5 files changed, 55 insertions, 31 deletions

diff --git a/crates/ra_hir/src/lib.rs b/crates/ra_hir/src/lib.rs
index 9cbd9a8ae..ca261e8f5 100644
--- a/crates/ra_hir/src/lib.rs
+++ b/crates/ra_hir/src/lib.rs
@@ -51,6 +51,7 @@ mod lang_item;
 mod generics;
 mod resolve;
 pub mod diagnostics;
+mod util;
 
 mod code_model;
 
diff --git a/crates/ra_hir/src/ty.rs b/crates/ra_hir/src/ty.rs
index d161735e8..fc4909d11 100644
--- a/crates/ra_hir/src/ty.rs
+++ b/crates/ra_hir/src/ty.rs
@@ -17,8 +17,8 @@ use std::sync::Arc;
 use std::{fmt, iter, mem};
 
 use crate::{
-    db::HirDatabase, expr::ExprId, type_ref::Mutability, Adt, Crate, DefWithBody, GenericParams,
-    HasGenericParams, Name, Trait, TypeAlias,
+    db::HirDatabase, expr::ExprId, type_ref::Mutability, util::make_mut_arc_slice, Adt, Crate,
+    DefWithBody, GenericParams, HasGenericParams, Name, Trait, TypeAlias,
 };
 use display::{HirDisplay, HirFormatter};
 
@@ -308,12 +308,11 @@ impl Substs {
     }
 
     pub fn walk_mut(&mut self, f: &mut impl FnMut(&mut Ty)) {
-        // Without an Arc::make_mut_slice, we can't avoid the clone here:
-        let mut v: Vec<_> = self.0.iter().cloned().collect();
-        for t in &mut v {
-            t.walk_mut(f);
-        }
-        self.0 = v.into();
+        make_mut_arc_slice(&mut self.0, |s| {
+            for t in s {
+                t.walk_mut(f);
+            }
+        });
     }
 
     pub fn as_single(&self) -> &Ty {
@@ -541,12 +540,11 @@ impl TypeWalk for FnSig {
     }
 
     fn walk_mut(&mut self, f: &mut impl FnMut(&mut Ty)) {
-        // Without an Arc::make_mut_slice, we can't avoid the clone here:
-        let mut v: Vec<_> = self.params_and_return.iter().cloned().collect();
-        for t in &mut v {
-            t.walk_mut(f);
-        }
-        self.params_and_return = v.into();
+        make_mut_arc_slice(&mut self.params_and_return, |s| {
+            for t in s {
+                t.walk_mut(f);
+            }
+        });
     }
 }
 
@@ -756,11 +754,11 @@ impl TypeWalk for Ty {
                 p_ty.parameters.walk_mut(f);
             }
             Ty::Dyn(predicates) | Ty::Opaque(predicates) => {
-                let mut v: Vec<_> = predicates.iter().cloned().collect();
-                for p in &mut v {
-                    p.walk_mut(f);
-                }
-                *predicates = v.into();
+                make_mut_arc_slice(predicates, |s| {
+                    for p in s {
+                        p.walk_mut(f);
+                    }
+                });
             }
             Ty::Param { .. } | Ty::Bound(_) | Ty::Infer(_) | Ty::Unknown => {}
         }
diff --git a/crates/ra_hir/src/ty/infer/unify.rs b/crates/ra_hir/src/ty/infer/unify.rs
index d161aa6b3..5e86ed260 100644
--- a/crates/ra_hir/src/ty/infer/unify.rs
+++ b/crates/ra_hir/src/ty/infer/unify.rs
@@ -6,6 +6,7 @@ use crate::ty::{
     Canonical, InEnvironment, InferTy, ProjectionPredicate, ProjectionTy, Substs, TraitRef, Ty,
     TypeWalk,
 };
+use crate::util::make_mut_arc_slice;
 
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     pub(super) fn canonicalizer<'b>(&'b mut self) -> Canonicalizer<'a, 'b, D>
@@ -74,10 +75,13 @@ where
         })
     }
 
-    fn do_canonicalize_trait_ref(&mut self, trait_ref: TraitRef) -> TraitRef {
-        let substs =
-            trait_ref.substs.iter().map(|ty| self.do_canonicalize_ty(ty.clone())).collect();
-        TraitRef { trait_: trait_ref.trait_, substs: Substs(substs) }
+    fn do_canonicalize_trait_ref(&mut self, mut trait_ref: TraitRef) -> TraitRef {
+        make_mut_arc_slice(&mut trait_ref.substs.0, |tys| {
+            for ty in tys {
+                *ty = self.do_canonicalize_ty(ty.clone());
+            }
+        });
+        trait_ref
     }
 
     fn into_canonicalized<T>(self, result: T) -> Canonicalized<T> {
@@ -87,10 +91,13 @@ where
         }
     }
 
-    fn do_canonicalize_projection_ty(&mut self, projection_ty: ProjectionTy) -> ProjectionTy {
-        let params =
-            projection_ty.parameters.iter().map(|ty| self.do_canonicalize_ty(ty.clone())).collect();
-        ProjectionTy { associated_ty: projection_ty.associated_ty, parameters: Substs(params) }
+    fn do_canonicalize_projection_ty(&mut self, mut projection_ty: ProjectionTy) -> ProjectionTy {
+        make_mut_arc_slice(&mut projection_ty.parameters.0, |params| {
+            for ty in params {
+                *ty = self.do_canonicalize_ty(ty.clone());
+            }
+        });
+        projection_ty
     }
 
     fn do_canonicalize_projection_predicate(
diff --git a/crates/ra_hir/src/ty/lower.rs b/crates/ra_hir/src/ty/lower.rs
index a604c02e2..003aa9bab 100644
--- a/crates/ra_hir/src/ty/lower.rs
+++ b/crates/ra_hir/src/ty/lower.rs
@@ -392,10 +392,9 @@ impl TraitRef {
     ) -> Self {
         let mut substs = TraitRef::substs_from_path(db, resolver, segment, resolved);
         if let Some(self_ty) = explicit_self_ty {
-            // FIXME this could be nicer
-            let mut substs_vec = substs.0.to_vec();
-            substs_vec[0] = self_ty;
-            substs.0 = substs_vec.into();
+            crate::util::make_mut_arc_slice(&mut substs.0, |substs| {
+                substs[0] = self_ty;
+            });
         }
         TraitRef { trait_: resolved, substs }
     }
diff --git a/crates/ra_hir/src/util.rs b/crates/ra_hir/src/util.rs
new file mode 100644
index 000000000..46f423c91
--- /dev/null
+++ b/crates/ra_hir/src/util.rs
@@ -0,0 +1,19 @@
+//! Internal utility functions.
+
+use std::sync::Arc;
+
+/// Helper for mutating `Arc<[T]>` (i.e. `Arc::make_mut` for Arc slices).
+/// The underlying values are cloned if there are other strong references.
+pub(crate) fn make_mut_arc_slice<T: Clone, R>(
+    a: &mut Arc<[T]>,
+    f: impl FnOnce(&mut [T]) -> R,
+) -> R {
+    if let Some(s) = Arc::get_mut(a) {
+        f(s)
+    } else {
+        let mut v = a.to_vec();
+        let r = f(&mut v);
+        *a = Arc::from(v);
+        r
+    }
+}