diff options
| -rw-r--r-- | src/librustc/ty/subst.rs | 11 |
1 files changed, 9 insertions, 2 deletions
diff --git a/src/librustc/ty/subst.rs b/src/librustc/ty/subst.rs index 7d7bbd93122..6beb2b968e2 100644 --- a/src/librustc/ty/subst.rs +++ b/src/librustc/ty/subst.rs @@ -304,8 +304,15 @@ impl<'a, 'gcx, 'tcx> Substs<'tcx> { impl<'tcx> TypeFoldable<'tcx> for &'tcx Substs<'tcx> { fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self { - let params = self.iter().map(|k| k.fold_with(folder)).collect(); - folder.tcx().mk_substs(params) + let params: Vec<_> = self.iter().map(|k| k.fold_with(folder)).collect(); + + // If folding doesn't change the substs, it's faster to avoid calling + // `mk_substs` and instead reuse the existing substs. + if params[..] == self[..] { + self + } else { + folder.tcx().mk_substs(params) + } } fn fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self { |