Complete derive helper attributes

Only their names, anything can go inside.

crates/hir/src/semantics.rs

@@ -510,6 +510,22 @@ impl<'db> SemanticsImpl<'db> {
         self.with_ctx(|ctx| ctx.has_derives(adt))
     }
 
+    pub fn derive_helpers_in_scope(&self, adt: &ast::Adt) -> Option<Vec<(Symbol, Symbol)>> {
+        let sa = self.analyze_no_infer(adt.syntax())?;
+        let id = self.db.ast_id_map(sa.file_id).ast_id(adt);
+        let result = sa
+            .resolver
+            .def_map()
+            .derive_helpers_in_scope(InFile::new(sa.file_id, id))?
+            .iter()
+            .map(|(name, macro_, _)| {
+                let macro_name = Macro::from(*macro_).name(self.db).symbol().clone();
+                (name.symbol().clone(), macro_name)
+            })
+            .collect();
+        Some(result)
+    }
+
     pub fn derive_helper(&self, attr: &ast::Attr) -> Option<Vec<(Macro, MacroFileId)>> {
         let adt = attr.syntax().ancestors().find_map(ast::Item::cast).and_then(|it| match it {
             ast::Item::Struct(it) => Some(ast::Adt::Struct(it)),

crates/ide-completion/src/completions/attribute.rs

@@ -86,10 +86,21 @@ pub(crate) fn complete_attribute_path(
     acc: &mut Completions,
     ctx: &CompletionContext<'_>,
     path_ctx @ PathCompletionCtx { qualified, .. }: &PathCompletionCtx,
-    &AttrCtx { kind, annotated_item_kind }: &AttrCtx,
+    &AttrCtx { kind, annotated_item_kind, ref derive_helpers }: &AttrCtx,
 ) {
     let is_inner = kind == AttrKind::Inner;
 
+    for (derive_helper, derive_name) in derive_helpers {
+        let mut item = CompletionItem::new(
+            SymbolKind::Attribute,
+            ctx.source_range(),
+            derive_helper.as_str(),
+            ctx.edition,
+        );
+        item.detail(format!("derive helper of `{derive_name}`"));
+        item.add_to(acc, ctx.db);
+    }
+
     match qualified {
         Qualified::With {
             resolution: Some(hir::PathResolution::Def(hir::ModuleDef::Module(module))),

crates/ide-completion/src/context.rs

@@ -7,8 +7,8 @@ mod tests;
 use std::{iter, ops::ControlFlow};
 
 use hir::{
-    HasAttrs, Local, ModuleSource, Name, PathResolution, ScopeDef, Semantics, SemanticsScope, Type,
-    TypeInfo,
+    HasAttrs, Local, ModuleSource, Name, PathResolution, ScopeDef, Semantics, SemanticsScope,
+    Symbol, Type, TypeInfo,
 };
 use ide_db::{
     base_db::SourceDatabase, famous_defs::FamousDefs, helpers::is_editable_crate, FilePosition,
@@ -133,6 +133,7 @@ pub(crate) type ExistingDerives = FxHashSet<hir::Macro>;
 pub(crate) struct AttrCtx {
     pub(crate) kind: AttrKind,
     pub(crate) annotated_item_kind: Option<SyntaxKind>,
+    pub(crate) derive_helpers: Vec<(Symbol, Symbol)>,
 }
 
 #[derive(Debug, PartialEq, Eq)]

crates/ide-completion/src/context/analysis.rs

@@ -1129,7 +1129,22 @@ fn classify_name_ref(
         let is_trailing_outer_attr = kind != AttrKind::Inner
             && non_trivia_sibling(attr.syntax().clone().into(), syntax::Direction::Next).is_none();
         let annotated_item_kind = if is_trailing_outer_attr { None } else { Some(attached.kind()) };
-        Some(PathKind::Attr { attr_ctx: AttrCtx { kind, annotated_item_kind } })
+        let derive_helpers = annotated_item_kind
+            .filter(|kind| {
+                matches!(
+                    kind,
+                    SyntaxKind::STRUCT
+                        | SyntaxKind::ENUM
+                        | SyntaxKind::UNION
+                        | SyntaxKind::VARIANT
+                        | SyntaxKind::TUPLE_FIELD
+                        | SyntaxKind::RECORD_FIELD
+                )
+            })
+            .and_then(|_| nameref.as_ref()?.syntax().ancestors().find_map(ast::Adt::cast))
+            .and_then(|adt| sema.derive_helpers_in_scope(&adt))
+            .unwrap_or_default();
+        Some(PathKind::Attr { attr_ctx: AttrCtx { kind, annotated_item_kind, derive_helpers } })
     };
 
     // Infer the path kind

crates/ide-completion/src/tests/attribute.rs

@@ -8,6 +8,70 @@ fn check(ra_fixture: &str, expect: Expect) {
     expect.assert_eq(&actual);
 }
 
+#[test]
+fn derive_helpers() {
+    check(
+        r#"
+//- /mac.rs crate:mac
+#![crate_type = "proc-macro"]
+
+#[proc_macro_derive(MyDerive, attributes(my_cool_helper_attribute))]
+pub fn my_derive() {}
+
+//- /lib.rs crate:lib deps:mac
+#[rustc_builtin_macro]
+pub macro derive($item:item) {}
+
+#[derive(mac::MyDerive)]
+pub struct Foo(#[m$0] i32);
+"#,
+        expect![[r#"
+            at allow(…)
+            at automatically_derived
+            at cfg(…)
+            at cfg_attr(…)
+            at cold
+            at deny(…)
+            at deprecated
+            at derive                 macro derive
+            at derive(…)
+            at doc = "…"
+            at doc(alias = "…")
+            at doc(hidden)
+            at expect(…)
+            at export_name = "…"
+            at forbid(…)
+            at global_allocator
+            at ignore = "…"
+            at inline
+            at link
+            at link_name = "…"
+            at link_section = "…"
+            at macro_export
+            at macro_use
+            at must_use
+            at my_cool_helper_attribute derive helper of `MyDerive`
+            at no_mangle
+            at non_exhaustive
+            at panic_handler
+            at path = "…"
+            at proc_macro
+            at proc_macro_attribute
+            at proc_macro_derive(…)
+            at repr(…)
+            at should_panic
+            at target_feature(enable = "…")
+            at test
+            at track_caller
+            at used
+            at warn(…)
+            md mac
+            kw crate::
+            kw self::
+        "#]],
+    )
+}
+
 #[test]
 fn proc_macros() {
     check(