Rollup merge of #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 https://github.com/rust-lang/unsafe-code-guidelines/issues/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`

4 files changed, 4 insertions, 4 deletions

diff --git a/src/test/ui/consts/const-eval/ub-uninhabit.32bit.stderr b/src/test/ui/consts/const-eval/ub-uninhabit.32bit.stderr
index 350bd941939..7873b3463c1 100644
--- a/src/test/ui/consts/const-eval/ub-uninhabit.32bit.stderr
+++ b/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) {
diff --git a/src/test/ui/consts/const-eval/ub-uninhabit.64bit.stderr b/src/test/ui/consts/const-eval/ub-uninhabit.64bit.stderr
index 13a4fde0830..47349750111 100644
--- a/src/test/ui/consts/const-eval/ub-uninhabit.64bit.stderr
+++ b/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) {
diff --git a/src/test/ui/consts/validate_never_arrays.32bit.stderr b/src/test/ui/consts/validate_never_arrays.32bit.stderr
index e2f5175bf0a..6280a7478e7 100644
--- a/src/test/ui/consts/validate_never_arrays.32bit.stderr
+++ b/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) {
diff --git a/src/test/ui/consts/validate_never_arrays.64bit.stderr b/src/test/ui/consts/validate_never_arrays.64bit.stderr
index c145eddef57..c5a71e5be51 100644
--- a/src/test/ui/consts/validate_never_arrays.64bit.stderr
+++ b/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) {