Rollup merge of rust-lang#97743 - RalfJung:const-err-future-breakage, r=estebank

make const_err show up in future breakage reports

As tracked in rust-lang#71800, const_err should become a hard error Any Day Now (TM). I'd love to move forward with that sooner rather than later; it has been deny-by-default for many years and a future incompat lint since rust-lang#80394 (landed more than a year ago). Some CTFE errors are already hard errors since rust-lang#86194. But before we truly make it a hard error in all cases, we now have one more intermediate step we can take -- to make it show up in future breakage reports.

Cc `@rust-lang/wg-const-eval`

Author: compiler-errors

compiler/rustc_lint_defs/src/builtin.rs

@@ -289,6 +289,7 @@ declare_lint! {
     "constant evaluation encountered erroneous expression",
     @future_incompatible = FutureIncompatibleInfo {
         reference: "issue #71800 <https://github.com/rust-lang/rust/issues/71800>",
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportNow,
     };
     report_in_external_macro
 }

src/test/ui/array-slice-vec/array_const_index-0.stderr

@@ -12,3 +12,16 @@ LL | const B: i32 = (&A)[1];
 
 error: aborting due to previous error
 
+Future incompatibility report: Future breakage diagnostic:
+error: any use of this value will cause an error
+  --> $DIR/array_const_index-0.rs:2:16
+   |
+LL | const B: i32 = (&A)[1];
+   | ---------------^^^^^^^-
+   |                |
+   |                index out of bounds: the length is 0 but the index is 1
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+