fix: resolve extern prelude for local mods in block modules

crates/hir-def/src/body/tests/block.rs

@@ -528,3 +528,65 @@ fn f() {$0
         "#]],
     )
 }
+
+#[test]
+fn resolve_extern_prelude_in_block() {
+    check_at(
+        r#"
+//- /main.rs crate:main deps:core
+fn main() {
+    mod f {
+        use core::S;
+        $0
+    }
+}
+
+//- /core.rs crate:core
+pub struct S;
+        "#,
+        expect![[r#"
+            block scope
+            f: t
+
+            block scope::f
+            S: ti vi
+
+            crate
+            main: v
+        "#]],
+    )
+}
+
+#[test]
+fn shadow_extern_prelude_in_block() {
+    check_at(
+        r#"
+//- /main.rs crate:main deps:core
+fn main() {
+    mod core { pub struct S; }
+    {
+        fn inner() {} // forces a block def map
+        use core::S; // should resolve to the local one
+        $0
+    }
+}
+
+//- /core.rs crate:core
+pub const S;
+        "#,
+        expect![[r#"
+            block scope
+            S: ti vi
+            inner: v
+
+            block scope
+            core: t
+
+            block scope::core
+            S: t v
+
+            crate
+            main: v
+        "#]],
+    )
+}

crates/hir-def/src/nameres/collector.rs

@@ -395,6 +395,8 @@ impl DefCollector<'_> {
             .cfg()
             .map_or(true, |cfg| self.cfg_options.check(&cfg) != Some(false));
         if is_cfg_enabled {
+            self.inject_prelude();
+
             ModCollector {
                 def_collector: self,
                 macro_depth: 0,

crates/hir-def/src/nameres/path_resolution.rs

@@ -221,7 +221,7 @@ impl DefMap {
                     None => return ResolvePathResult::empty(ReachedFixedPoint::Yes),
                 };
                 tracing::debug!("resolving {:?} in crate root (+ extern prelude)", segment);
-                self.resolve_name_in_crate_root_or_extern_prelude(db, segment)
+                self.resolve_name_in_crate_root_or_extern_prelude(db, original_module, segment)
             }
             PathKind::Plain => {
                 let (_, segment) = match segments.next() {
@@ -470,9 +470,9 @@ impl DefMap {
         };
 
         let extern_prelude = || {
-            if self.block.is_some() {
-                // Don't resolve extern prelude in block `DefMap`s, defer it to the crate def map so
-                // that blocks can properly shadow them
+            if self.block.is_some() && module == DefMap::ROOT {
+                // Don't resolve extern prelude in pseudo-modules of blocks, because
+                // they might been shadowed by local names.
                 return PerNs::none();
             }
             self.data.extern_prelude.get(name).map_or(PerNs::none(), |&(it, extern_crate)| {
@@ -505,6 +505,7 @@ impl DefMap {
     fn resolve_name_in_crate_root_or_extern_prelude(
         &self,
         db: &dyn DefDatabase,
+        module: LocalModuleId,
         name: &Name,
     ) -> PerNs {
         let from_crate_root = match self.block {
@@ -515,8 +516,8 @@ impl DefMap {
             None => self[Self::ROOT].scope.get(name),
         };
         let from_extern_prelude = || {
-            if self.block.is_some() {
-                // Don't resolve extern prelude in block `DefMap`s.
+            if self.block.is_some() && module == DefMap::ROOT {
+                // Don't resolve extern prelude in pseudo-module of a block.
                 return PerNs::none();
             }
             self.data.extern_prelude.get(name).copied().map_or(