3 files changed, 9 insertions, 2 deletions

diff --git a/crates/ra_hir/Cargo.toml b/crates/ra_hir/Cargo.toml
index d9bed4dda..f955dc3e6 100644
--- a/crates/ra_hir/Cargo.toml
+++ b/crates/ra_hir/Cargo.toml
@@ -6,6 +6,7 @@ authors = ["rust-analyzer developers"]
 
 [dependencies]
 arrayvec = "0.4.10"
+im = "13.0.0"
 log = "0.4.5"
 relative-path = "0.4.0"
 rustc-hash = "1.0"
diff --git a/crates/ra_hir/src/nameres.rs b/crates/ra_hir/src/nameres.rs
index befbb2a9b..7e5138d05 100644
--- a/crates/ra_hir/src/nameres.rs
+++ b/crates/ra_hir/src/nameres.rs
@@ -54,6 +54,7 @@ mod mod_resolution;
 #[cfg(test)]
 mod tests;
 
+use std::hash::BuildHasherDefault;
 use std::sync::Arc;
 
 use once_cell::sync::Lazy;
@@ -61,7 +62,7 @@ use ra_arena::{impl_arena_id, Arena, RawId};
 use ra_db::{Edition, FileId};
 use ra_prof::profile;
 use ra_syntax::ast;
-use rustc_hash::{FxHashMap, FxHashSet};
+use rustc_hash::{FxHashMap, FxHashSet, FxHasher};
 use test_utils::tested_by;
 
 use crate::{
@@ -73,6 +74,8 @@ use crate::{
     AstId, BuiltinType, Crate, HirFileId, MacroDef, Module, ModuleDef, Name, Path, PathKind, Trait,
 };
 
+pub(crate) type ImmFxHashMap<K, V> = im::HashMap<K, V, BuildHasherDefault<FxHasher>>;
+
 pub(crate) use self::raw::{ImportSourceMap, RawItems};
 
 pub use self::{
@@ -139,7 +142,7 @@ pub(crate) struct ModuleData {
 pub struct ModuleScope {
     items: FxHashMap<Name, Resolution>,
     macros: FxHashMap<Name, MacroDef>,
-    textual_macros: FxHashMap<Name, MacroDef>,
+    textual_macros: ImmFxHashMap<Name, MacroDef>,
 }
 
 static BUILTIN_SCOPE: Lazy<FxHashMap<Name, Resolution>> = Lazy::new(|| {
diff --git a/crates/ra_hir/src/nameres/collector.rs b/crates/ra_hir/src/nameres/collector.rs
index 10c32ffa1..3803c7185 100644
--- a/crates/ra_hir/src/nameres/collector.rs
+++ b/crates/ra_hir/src/nameres/collector.rs
@@ -631,6 +631,7 @@ where
         modules[res].parent = Some(self.module_id);
         modules[res].declaration = Some(declaration);
         modules[res].definition = definition;
+        // Cloning immutable map is lazy and fast
         modules[res].scope.textual_macros = modules[self.module_id].scope.textual_macros.clone();
         modules[self.module_id].children.insert(name.clone(), res);
         let resolution = Resolution {
@@ -707,6 +708,8 @@ where
     }
 
     fn import_all_textual_macros(&mut self, module_id: CrateModuleId) {
+        // `clone()` is needed here to avoid mutable borrow `self.def_collector` when first borrow is alive
+        // Cloning immutable map is lazy and fast
         let macros = self.def_collector.def_map[module_id].scope.textual_macros.clone();
         for (name, macro_) in macros {
             self.def_collector.define_textual_macro(self.module_id, name.clone(), macro_.id);