Auto merge of #55549 - RalfJung:miri-visitor, r=oli-obk

Value visitors for miri

Generalize the traversal part of validation to a `ValueVisitor`.

~~This includes https://github.com/rust-lang/rust/pull/55316, [click here](https://github.com/RalfJung/rust/compare/retagging...RalfJung:miri-visitor) for just the new commits.~~

src/test/ui/consts/const-eval/double_check2.stderr

@@ -5,7 +5,7 @@ LL | / static FOO: (&Foo, &Bar) = unsafe {( //~ undefined behavior
 LL | |     Union { u8: &BAR }.foo,
 LL | |     Union { u8: &BAR }.bar,
 LL | | )};
-   | |___^ type validation failed: encountered invalid enum discriminant 5 at .1.<deref>
+   | |___^ type validation failed: encountered 5 at .1.<deref>, but expected a valid enum discriminant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 

src/test/ui/consts/const-eval/ub-enum.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![allow(const_err)] // make sure we cannot allow away the errors tested here
+
 #[repr(usize)]
 #[derive(Copy, Clone)]
 enum Enum {
@@ -31,10 +33,15 @@ enum Enum2 {
 union TransmuteEnum2 {
     a: usize,
     b: Enum2,
+    c: (),
 }
 const BAD_ENUM2 : Enum2 = unsafe { TransmuteEnum2 { a: 0 }.b };
 //~^ ERROR is undefined behavior
 
+// Undef enum discriminant. In an arry to avoid `Scalar` layout.
+const BAD_ENUM3 : [Enum2; 2] = [unsafe { TransmuteEnum2 { c: () }.b }; 2];
+//~^ ERROR is undefined behavior
+
 // Invalid enum field content (mostly to test printing of apths for enum tuple
 // variants and tuples).
 union TransmuteChar {

src/test/ui/consts/const-eval/ub-enum.stderr

@@ -1,27 +1,35 @@
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-enum.rs:22:1
+  --> $DIR/ub-enum.rs:24:1
    |
 LL | const BAD_ENUM: Enum = unsafe { TransmuteEnum { a: &1 }.b };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered non-integer enum discriminant
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a pointer, but expected a valid enum discriminant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-enum.rs:35:1
+  --> $DIR/ub-enum.rs:38:1
    |
 LL | const BAD_ENUM2 : Enum2 = unsafe { TransmuteEnum2 { a: 0 }.b };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered invalid enum discriminant 0
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected a valid enum discriminant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-enum.rs:45:1
+  --> $DIR/ub-enum.rs:42:1
+   |
+LL | const BAD_ENUM3 : [Enum2; 2] = [unsafe { TransmuteEnum2 { c: () }.b }; 2];
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempted to read undefined bytes
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
+
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/ub-enum.rs:52:1
    |
 LL | const BAD_ENUM_CHAR : Option<(char, char)> = Some(('x', unsafe { TransmuteChar { a: !0 }.b }));
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 4294967295 at .Some.0.1, but expected something in the range 0..=1114111
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/const-eval/ub-nonnull.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(const_transmute)]
+#![allow(const_err)] // make sure we cannot allow away the errors tested here
 
 use std::mem;
 use std::ptr::NonNull;

src/test/ui/consts/const-eval/ub-nonnull.stderr

@@ -1,5 +1,5 @@
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:17:1
+  --> $DIR/ub-nonnull.rs:18:1
    |
 LL | const NULL_PTR: NonNull<u8> = unsafe { mem::transmute(0usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
@@ -7,7 +7,7 @@ LL | const NULL_PTR: NonNull<u8> = unsafe { mem::transmute(0usize) };
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:20:1
+  --> $DIR/ub-nonnull.rs:21:1
    |
 LL | const NULL_U8: NonZeroU8 = unsafe { mem::transmute(0u8) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
@@ -15,7 +15,7 @@ LL | const NULL_U8: NonZeroU8 = unsafe { mem::transmute(0u8) };
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-nonnull.rs:22:1
+  --> $DIR/ub-nonnull.rs:23:1
    |
 LL | const NULL_USIZE: NonZeroUsize = unsafe { mem::transmute(0usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1

src/test/ui/consts/const-eval/ub-ref.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(const_transmute)]
+#![allow(const_err)] // make sure we cannot allow away the errors tested here
 
 use std::mem;
 

src/test/ui/consts/const-eval/ub-ref.stderr

@@ -1,5 +1,5 @@
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref.rs:15:1
+  --> $DIR/ub-ref.rs:16:1
    |
 LL | const UNALIGNED: &u16 = unsafe { mem::transmute(&[0u8; 4]) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered unaligned reference
@@ -7,7 +7,7 @@ LL | const UNALIGNED: &u16 = unsafe { mem::transmute(&[0u8; 4]) };
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref.rs:18:1
+  --> $DIR/ub-ref.rs:19:1
    |
 LL | const NULL: &u16 = unsafe { mem::transmute(0usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
@@ -15,7 +15,7 @@ LL | const NULL: &u16 = unsafe { mem::transmute(0usize) };
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref.rs:21:1
+  --> $DIR/ub-ref.rs:22:1
    |
 LL | const REF_AS_USIZE: usize = unsafe { mem::transmute(&0) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a pointer, but expected initialized plain bits
@@ -23,15 +23,15 @@ LL | const REF_AS_USIZE: usize = unsafe { mem::transmute(&0) };
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref.rs:24:1
+  --> $DIR/ub-ref.rs:25:1
    |
 LL | const REF_AS_USIZE_SLICE: &[usize] = &[unsafe { mem::transmute(&0) }];
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ a raw memory access tried to access part of a pointer value as raw bytes
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a pointer at .<deref>, but expected plain bytes
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref.rs:27:1
+  --> $DIR/ub-ref.rs:28:1
    |
 LL | const USIZE_AS_REF: &'static u8 = unsafe { mem::transmute(1337usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered integer pointer in non-ZST reference

src/test/ui/consts/const-eval/ub-uninhabit.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(const_transmute)]
+#![allow(const_err)] // make sure we cannot allow away the errors tested here
 
 use std::mem;
 

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

@@ -1,5 +1,5 @@
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-uninhabit.rs:18:1
+  --> $DIR/ub-uninhabit.rs:19:1
    |
 LL | const BAD_BAD_BAD: Bar = unsafe { mem::transmute(()) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a value of an uninhabited type
@@ -7,7 +7,7 @@ LL | const BAD_BAD_BAD: Bar = unsafe { mem::transmute(()) };
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-uninhabit.rs:21:1
+  --> $DIR/ub-uninhabit.rs:22:1
    |
 LL | const BAD_BAD_REF: &Bar = unsafe { mem::transmute(1usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a value of an uninhabited type at .<deref>
@@ -15,7 +15,7 @@ LL | const BAD_BAD_REF: &Bar = unsafe { mem::transmute(1usize) };
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-uninhabit.rs:24:1
+  --> $DIR/ub-uninhabit.rs:25:1
    |
 LL | const BAD_BAD_ARRAY: [Bar; 1] = unsafe { mem::transmute(()) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a value of an uninhabited type at [0]

src/test/ui/consts/const-eval/ub-upvars.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(const_transmute,const_let)]
+#![allow(const_err)] // make sure we cannot allow away the errors tested here
 
 use std::mem;
 

src/test/ui/consts/const-eval/ub-upvars.stderr

@@ -1,12 +1,12 @@
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-upvars.rs:15:1
+  --> $DIR/ub-upvars.rs:16:1
    |
 LL | / const BAD_UPVAR: &FnOnce() = &{ //~ ERROR it is undefined behavior to use this value
 LL | |     let bad_ref: &'static u16 = unsafe { mem::transmute(0usize) };
 LL | |     let another_var = 13;
 LL | |     move || { let _ = bad_ref; let _ = another_var; }
 LL | | };
-   | |__^ type validation failed: encountered 0 at .<deref>.<closure-var(bad_ref)>, but expected something greater or equal to 1
+   | |__^ type validation failed: encountered 0 at .<deref>.<dyn-downcast>.<closure-var(bad_ref)>, but expected something greater or equal to 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 rust compiler repository if you believe it should not be considered undefined behavior
 

src/test/ui/consts/const-eval/union-ub-fat-ptr.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![allow(unused)]
+#![allow(const_err)] // make sure we cannot allow away the errors tested here
 
 // normalize-stderr-test "alignment \d+" -> "alignment N"
 // normalize-stderr-test "offset \d+" -> "offset N"

src/test/ui/consts/const-eval/union-ub-fat-ptr.stderr

@@ -1,5 +1,5 @@
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:87:1
+  --> $DIR/union-ub-fat-ptr.rs:88:1
    |
 LL | const B: &str = unsafe { SliceTransmute { repr: SliceRepr { ptr: &42, len: 999 } }.str};
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered dangling (not entirely in bounds) reference
@@ -7,7 +7,7 @@ LL | const B: &str = unsafe { SliceTransmute { repr: SliceRepr { ptr: &42, len:
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:90:1
+  --> $DIR/union-ub-fat-ptr.rs:91:1
    |
 LL | const C: &str = unsafe { SliceTransmute { bad: BadSliceRepr { ptr: &42, len: &3 } }.str};
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered non-integer slice length in fat pointer
@@ -15,7 +15,7 @@ LL | const C: &str = unsafe { SliceTransmute { bad: BadSliceRepr { ptr: &42, len
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:93:1
+  --> $DIR/union-ub-fat-ptr.rs:94:1
    |
 LL | const C2: &MyStr = unsafe { SliceTransmute { bad: BadSliceRepr { ptr: &42, len: &3 } }.my_str};
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered non-integer slice length in fat pointer
@@ -23,7 +23,7 @@ LL | const C2: &MyStr = unsafe { SliceTransmute { bad: BadSliceRepr { ptr: &42,
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:99:1
+  --> $DIR/union-ub-fat-ptr.rs:100:1
    |
 LL | const B2: &[u8] = unsafe { SliceTransmute { repr: SliceRepr { ptr: &42, len: 999 } }.slice};
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered dangling (not entirely in bounds) reference
@@ -31,7 +31,7 @@ LL | const B2: &[u8] = unsafe { SliceTransmute { repr: SliceRepr { ptr: &42, len
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:102:1
+  --> $DIR/union-ub-fat-ptr.rs:103:1
    |
 LL | const C3: &[u8] = unsafe { SliceTransmute { bad: BadSliceRepr { ptr: &42, len: &3 } }.slice};
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered non-integer slice length in fat pointer
@@ -39,7 +39,7 @@ LL | const C3: &[u8] = unsafe { SliceTransmute { bad: BadSliceRepr { ptr: &42, l
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:106:1
+  --> $DIR/union-ub-fat-ptr.rs:107:1
    |
 LL | const D: &Trait = unsafe { DynTransmute { repr: DynRepr { ptr: &92, vtable: &3 } }.rust};
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered invalid drop fn in vtable
@@ -47,7 +47,7 @@ LL | const D: &Trait = unsafe { DynTransmute { repr: DynRepr { ptr: &92, vtable:
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:109:1
+  --> $DIR/union-ub-fat-ptr.rs:110:1
    |
 LL | const E: &Trait = unsafe { DynTransmute { repr2: DynRepr2 { ptr: &92, vtable: &3 } }.rust};
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered invalid drop fn in vtable
@@ -55,7 +55,7 @@ LL | const E: &Trait = unsafe { DynTransmute { repr2: DynRepr2 { ptr: &92, vtabl
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:112:1
+  --> $DIR/union-ub-fat-ptr.rs:113:1
    |
 LL | const F: &Trait = unsafe { DynTransmute { bad: BadDynRepr { ptr: &92, vtable: 3 } }.rust};
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered non-pointer vtable in fat pointer
@@ -63,15 +63,15 @@ LL | const F: &Trait = unsafe { DynTransmute { bad: BadDynRepr { ptr: &92, vtabl
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:116:1
+  --> $DIR/union-ub-fat-ptr.rs:117:1
    |
 LL | const G: &Trait = &unsafe { BoolTransmute { val: 3 }.bl };
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 3 at .<deref>, but expected something in the range 0..=1
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 3 at .<deref>.<dyn-downcast>, but expected something in the range 0..=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 rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:120:1
+  --> $DIR/union-ub-fat-ptr.rs:121:1
    |
 LL | const H: &[bool] = &[unsafe { BoolTransmute { val: 3 }.bl }];
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 3 at .<deref>[0], but expected something in the range 0..=1
@@ -79,7 +79,7 @@ LL | const H: &[bool] = &[unsafe { BoolTransmute { val: 3 }.bl }];
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:126:1
+  --> $DIR/union-ub-fat-ptr.rs:127:1
    |
 LL | const I2: &MySliceBool = &MySlice(unsafe { BoolTransmute { val: 3 }.bl }, [false]);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 3 at .<deref>.0, but expected something in the range 0..=1
@@ -87,7 +87,7 @@ LL | const I2: &MySliceBool = &MySlice(unsafe { BoolTransmute { val: 3 }.bl }, [
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:129:1
+  --> $DIR/union-ub-fat-ptr.rs:130:1
    |
 LL | const I3: &MySliceBool = &MySlice(true, [unsafe { BoolTransmute { val: 3 }.bl }]);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 3 at .<deref>.1[0], but expected something in the range 0..=1
@@ -95,7 +95,7 @@ LL | const I3: &MySliceBool = &MySlice(true, [unsafe { BoolTransmute { val: 3 }.
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:133:1
+  --> $DIR/union-ub-fat-ptr.rs:134:1
    |
 LL | const J1: &str = unsafe { SliceTransmute { slice: &[0xFF] }.str };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized or non-UTF-8 data in str at .<deref>
@@ -103,7 +103,7 @@ LL | const J1: &str = unsafe { SliceTransmute { slice: &[0xFF] }.str };
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub-fat-ptr.rs:136:1
+  --> $DIR/union-ub-fat-ptr.rs:137:1
    |
 LL | const J2: &MyStr = unsafe { SliceTransmute { slice: &[0xFF] }.my_str };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered uninitialized or non-UTF-8 data in str at .<deref>.0

src/test/ui/consts/const-eval/union-ub.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![allow(const_err)] // make sure we cannot allow away the errors tested here
+
 union DummyUnion {
     u8: u8,
     bool: bool,

src/test/ui/consts/const-eval/union-ub.stderr

@@ -1,5 +1,5 @@
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/union-ub.rs:36:1
+  --> $DIR/union-ub.rs:38:1
    |
 LL | const BAD_BOOL: bool = unsafe { DummyUnion { u8: 42 }.bool};
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 42, but expected something in the range 0..=1