Add explanation of test for validation mismatch

Author: ecstatic-morse

src/test/ui/consts/const-eval/issue-65394.rs

@@ -1,7 +1,10 @@
-// Test for absence of validation mismatch ICE in #65394
+// This test originated from #65394. We conservatively assume that `x` is still `LiveDrop` even
+// after it has been moved because a mutable reference to it exists at some point in the const body.
+//
+// We will likely have to change this behavior before we allow `&mut` in a `const`.
 
 const _: Vec<i32> = {
-    let mut x = Vec::<i32>::new();
+    let mut x = Vec::<i32>::new(); //~ ERROR destructors cannot be evaluated at compile-time
     let r = &mut x; //~ ERROR references in constants may only refer to immutable values
     let y = x;
     y

src/test/ui/consts/const-eval/issue-65394.stderr

@@ -1,11 +1,16 @@
 error[E0017]: references in constants may only refer to immutable values
-  --> $DIR/issue-65394.rs:5:13
+  --> $DIR/issue-65394.rs:8:13
    |
 LL |     let r = &mut x;
    |             ^^^^^^ constants require immutable values
 
-[ERROR rustc_mir::transform::qualify_consts] old validator: [($DIR/issue-65394.rs:5:13: 5:19, "MutBorrow(Mut { allow_two_phase_borrow: false })")]
-[ERROR rustc_mir::transform::qualify_consts] new validator: [($DIR/issue-65394.rs:5:13: 5:19, "MutBorrow(Mut { allow_two_phase_borrow: false })"), ($DIR/issue-65394.rs:4:9: 4:14, "LiveDrop")]
-error: aborting due to previous error
+error[E0493]: destructors cannot be evaluated at compile-time
+  --> $DIR/issue-65394.rs:7:9
+   |
+LL |     let mut x = Vec::<i32>::new();
+   |         ^^^^^ constants cannot evaluate destructors
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0017`.
+Some errors have detailed explanations: E0017, E0493.
+For more information about an error, try `rustc --explain E0017`.