diff --git a/crates/ra_hir/src/ty.rs b/crates/ra_hir/src/ty.rs
index e6ecbe1ea7f..fae9c1e2285 100644
--- a/crates/ra_hir/src/ty.rs
+++ b/crates/ra_hir/src/ty.rs
@@ -223,7 +223,7 @@ impl Substs {
     }
 
     pub fn prefix(&self, n: usize) -> Substs {
-        Substs(self.0.iter().cloned().take(n).collect::<Vec<_>>().into())
+        Substs(self.0[..std::cmp::min(self.0.len(), n)].into())
     }
 
     pub fn walk(&self, f: &mut impl FnMut(&Ty)) {