Do not apply #[do_not_recommend] if the feature flag is not set

This commit adds additional checks for the feature flag as apparently it
is possible to use this on a beta compiler without feature flags. This
PR might be a candidate for backporting.

Reported in the bevy issue tracker: bevyengine/bevy#14591 (comment)

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

@@ -682,6 +682,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         &self,
         obligation: &mut PredicateObligation<'tcx>,
     ) -> bool {
+        if !self.tcx.features().do_not_recommend {
+            return false;
+        }
+
         let mut base_cause = obligation.cause.code().clone();
         let mut applied_do_not_recommend = false;
         loop {
@@ -1782,10 +1786,11 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             .into_iter()
             .cloned()
             .filter(|cand| {
-                !self.tcx.has_attrs_with_path(
-                    cand.impl_def_id,
-                    &[sym::diagnostic, sym::do_not_recommend],
-                )
+                !self.tcx.features().do_not_recommend
+                    || !self.tcx.has_attrs_with_path(
+                        cand.impl_def_id,
+                        &[sym::diagnostic, sym::do_not_recommend],
+                    )
             })
             .collect::<Vec<_>>();
 
@@ -1803,9 +1808,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 .all_impls(def_id)
                 // ignore `do_not_recommend` items
                 .filter(|def_id| {
-                    !self
-                        .tcx
-                        .has_attrs_with_path(*def_id, &[sym::diagnostic, sym::do_not_recommend])
+                    !self.tcx.features().do_not_recommend
+                        || !self
+                            .tcx
+                            .has_attrs_with_path(*def_id, &[sym::diagnostic, sym::do_not_recommend])
                 })
                 // Ignore automatically derived impls and `!Trait` impls.
                 .filter_map(|def_id| self.tcx.impl_trait_header(def_id))

tests/ui/diagnostic_namespace/do_not_recommend/do_not_apply_attribute_without_feature_flag.rs

@@ -0,0 +1,21 @@
+#![allow(unknown_or_malformed_diagnostic_attributes)]
+
+trait Foo {}
+
+#[diagnostic::do_not_recommend]
+impl<A> Foo for (A,) {}
+
+#[diagnostic::do_not_recommend]
+impl<A, B> Foo for (A, B) {}
+
+#[diagnostic::do_not_recommend]
+impl<A, B, C> Foo for (A, B, C) {}
+
+impl Foo for i32 {}
+
+fn check(a: impl Foo) {}
+
+fn main() {
+    check(());
+    //~^ ERROR the trait bound `(): Foo` is not satisfied
+}

tests/ui/diagnostic_namespace/do_not_recommend/do_not_apply_attribute_without_feature_flag.stderr

@@ -0,0 +1,21 @@
+error[E0277]: the trait bound `(): Foo` is not satisfied
+  --> $DIR/do_not_apply_attribute_without_feature_flag.rs:19:11
+   |
+LL |     check(());
+   |     ----- ^^ the trait `Foo` is not implemented for `()`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the following other types implement trait `Foo`:
+             (A, B)
+             (A, B, C)
+             (A,)
+note: required by a bound in `check`
+  --> $DIR/do_not_apply_attribute_without_feature_flag.rs:16:18
+   |
+LL | fn check(a: impl Foo) {}
+   |                  ^^^ required by this bound in `check`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.