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};
@@ -275,6 +276,10 @@ impl CStore {
             .filter_map(|(cnum, data)| data.as_deref().map(|data| (cnum, data)))
     }
 
+    pub fn all_proc_macro_def_ids(&self) -> impl Iterator<Item = DefId> {
+        self.iter_crate_data().flat_map(|(krate, data)| data.proc_macros_for_crate(krate, self))
+    }
+
     fn push_dependencies_in_postorder(&self, deps: &mut IndexSet<CrateNum>, cnum: CrateNum) {
         if !deps.contains(&cnum) {
             let data = self.get_crate_data(cnum);

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -2014,6 +2014,22 @@ impl CrateMetadata {
         self.root.is_proc_macro_crate()
     }
 
+    pub(crate) fn proc_macros_for_crate(
+        &self,
+        krate: CrateNum,
+        cstore: &CStore,
+    ) -> impl Iterator<Item = DefId> {
+        gen move {
+            for def_id in self.root.proc_macro_data.as_ref().into_iter().flat_map(move |data| {
+                data.macros
+                    .decode(CrateMetadataRef { cdata: self, cstore })
+                    .map(move |index| DefId { index, krate })
+            }) {
+                yield def_id;
+            }
+        }
+    }
+
     pub(crate) fn name(&self) -> Symbol {
         self.root.header.name
     }

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -210,6 +210,17 @@ 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) {
+        if !self.all_crate_macros_already_registered {
+            for def_id in self.cstore().all_proc_macro_def_ids() {
+                self.get_macro_by_def_id(def_id);
+            }
+            self.all_crate_macros_already_registered = true;
+        }
+    }
+
     pub(crate) fn build_reduced_graph(
         &mut self,
         fragment: &AstFragment,

compiler/rustc_resolve/src/diagnostics.rs

@@ -1469,33 +1469,9 @@ 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.cm().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().best_binding() else {
-                        continue;
-                    };
-                    let Res::Def(DefKind::Macro(kinds), def_id) = binding.res() else {
-                        continue;
-                    };
-                    if !kinds.intersects(MacroKinds::ATTR | MacroKinds::DERIVE) {
-                        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_kinds().is_some_and(|k| k.contains(macro_kind.into()));

compiler/rustc_resolve/src/lib.rs

@@ -1267,6 +1267,10 @@ pub struct Resolver<'ra, 'tcx> {
 
     mods_with_parse_errors: FxHashSet<DefId>,
 
+    /// Whether `Resolver::register_macros_for_all_crates` has been called once already, as we
+    /// don't need to run it more than once.
+    all_crate_macros_already_registered: bool = false,
+
     // Stores pre-expansion and pre-placeholder-fragment-insertion names for `impl Trait` types
     // that were encountered during resolution. These names are used to generate item names
     // for APITs, so we don't want to leak details of resolution into these names.

tests/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr

@@ -559,6 +559,12 @@ error: cannot find attribute `error` in this scope
    |
 LL | #[error(no_crate_example, code = E0123)]
    |   ^^^^^
+   |
+help: `error` is an attribute that can be used by the derive macro `Error`, you might be missing a `derive` attribute
+   |
+LL + #[derive(Error)]
+LL | struct ErrorAttribute {}
+   |
 
 error: cannot find attribute `warn_` in this scope
   --> $DIR/diagnostic-derive.rs:590:3

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