Structured suggestion for "missing `feature` in unstable fn call"

When encountering a call corresponding to an item marked as unstable behind a feature flag, provide a structured suggestion pointing at where in the crate the `#![feature(..)]` needs to be written.

```
error: `foobar` is not yet stable as a const fn
  --> $DIR/const-stability-attribute-implies-no-feature.rs:12:5
   |
LL |     foobar();
   |     ^^^^^^^^
   |
help: add `#![feature(const_foobar)]` to the crate attributes to enable
   |
LL + #![feature(const_foobar)]
   |
```

Fix #81370.

1 files changed, 8 insertions, 1 deletions

diff --git a/compiler/rustc_const_eval/src/check_consts/check.rs b/compiler/rustc_const_eval/src/check_consts/check.rs
index 6c940124193..94fb039512a 100644
--- a/compiler/rustc_const_eval/src/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/check_consts/check.rs
@@ -911,16 +911,23 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                                 // regular stability.
                                 feature == sym::rustc_private
                                     && issue == NonZero::new(27812)
-                                    && self.tcx.sess.opts.unstable_opts.force_unstable_if_unmarked
+                                    && tcx.sess.opts.unstable_opts.force_unstable_if_unmarked
                             };
                         // Even if the feature is enabled, we still need check_op to double-check
                         // this if the callee is not safe to expose on stable.
                         if !feature_enabled || !callee_safe_to_expose_on_stable {
+                            let suggestion_span =
+                                tcx.hir_crate_items(()).definitions().next().and_then(|id| {
+                                    tcx.crate_level_attribute_injection_span(
+                                        tcx.local_def_id_to_hir_id(id),
+                                    )
+                                });
                             self.check_op(ops::FnCallUnstable {
                                 def_id: callee,
                                 feature,
                                 feature_enabled,
                                 safe_to_expose_on_stable: callee_safe_to_expose_on_stable,
+                                suggestion_span,
                             });
                         }
                     }