Refactor `#[diagnostic::do_not_recommend]` support

This commit refactors the `#[do_not_recommend]` support in the old
parser to also apply to projection errors and not only to selection
errors. This allows the attribute to be used more widely.

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -414,7 +414,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_predicate)) => {
                         let trait_predicate = bound_predicate.rebind(trait_predicate);
                         let trait_predicate = self.resolve_vars_if_possible(trait_predicate);
-                        let trait_predicate = self.apply_do_not_recommend(trait_predicate, &mut obligation);
 
                         // Let's use the root obligation as the main message, when we care about the
                         // most general case ("X doesn't implement Pattern<'_>") over the case that
@@ -996,12 +995,9 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         err.emit()
     }
 
-    fn apply_do_not_recommend(
-        &self,
-        mut trait_predicate: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
-        obligation: &'_ mut PredicateObligation<'tcx>,
-    ) -> ty::Binder<'tcx, ty::TraitPredicate<'tcx>> {
+    fn apply_do_not_recommend(&self, obligation: &mut PredicateObligation<'tcx>) -> bool {
         let mut base_cause = obligation.cause.code().clone();
+        let mut applied_do_not_recommend = false;
         loop {
             if let ObligationCauseCode::ImplDerived(ref c) = base_cause {
                 if self.tcx.has_attrs_with_path(
@@ -1011,7 +1007,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     let code = (*c.derived.parent_code).clone();
                     obligation.cause.map_code(|_| code);
                     obligation.predicate = c.derived.parent_trait_pred.upcast(self.tcx);
-                    trait_predicate = c.derived.parent_trait_pred.clone();
+                    applied_do_not_recommend = true;
                 }
             }
             if let Some((parent_cause, _parent_pred)) = base_cause.parent() {
@@ -1021,7 +1017,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             }
         }
 
-        trait_predicate
+        applied_do_not_recommend
     }
 
     fn emit_specialized_closure_kind_error(
@@ -1521,6 +1517,20 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
     #[instrument(skip(self), level = "debug")]
     fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>) -> ErrorGuaranteed {
+        let mut error = FulfillmentError {
+            obligation: error.obligation.clone(),
+            code: error.code.clone(),
+            root_obligation: error.root_obligation.clone(),
+        };
+        if matches!(
+            error.code,
+            FulfillmentErrorCode::Select(crate::traits::SelectionError::Unimplemented)
+                | FulfillmentErrorCode::Project(_)
+        ) && self.apply_do_not_recommend(&mut error.obligation)
+        {
+            error.code = FulfillmentErrorCode::Select(SelectionError::Unimplemented);
+        }
+
         match error.code {
             FulfillmentErrorCode::Select(ref selection_error) => self.report_selection_error(
                 error.obligation.clone(),

tests/ui/diagnostic_namespace/do_not_recommend/type_mismatch.current.stderr

@@ -0,0 +1,15 @@
+error[E0277]: Very important message!
+  --> $DIR/type_mismatch.rs:25:14
+   |
+LL |     verify::<u8>();
+   |              ^^ the trait `TheImportantOne` is not implemented for `u8`
+   |
+note: required by a bound in `verify`
+  --> $DIR/type_mismatch.rs:22:14
+   |
+LL | fn verify<T: TheImportantOne>() {}
+   |              ^^^^^^^^^^^^^^^ required by this bound in `verify`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/diagnostic_namespace/do_not_recommend/type_mismatch.next.stderr

@@ -0,0 +1,15 @@
+error[E0277]: Very important message!
+  --> $DIR/type_mismatch.rs:25:14
+   |
+LL |     verify::<u8>();
+   |              ^^ the trait `TheImportantOne` is not implemented for `u8`
+   |
+note: required by a bound in `verify`
+  --> $DIR/type_mismatch.rs:22:14
+   |
+LL | fn verify<T: TheImportantOne>() {}
+   |              ^^^^^^^^^^^^^^^ required by this bound in `verify`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/diagnostic_namespace/do_not_recommend/type_mismatch.rs

@@ -0,0 +1,27 @@
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+#![feature(do_not_recommend)]
+
+#[diagnostic::on_unimplemented(message = "Very important message!")]
+trait TheImportantOne {}
+
+trait ImplementationDetail {
+    type Restriction;
+}
+
+#[diagnostic::do_not_recommend]
+impl<T: ImplementationDetail<Restriction = ()>> TheImportantOne for T {}
+
+// Comment out this `impl` to show the expected error message.
+impl ImplementationDetail for u8 {
+    type Restriction = u8;
+}
+
+fn verify<T: TheImportantOne>() {}
+
+pub fn main() {
+    verify::<u8>();
+    //~^ERROR: Very important message! [E0277]
+}