a null {kind}" },
             err_ub!(DanglingIntPointer(i, _)) =>
-                { "a dangling {} (address 0x{:x} is unallocated)", kind, i },
+                { "a dangling {kind} (address 0x{i:x} is unallocated)" },
             err_ub!(PointerOutOfBounds { .. }) =>
-                { "a dangling {} (going beyond the bounds of its allocation)", kind },
+                { "a dangling {kind} (going beyond the bounds of its allocation)" },
             // This cannot happen during const-eval (because interning already detects
             // dangling pointers), but it can happen in Miri.
             err_ub!(PointerUseAfterFree(..)) =>
-                { "a dangling {} (use-after-free)", kind },
+                { "a dangling {kind} (use-after-free)" },
         );
+        // Do not allow pointers to uninhabited types.
+        if place.layout.abi.is_uninhabited() {
+            throw_validation_failure!(self.path,
+                { "a {kind} pointing to uninhabited type {}", place.layout.ty }
+            )
+        }
         // Recursive checking
         if let Some(ref mut ref_tracking) = self.ref_tracking {
             // Proceed recursively even for ZST, no reason to skip them!

src/test/ui/consts/const-eval/ub-uninhabit.32bit.stderr

@@ -11,7 +11,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-uninhabit.rs:18:1
    |
 LL | const BAD_BAD_REF: &Bar = unsafe { mem::transmute(1usize) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .<deref>: encountered a value of uninhabited type Bar
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a reference pointing to uninhabited type Bar
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 4, align: 4) {

src/test/ui/consts/const-eval/ub-uninhabit.64bit.stderr

@@ -11,7 +11,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-uninhabit.rs:18:1
    |
 LL | const BAD_BAD_REF: &Bar = unsafe { mem::transmute(1usize) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .<deref>: encountered a value of uninhabited type Bar
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a reference pointing to uninhabited type Bar
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 8, align: 8) {

src/test/ui/consts/validate_never_arrays.32bit.stderr

@@ -2,7 +2,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/validate_never_arrays.rs:4:1
    |
 LL | const _: &[!; 1] = unsafe { &*(1_usize as *const [!; 1]) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .<deref>[0]: encountered a value of the never type `!`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a reference pointing to uninhabited type [!; 1]
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 4, align: 4) {

src/test/ui/consts/validate_never_arrays.64bit.stderr

@@ -2,7 +2,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/validate_never_arrays.rs:4:1
    |
 LL | const _: &[!; 1] = unsafe { &*(1_usize as *const [!; 1]) };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed at .<deref>[0]: encountered a value of the never type `!`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a reference pointing to uninhabited type [!; 1]
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 8, align: 8) {