Detect missing derive on unresolved attribute even when not imported

compiler/rustc_metadata/src/creader.rs

@@ -32,6 +32,7 @@ use rustc_session::cstore::{CrateDepKind, CrateSource, ExternCrate, ExternCrateS
 use rustc_session::lint::{self, BuiltinLintDiag};
 use rustc_session::output::validate_crate_name;
 use rustc_session::search_paths::PathKind;
+use rustc_span::def_id::DefId;
 use rustc_span::edition::Edition;
 use rustc_span::{DUMMY_SP, Ident, Span, Symbol, sym};
 use rustc_target::spec::{PanicStrategy, Target, TargetTuple};
@@ -276,6 +277,12 @@ impl CStore {
             .filter_map(|(cnum, data)| data.as_deref().map(|data| (cnum, data)))
     }
 
+    pub fn all_proc_macro_def_ids(&self) -> Vec<DefId> {
+        self.iter_crate_data()
+            .flat_map(|(krate, data)| data.proc_macros_for_crate(krate, self))
+            .collect()
+    }
+
     fn iter_crate_data_mut(&mut self) -> impl Iterator<Item = (CrateNum, &mut CrateMetadata)> {
         self.metas
             .iter_enumerated_mut()

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1949,6 +1949,16 @@ impl CrateMetadata {
         self.root.is_proc_macro_crate()
     }
 
+    pub(crate) fn proc_macros_for_crate(&self, krate: CrateNum, cstore: &CStore) -> Vec<DefId> {
+        let Some(data) = self.root.proc_macro_data.as_ref() else {
+            return vec![];
+        };
+        data.macros
+            .decode(CrateMetadataRef { cdata: self, cstore })
+            .map(|index| DefId { index, krate })
+            .collect()
+    }
+
     pub(crate) fn name(&self) -> Symbol {
         self.root.header.name
     }

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -169,6 +169,15 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         }
     }
 
+    /// Add every proc macro accessible from the current crate to the `macro_map` so diagnostics can
+    /// find them for suggestions.
+    pub(crate) fn register_macros_for_all_crates(&mut self) {
+        let def_ids = self.cstore().all_proc_macro_def_ids();
+        for def_id in def_ids {
+            self.get_macro_by_def_id(def_id);
+        }
+    }
+
     pub(crate) fn get_macro_by_def_id(&mut self, def_id: DefId) -> &MacroData {
         if self.macro_map.contains_key(&def_id) {
             return &self.macro_map[&def_id];

compiler/rustc_resolve/src/diagnostics.rs

@@ -1485,32 +1485,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         krate: &Crate,
         sugg_span: Option<Span>,
     ) {
-        // Bring imported but unused `derive` macros into `macro_map` so we ensure they can be used
-        // for suggestions.
-        self.visit_scopes(
-            ScopeSet::Macro(MacroKind::Derive),
-            &parent_scope,
-            ident.span.ctxt(),
-            |this, scope, _use_prelude, _ctxt| {
-                let Scope::Module(m, _) = scope else {
-                    return None;
-                };
-                for (_, resolution) in this.resolutions(m).borrow().iter() {
-                    let Some(binding) = resolution.borrow().binding else {
-                        continue;
-                    };
-                    let Res::Def(DefKind::Macro(MacroKind::Derive | MacroKind::Attr), def_id) =
-                        binding.res()
-                    else {
-                        continue;
-                    };
-                    // By doing this all *imported* macros get added to the `macro_map` even if they
-                    // are *unused*, which makes the later suggestions find them and work.
-                    let _ = this.get_macro_by_def_id(def_id);
-                }
-                None::<()>
-            },
-        );
+        // Bring all unused `derive` macros into `macro_map` so we ensure they can be used for
+        // suggestions.
+        self.register_macros_for_all_crates();
 
         let is_expected = &|res: Res| res.macro_kind() == Some(macro_kind);
         let suggestion = self.early_lookup_typo_candidate(

tests/ui/macros/missing-derive-3.stderr

@@ -3,6 +3,11 @@ error: cannot find attribute `sede` in this scope
    |
 LL |     #[sede(untagged)]
    |       ^^^^
+   |
+help: the derive macros `Deserialize` and `Serialize` accept the similarly named `serde` attribute
+   |
+LL |     #[serde(untagged)]
+   |         +
 
 error: cannot find attribute `serde` in this scope
   --> $DIR/missing-derive-3.rs:14:7
@@ -15,6 +20,11 @@ note: `serde` is imported here, but it is a crate, not an attribute
    |
 LL | extern crate serde;
    | ^^^^^^^^^^^^^^^^^^^
+help: `serde` is an attribute that can be used by the derive macros `Deserialize` and `Serialize`, you might be missing a `derive` attribute
+   |
+LL + #[derive(Deserialize, Serialize)]
+LL | enum B {
+   |
 
 error: cannot find attribute `serde` in this scope
   --> $DIR/missing-derive-3.rs:6:3
@@ -27,6 +37,11 @@ note: `serde` is imported here, but it is a crate, not an attribute
    |
 LL | extern crate serde;
    | ^^^^^^^^^^^^^^^^^^^
+help: `serde` is an attribute that can be used by the derive macros `Deserialize` and `Serialize`, you might be missing a `derive` attribute
+   |
+LL + #[derive(Deserialize, Serialize)]
+LL | enum A {
+   |
 
 error: aborting due to 3 previous errors
 

tests/ui/proc-macro/derive-helper-legacy-spurious.stderr

@@ -9,6 +9,12 @@ error: cannot find attribute `empty_helper` in this scope
    |
 LL | #[empty_helper]
    |   ^^^^^^^^^^^^
+   |
+help: `empty_helper` is an attribute that can be used by the derive macro `Empty`, you might be missing a `derive` attribute
+   |
+LL + #[derive(Empty)]
+LL | struct Foo {}
+   |
 
 error: aborting due to 2 previous errors