Auto merge of #133321 - compiler-errors:const-checker, r=wesleywiser

Get rid of HIR const checker

As far as I can tell, the HIR const checker was implemented in https://github.com/rust-lang/rust/pull/66170 because we were not able to issue useful const error messages in the MIR const checker.

This seems to have changed in the last 5 years, probably due to work like #90532. I've tweaked the diagnostics slightly and think the error messages have gotten *better* in fact.

Thus I think the HIR const checker has reached the end of its usefulness, and we can retire it.

cc `@RalfJung`

2 files changed, 9 insertions, 10 deletions

diff --git a/compiler/rustc_const_eval/messages.ftl b/compiler/rustc_const_eval/messages.ftl
index f93f4d36e45..c31c94495d0 100644
--- a/compiler/rustc_const_eval/messages.ftl
+++ b/compiler/rustc_const_eval/messages.ftl
@@ -110,7 +110,7 @@ const_eval_extern_type_field = `extern type` field does not have a known offset
 const_eval_fn_ptr_call =
     function pointers need an RFC before allowed to be called in {const_eval_const_context}s
 const_eval_for_loop_into_iter_non_const =
-    cannot convert `{$ty}` into an iterator in {const_eval_const_context}s
+    cannot use `for` loop on `{$ty}` in {const_eval_const_context}s
 
 const_eval_frame_note = {$times ->
     [0] {const_eval_frame_note_inner}
@@ -324,11 +324,11 @@ const_eval_ptr_as_bytes_1 =
     this code performed an operation that depends on the underlying bytes representing a pointer
 const_eval_ptr_as_bytes_2 =
     the absolute address of a pointer is not known at compile-time, so such operations are not supported
-const_eval_question_branch_non_const =
-    `?` cannot determine the branch of `{$ty}` in {const_eval_const_context}s
 
+const_eval_question_branch_non_const =
+    `?` is not allowed on `{$ty}` in {const_eval_const_context}s
 const_eval_question_from_residual_non_const =
-    `?` cannot convert from residual of `{$ty}` in {const_eval_const_context}s
+    `?` is not allowed on `{$ty}` in {const_eval_const_context}s
 
 const_eval_range = in the range {$lo}..={$hi}
 const_eval_range_lower = greater or equal to {$lo}
diff --git a/compiler/rustc_const_eval/src/check_consts/ops.rs b/compiler/rustc_const_eval/src/check_consts/ops.rs
index ab81e60a33f..489bb54a6f9 100644
--- a/compiler/rustc_const_eval/src/check_consts/ops.rs
+++ b/compiler/rustc_const_eval/src/check_consts/ops.rs
@@ -180,8 +180,10 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                     };
                 }
 
-                let mut err = match kind {
-                    CallDesugaringKind::ForLoopIntoIter => {
+                // Don't point at the trait if this is a desugaring...
+                // FIXME(const_trait_impl): we could perhaps do this for `Iterator`.
+                match kind {
+                    CallDesugaringKind::ForLoopIntoIter | CallDesugaringKind::ForLoopNext => {
                         error!(NonConstForLoopIntoIter)
                     }
                     CallDesugaringKind::QuestionBranch => {
@@ -196,10 +198,7 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                     CallDesugaringKind::Await => {
                         error!(NonConstAwait)
                     }
-                };
-
-                diag_trait(&mut err, self_ty, kind.trait_def_id(tcx));
-                err
+                }
             }
             CallKind::FnCall { fn_trait_id, self_ty } => {
                 let note = match self_ty.kind() {