Collect trait impls inside unnamed consts

crates/hir_def/src/item_scope.rs

@@ -107,6 +107,10 @@ pub fn visibility_of(&self, def: ModuleDefId) -> Option<Visibility> {
             .map(|(_, v)| v)
     }
 
+    pub fn unnamed_consts(&self) -> impl Iterator<Item = ConstId> + '_ {
+        self.unnamed_consts.iter().copied()
+    }
+
     /// Iterate over all module scoped macros
     pub(crate) fn macros<'a>(&'a self) -> impl Iterator<Item = (&'a Name, MacroDefId)> + 'a {
         self.entries().filter_map(|(name, def)| def.take_macros().map(|macro_| (name, macro_)))

crates/hir_ty/src/method_resolution.rs

@@ -8,8 +8,8 @@
 use base_db::CrateId;
 use chalk_ir::{cast::Cast, Mutability, UniverseIndex};
 use hir_def::{
-    lang_item::LangItemTarget, AssocContainerId, AssocItemId, FunctionId, GenericDefId, HasModule,
-    ImplId, Lookup, ModuleId, TraitId,
+    lang_item::LangItemTarget, nameres::DefMap, AssocContainerId, AssocItemId, FunctionId,
+    GenericDefId, HasModule, ImplId, Lookup, ModuleId, TraitId,
 };
 use hir_expand::name::Name;
 use rustc_hash::{FxHashMap, FxHashSet};
@@ -100,7 +100,12 @@ pub(crate) fn trait_impls_in_crate_query(db: &dyn HirDatabase, krate: CrateId) -
         let mut impls = Self { map: FxHashMap::default() };
 
         let crate_def_map = db.crate_def_map(krate);
-        for (_module_id, module_data) in crate_def_map.modules() {
+        collect_def_map(db, &crate_def_map, &mut impls);
+
+        return Arc::new(impls);
+
+        fn collect_def_map(db: &dyn HirDatabase, def_map: &DefMap, impls: &mut TraitImpls) {
+            for (_module_id, module_data) in def_map.modules() {
                 for impl_id in module_data.scope.impls() {
                     let target_trait = match db.impl_trait(impl_id) {
                         Some(tr) => tr.skip_binders().hir_trait_id(),
@@ -116,9 +121,17 @@ pub(crate) fn trait_impls_in_crate_query(db: &dyn HirDatabase, krate: CrateId) -
                         .or_default()
                         .push(impl_id);
                 }
-        }
 
-        Arc::new(impls)
+                // To better support custom derives, collect impls in all unnamed const items.
+                // const _: () = { ... };
+                for konst in module_data.scope.unnamed_consts() {
+                    let body = db.body(konst.into());
+                    for (_, block_def_map) in body.blocks(db.upcast()) {
+                        collect_def_map(db, &block_def_map, impls);
+                    }
+                }
+            }
+        }
     }
 
     pub(crate) fn trait_impls_in_deps_query(db: &dyn HirDatabase, krate: CrateId) -> Arc<Self> {
@@ -208,6 +221,9 @@ pub(crate) fn inherent_impls_in_crate_query(db: &dyn HirDatabase, krate: CrateId
             }
         }
 
+        // NOTE: We're not collecting inherent impls from unnamed consts here, we intentionally only
+        // support trait impls there.
+
         Arc::new(Self { map })
     }
 

crates/hir_ty/src/tests/method_resolution.rs

@@ -1292,3 +1292,25 @@ fn foo() {
         "#]],
     )
 }
+
+#[test]
+fn impl_in_unnamed_const() {
+    check_types(
+        r#"
+struct S;
+
+trait Tr {
+    fn method(&self) -> u16;
+}
+
+const _: () = {
+    impl Tr for S {}
+};
+
+fn f() {
+    S.method();
+  //^^^^^^^^^^ u16
+}
+    "#,
+    );
+}