Correctly handle stability of `#[diagnostic]` attributes

This commit changes the way we treat the stability of attributes in the
`#[diagnostic]` namespace. Instead of relaying on ad-hoc checks to
ensure at call side that a certain attribute is really usable at that
location it centralises the logic to one place. For diagnostic
attributes comming from other crates it just skips serializing
attributes that are not stable and that do not have the corresponding
feature enabled. For attributes from the current crate we can just use
the feature information provided by `TyCtx`.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1186,3 +1186,11 @@ pub static BUILTIN_ATTRIBUTE_MAP: LazyLock<FxHashMap<Symbol, &BuiltinAttribute>>
         }
         map
     });
+
+pub fn is_stable_diagnostic_attribute(sym: Symbol, features: &Features) -> bool {
+    match sym {
+        sym::on_unimplemented => true,
+        sym::do_not_recommend => features.do_not_recommend,
+        _ => false,
+    }
+}

compiler/rustc_feature/src/lib.rs

@@ -130,8 +130,9 @@ pub fn find_feature_issue(feature: Symbol, issue: GateIssue) -> Option<NonZero<u
 pub use accepted::ACCEPTED_FEATURES;
 pub use builtin_attrs::{
     deprecated_attributes, encode_cross_crate, find_gated_cfg, is_builtin_attr_name,
-    is_valid_for_get_attr, AttributeDuplicates, AttributeGate, AttributeSafety, AttributeTemplate,
-    AttributeType, BuiltinAttribute, GatedCfg, BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
+    is_stable_diagnostic_attribute, is_valid_for_get_attr, AttributeDuplicates, AttributeGate,
+    AttributeSafety, AttributeTemplate, AttributeType, BuiltinAttribute, GatedCfg,
+    BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
 };
 pub use removed::REMOVED_FEATURES;
 pub use unstable::{Features, INCOMPATIBLE_FEATURES, UNSTABLE_FEATURES};

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -9,6 +9,7 @@ use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_data_structures::memmap::{Mmap, MmapMut};
 use rustc_data_structures::sync::{join, par_for_each_in, Lrc};
 use rustc_data_structures::temp_dir::MaybeTempDir;
+use rustc_feature::Features;
 use rustc_hir as hir;
 use rustc_hir::def_id::{LocalDefId, LocalDefIdSet, CRATE_DEF_ID, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_hir::definitions::DefPathData;
@@ -797,9 +798,10 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
     }
 }
 
-struct AnalyzeAttrState {
+struct AnalyzeAttrState<'a> {
     is_exported: bool,
     is_doc_hidden: bool,
+    features: &'a Features,
 }
 
 /// Returns whether an attribute needs to be recorded in metadata, that is, if it's usable and
@@ -812,7 +814,7 @@ struct AnalyzeAttrState {
 /// visibility: this is a piece of data that can be computed once per defid, and not once per
 /// attribute. Some attributes would only be usable downstream if they are public.
 #[inline]
-fn analyze_attr(attr: &Attribute, state: &mut AnalyzeAttrState) -> bool {
+fn analyze_attr(attr: &Attribute, state: &mut AnalyzeAttrState<'_>) -> bool {
     let mut should_encode = false;
     if !rustc_feature::encode_cross_crate(attr.name_or_empty()) {
         // Attributes not marked encode-cross-crate don't need to be encoded for downstream crates.
@@ -837,6 +839,9 @@ fn analyze_attr(attr: &Attribute, state: &mut AnalyzeAttrState) -> bool {
                 }
             }
         }
+    } else if attr.path().starts_with(&[sym::diagnostic]) && attr.path().len() == 2 {
+        should_encode =
+            rustc_feature::is_stable_diagnostic_attribute(attr.path()[1], state.features);
     } else {
         should_encode = true;
     }
@@ -1343,6 +1348,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         let mut state = AnalyzeAttrState {
             is_exported: tcx.effective_visibilities(()).is_exported(def_id),
             is_doc_hidden: false,
+            features: &tcx.features(),
         };
         let attr_iter = tcx
             .hir()

compiler/rustc_middle/src/ty/context.rs

@@ -3183,8 +3183,7 @@ impl<'tcx> TyCtxt<'tcx> {
 
     /// Whether this is a trait implementation that has `#[diagnostic::do_not_recommend]`
     pub fn do_not_recommend_impl(self, def_id: DefId) -> bool {
-        matches!(self.def_kind(def_id), DefKind::Impl { of_trait: true })
-            && self.impl_trait_header(def_id).is_some_and(|header| header.do_not_recommend)
+        self.get_diagnostic_attr(def_id, sym::do_not_recommend).is_some()
     }
 }
 

compiler/rustc_middle/src/ty/mod.rs

@@ -1797,6 +1797,37 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
+    /// Get an attribute from the diagnostic attribute namespace
+    ///
+    /// This function requests an attribute with the following structure:
+    ///
+    /// `#[diagnostic::$attr]`
+    ///
+    /// This function performs feature checking, so if an attribute is returned
+    /// it can be used by the consumer
+    pub fn get_diagnostic_attr(
+        self,
+        did: impl Into<DefId>,
+        attr: Symbol,
+    ) -> Option<&'tcx ast::Attribute> {
+        let did: DefId = did.into();
+        if did.as_local().is_some() {
+            // it's a crate local item, we need to check feature flags
+            if rustc_feature::is_stable_diagnostic_attribute(attr, self.features()) {
+                self.get_attrs_by_path(did, &[sym::diagnostic, sym::do_not_recommend]).next()
+            } else {
+                None
+            }
+        } else {
+            // we filter out unstable diagnostic attributes before
+            // encoding attributes
+            debug_assert!(rustc_feature::encode_cross_crate(attr));
+            self.item_attrs(did)
+                .iter()
+                .find(|a| matches!(a.path().as_ref(), [sym::diagnostic, a] if *a == attr))
+        }
+    }
+
     pub fn get_attrs_by_path<'attr>(
         self,
         did: DefId,