Rollup merge of #81128 - RalfJung:validation-testing, r=oli-obk

validation test: turn some const_err back into validation failures

This resolves the problem I raised at #78407 (comment).
r? `@oli-obk`

Author: KodrAus

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

@@ -8,6 +8,12 @@ use std::mem;
 // normalize-stderr-test "alloc\d+" -> "allocN"
 // normalize-stderr-test "size \d+" -> "size N"
 
+/// A newtype wrapper to prevent MIR generation from inserting reborrows that would affect the error
+/// message. Use this whenever the message is "any use of this value will cause an error" instead of
+/// "it is undefined behavior to use this value".
+#[repr(transparent)]
+struct W<T>(T);
+
 #[repr(C)]
 union MaybeUninit<T: Copy> {
     uninit: (),
@@ -95,26 +101,22 @@ const RAW_SLICE_LENGTH_UNINIT: *const [u8] = unsafe {
 
 // # trait object
 // bad trait object
-#[warn(const_err)]
-const TRAIT_OBJ_SHORT_VTABLE_1: &dyn Trait = unsafe { mem::transmute((&92u8, &3u8)) };
-//~^ WARN any use of this value will cause an error [const_err]
+const TRAIT_OBJ_SHORT_VTABLE_1: W<&dyn Trait> = unsafe { mem::transmute(W((&92u8, &3u8))) };
+//~^ ERROR it is undefined behavior to use this value
 // bad trait object
-#[warn(const_err)]
-const TRAIT_OBJ_SHORT_VTABLE_2: &dyn Trait = unsafe { mem::transmute((&92u8, &3u64)) };
-//~^ WARN any use of this value will cause an error [const_err]
+const TRAIT_OBJ_SHORT_VTABLE_2: W<&dyn Trait> = unsafe { mem::transmute(W((&92u8, &3u64))) };
+//~^ ERROR it is undefined behavior to use this value
 // bad trait object
-#[warn(const_err)]
-const TRAIT_OBJ_INT_VTABLE: &dyn Trait = unsafe { mem::transmute((&92u8, 4usize)) };
-//~^ WARN any use of this value will cause an error [const_err]
+const TRAIT_OBJ_INT_VTABLE: W<&dyn Trait> = unsafe { mem::transmute(W((&92u8, 4usize))) };
+//~^ ERROR it is undefined behavior to use this value
 const TRAIT_OBJ_UNALIGNED_VTABLE: &dyn Trait = unsafe { mem::transmute((&92u8, &[0u8; 128])) };
 //~^ ERROR it is undefined behavior to use this value
 const TRAIT_OBJ_BAD_DROP_FN_NULL: &dyn Trait = unsafe { mem::transmute((&92u8, &[0usize; 8])) };
 //~^ ERROR it is undefined behavior to use this value
 const TRAIT_OBJ_BAD_DROP_FN_INT: &dyn Trait = unsafe { mem::transmute((&92u8, &[1usize; 8])) };
 //~^ ERROR it is undefined behavior to use this value
-#[warn(const_err)]
-const TRAIT_OBJ_BAD_DROP_FN_NOT_FN_PTR: &dyn Trait = unsafe { mem::transmute((&92u8, &[&42u8; 8])) };
-//~^ WARN any use of this value will cause an error [const_err]
+const TRAIT_OBJ_BAD_DROP_FN_NOT_FN_PTR: W<&dyn Trait> = unsafe { mem::transmute(W((&92u8, &[&42u8; 8]))) };
+//~^ ERROR it is undefined behavior to use this value
 
 // bad data *inside* the trait object
 const TRAIT_OBJ_CONTENT_INVALID: &dyn Trait = unsafe { mem::transmute::<_, &bool>(&3u8) };