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

make const_err show up in future breakage reports

As tracked in https://github.com/rust-lang/rust/issues/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 https://github.com/rust-lang/rust/pull/80394 (landed more than a year ago). Some CTFE errors are already hard errors since https://github.com/rust-lang/rust/pull/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`````

1 files changed, 33 insertions, 0 deletions

diff --git a/src/test/ui/consts/const-eval/conditional_array_execution.stderr b/src/test/ui/consts/const-eval/conditional_array_execution.stderr
index 9dc40030a6f..f88bf445426 100644
--- a/src/test/ui/consts/const-eval/conditional_array_execution.stderr
+++ b/src/test/ui/consts/const-eval/conditional_array_execution.stderr
@@ -33,3 +33,36 @@ LL |     println!("{}", FOO);
 error: aborting due to previous error; 2 warnings emitted
 
 For more information about this error, try `rustc --explain E0080`.
+Future incompatibility report: Future breakage diagnostic:
+warning: any use of this value will cause an error
+  --> $DIR/conditional_array_execution.rs:7:19
+   |
+LL | const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
+   | ------------------^^^^^---------------------------
+   |                   |
+   |                   attempt to compute `5_u32 - 6_u32`, which would overflow
+   |
+note: the lint level is defined here
+  --> $DIR/conditional_array_execution.rs:3:9
+   |
+LL | #![warn(const_err)]
+   |         ^^^^^^^^^
+   = 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>
+
+Future breakage diagnostic:
+warning: erroneous constant used
+  --> $DIR/conditional_array_execution.rs:12:20
+   |
+LL |     println!("{}", FOO);
+   |                    ^^^ referenced constant has errors
+   |
+note: the lint level is defined here
+  --> $DIR/conditional_array_execution.rs:3:9
+   |
+LL | #![warn(const_err)]
+   |         ^^^^^^^^^
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+   = note: this warning originates in the macro `$crate::format_args_nl` (in Nightly builds, run with -Z macro-backtrace for more info)
+