Rollup merge of #141698 - oli-obk:ctfe-err-flip, r=RalfJung

Use the informative error as the main const eval error message

r? `@RalfJung`

I only did the minimal changes necessary to the const eval error machinery. I'd prefer not to mix test changes with refactorings 😆

Author: matthiaskrgr

tests/fail/const-ub-checks.rs

@@ -1,7 +1,7 @@
 const UNALIGNED_READ: () = unsafe {
     let x = &[0u8; 4];
     let ptr = x.as_ptr().cast::<u32>();
-    ptr.read(); //~ERROR: evaluation of constant value failed
+    ptr.read(); //~ERROR: accessing memory based on pointer with alignment 1, but alignment 4 is required
 };
 
 fn main() {

tests/fail/const-ub-checks.stderr

@@ -1,8 +1,8 @@
-error[E0080]: evaluation of constant value failed
+error[E0080]: accessing memory based on pointer with alignment ALIGN, but alignment ALIGN is required
   --> tests/fail/const-ub-checks.rs:LL:CC
    |
 LL |     ptr.read();
-   |     ^^^^^^^^^^ accessing memory based on pointer with alignment ALIGN, but alignment ALIGN is required
+   |     ^^^^^^^^^^ evaluation of constant value failed here
 
 note: erroneous constant encountered
   --> tests/fail/const-ub-checks.rs:LL:CC

tests/fail/erroneous_const.rs

@@ -4,7 +4,7 @@
 
 struct PrintName<T>(T);
 impl<T> PrintName<T> {
-    const VOID: ! = panic!(); //~ERROR: evaluation of `PrintName::<i32>::VOID` failed
+    const VOID: ! = panic!(); //~ERROR: explicit panic
 }
 
 fn no_codegen<T>() {

tests/fail/erroneous_const.stderr

@@ -1,8 +1,8 @@
-error[E0080]: evaluation of `PrintName::<i32>::VOID` failed
+error[E0080]: evaluation panicked: explicit panic
   --> tests/fail/erroneous_const.rs:LL:CC
    |
 LL |     const VOID: ! = panic!();
-   |                     ^^^^^^^^ evaluation panicked: explicit panic
+   |                     ^^^^^^^^ evaluation of `PrintName::<i32>::VOID` failed here
 
 note: erroneous constant encountered
   --> tests/fail/erroneous_const.rs:LL:CC

tests/fail/erroneous_const2.rs

@@ -1,7 +1,7 @@
 const X: u32 = 5;
 const Y: u32 = 6;
 const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
-//~^ERROR: evaluation of constant value failed
+//~^ERROR: overflow
 
 #[rustfmt::skip] // rustfmt bug: https://github.com/rust-lang/rustfmt/issues/5391
 fn main() {

tests/fail/erroneous_const2.stderr

@@ -1,8 +1,8 @@
-error[E0080]: evaluation of constant value failed
+error[E0080]: attempt to compute `5_u32 - 6_u32`, which would overflow
   --> tests/fail/erroneous_const2.rs:LL:CC
    |
 LL | const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
-   |                   ^^^^^ attempt to compute `5_u32 - 6_u32`, which would overflow
+   |                   ^^^^^ evaluation of constant value failed here
 
 note: erroneous constant encountered
   --> tests/fail/erroneous_const2.rs:LL:CC