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`.

Author: weiznich

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1186,3 +1186,11 @@ pub static BUILTIN_ATTRIBUTE_MAP: LazyLock<FxHashMap<Symbol, &BuiltinAttribute>>
 }
  map
 });
+
+pubfnis_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
 pubuse accepted::ACCEPTED_FEATURES;
 pubuse 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,
 };
 pubuse removed::REMOVED_FEATURES;
 pubuse 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> {
 }
 }
 
-structAnalyzeAttrState{
+structAnalyzeAttrState<'a>{
 is_exported:bool,
 is_doc_hidden:bool,
+features:&'aFeatures,
 }
 
 /// 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]
-fnanalyze_attr(attr:&Attribute,state:&mutAnalyzeAttrState) -> bool{
+fnanalyze_attr(attr:&Attribute,state:&mutAnalyzeAttrState<'_>) -> bool{
 letmut 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 {
 }
 }
 }
+}elseif 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> {
 letmut 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]`
 pubfndo_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
+pubfnget_diagnostic_attr(
+self,
+did:implInto<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))
+}
+}
+
 pubfnget_attrs_by_path<'attr>(
 self,
 did:DefId,