diff --git a/crates/ra_hir/src/lib.rs b/crates/ra_hir/src/lib.rs
index 9cbd9a8aef..ca261e8f54 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 d161735e8f..cc9746f6d7 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_slice, Adt, Crate,
+    DefWithBody, GenericParams, HasGenericParams, Name, Trait, TypeAlias,
 };
 use display::{HirDisplay, HirFormatter};
 
@@ -308,12 +308,9 @@ 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 {
+        for t in make_mut_slice(&mut self.0) {
             t.walk_mut(f);
         }
-        self.0 = v.into();
     }
 
     pub fn as_single(&self) -> &Ty {
@@ -330,8 +327,7 @@ impl Substs {
                 .params_including_parent()
                 .into_iter()
                 .map(|p| Ty::Param { idx: p.idx, name: p.name.clone() })
-                .collect::<Vec<_>>()
-                .into(),
+                .collect(),
         )
     }
 
@@ -342,8 +338,7 @@ impl Substs {
                 .params_including_parent()
                 .into_iter()
                 .map(|p| Ty::Bound(p.idx))
-                .collect::<Vec<_>>()
-                .into(),
+                .collect(),
         )
     }
 
@@ -541,12 +536,9 @@ 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 {
+        for t in make_mut_slice(&mut self.params_and_return) {
             t.walk_mut(f);
         }
-        self.params_and_return = v.into();
     }
 }
 
@@ -756,11 +748,9 @@ 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 {
+                for p in make_mut_slice(predicates) {
                     p.walk_mut(f);
                 }
-                *predicates = v.into();
             }
             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 d161aa6b39..014c7981fe 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_slice;
 
 impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     pub(super) fn canonicalizer<'b>(&'b mut self) -> Canonicalizer<'a, 'b, D>
@@ -74,10 +75,11 @@ 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 {
+        for ty in make_mut_slice(&mut trait_ref.substs.0) {
+            *ty = self.do_canonicalize_ty(ty.clone());
+        }
+        trait_ref
     }
 
     fn into_canonicalized<T>(self, result: T) -> Canonicalized<T> {
@@ -87,10 +89,11 @@ 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 {
+        for ty in make_mut_slice(&mut projection_ty.parameters.0) {
+            *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 a604c02e2d..366556134b 100644
--- a/crates/ra_hir/src/ty/lower.rs
+++ b/crates/ra_hir/src/ty/lower.rs
@@ -22,6 +22,7 @@ use crate::{
     resolve::{Resolver, TypeNs},
     ty::Adt,
     type_ref::{TypeBound, TypeRef},
+    util::make_mut_slice,
     BuiltinType, Const, Enum, EnumVariant, Function, ModuleDef, Path, Static, Struct, StructField,
     Trait, TypeAlias, Union,
 };
@@ -31,11 +32,11 @@ impl Ty {
         match type_ref {
             TypeRef::Never => Ty::simple(TypeCtor::Never),
             TypeRef::Tuple(inner) => {
-                let inner_tys =
-                    inner.iter().map(|tr| Ty::from_hir(db, resolver, tr)).collect::<Vec<_>>();
+                let inner_tys: Arc<[Ty]> =
+                    inner.iter().map(|tr| Ty::from_hir(db, resolver, tr)).collect();
                 Ty::apply(
                     TypeCtor::Tuple { cardinality: inner_tys.len() as u16 },
-                    Substs(inner_tys.into()),
+                    Substs(inner_tys),
                 )
             }
             TypeRef::Path(path) => Ty::from_hir_path(db, resolver, path),
@@ -57,9 +58,7 @@ impl Ty {
             }
             TypeRef::Placeholder => Ty::Unknown,
             TypeRef::Fn(params) => {
-                let inner_tys =
-                    params.iter().map(|tr| Ty::from_hir(db, resolver, tr)).collect::<Vec<_>>();
-                let sig = Substs(inner_tys.into());
+                let sig = Substs(params.iter().map(|tr| Ty::from_hir(db, resolver, tr)).collect());
                 Ty::apply(TypeCtor::FnPtr { num_args: sig.len() as u16 - 1 }, sig)
             }
             TypeRef::DynTrait(bounds) => {
@@ -69,8 +68,8 @@ impl Ty {
                     .flat_map(|b| {
                         GenericPredicate::from_type_bound(db, resolver, b, self_ty.clone())
                     })
-                    .collect::<Vec<_>>();
-                Ty::Dyn(predicates.into())
+                    .collect();
+                Ty::Dyn(predicates)
             }
             TypeRef::ImplTrait(bounds) => {
                 let self_ty = Ty::Bound(0);
@@ -79,8 +78,8 @@ impl Ty {
                     .flat_map(|b| {
                         GenericPredicate::from_type_bound(db, resolver, b, self_ty.clone())
                     })
-                    .collect::<Vec<_>>();
-                Ty::Opaque(predicates.into())
+                    .collect();
+                Ty::Opaque(predicates)
             }
             TypeRef::Error => Ty::Unknown,
         }
@@ -392,10 +391,7 @@ 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();
+            make_mut_slice(&mut substs.0)[0] = self_ty;
         }
         TraitRef { trait_: resolved, substs }
     }
@@ -558,13 +554,12 @@ pub(crate) fn generic_predicates_for_param_query(
     param_idx: u32,
 ) -> Arc<[GenericPredicate]> {
     let resolver = def.resolver(db);
-    let predicates = resolver
+    resolver
         .where_predicates_in_scope()
         // we have to filter out all other predicates *first*, before attempting to lower them
         .filter(|pred| Ty::from_hir_only_param(db, &resolver, &pred.type_ref) == Some(param_idx))
         .flat_map(|pred| GenericPredicate::from_where_predicate(db, &resolver, pred))
-        .collect::<Vec<_>>();
-    predicates.into()
+        .collect()
 }
 
 pub(crate) fn trait_env(
@@ -585,11 +580,10 @@ pub(crate) fn generic_predicates_query(
     def: GenericDef,
 ) -> Arc<[GenericPredicate]> {
     let resolver = def.resolver(db);
-    let predicates = resolver
+    resolver
         .where_predicates_in_scope()
         .flat_map(|pred| GenericPredicate::from_where_predicate(db, &resolver, pred))
-        .collect::<Vec<_>>();
-    predicates.into()
+        .collect()
 }
 
 /// Resolve the default type params from generics
@@ -603,9 +597,9 @@ pub(crate) fn generic_defaults_query(db: &impl HirDatabase, def: GenericDef) ->
         .map(|p| {
             p.default.as_ref().map_or(Ty::Unknown, |path| Ty::from_hir_path(db, &resolver, path))
         })
-        .collect::<Vec<_>>();
+        .collect();
 
-    Substs(defaults.into())
+    Substs(defaults)
 }
 
 fn fn_sig_for_fn(db: &impl HirDatabase, def: Function) -> FnSig {
diff --git a/crates/ra_hir/src/ty/traits.rs b/crates/ra_hir/src/ty/traits.rs
index b0f67ae501..0cb5c37984 100644
--- a/crates/ra_hir/src/ty/traits.rs
+++ b/crates/ra_hir/src/ty/traits.rs
@@ -89,7 +89,7 @@ pub(crate) fn impls_for_trait_query(
     }
     let crate_impl_blocks = db.impls_in_crate(krate);
     impls.extend(crate_impl_blocks.lookup_impl_blocks_for_trait(trait_));
-    impls.into_iter().collect::<Vec<_>>().into()
+    impls.into_iter().collect()
 }
 
 /// A set of clauses that we assume to be true. E.g. if we are inside this function:
diff --git a/crates/ra_hir/src/ty/traits/chalk.rs b/crates/ra_hir/src/ty/traits/chalk.rs
index 9168de7092..00aaf65d92 100644
--- a/crates/ra_hir/src/ty/traits/chalk.rs
+++ b/crates/ra_hir/src/ty/traits/chalk.rs
@@ -126,8 +126,7 @@ impl ToChalk for Substs {
                 chalk_ir::Parameter(chalk_ir::ParameterKind::Ty(ty)) => from_chalk(db, ty),
                 chalk_ir::Parameter(chalk_ir::ParameterKind::Lifetime(_)) => unimplemented!(),
             })
-            .collect::<Vec<_>>()
-            .into();
+            .collect();
         Substs(tys)
     }
 }
diff --git a/crates/ra_hir/src/util.rs b/crates/ra_hir/src/util.rs
new file mode 100644
index 0000000000..0095ee45d3
--- /dev/null
+++ b/crates/ra_hir/src/util.rs
@@ -0,0 +1,12 @@
+//! 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_slice<T: Clone>(a: &mut Arc<[T]>) -> &mut [T] {
+    if Arc::get_mut(a).is_none() {
+        *a = a.iter().cloned().collect();
+    }
+    Arc::get_mut(a).unwrap()
+}