Move diagnostics code out of the critical path

src/librustc_typeck/check/method/mod.rs

@@ -213,27 +213,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  segment,
  );
 
- let mut needs_mut = false;
- if let ty::Ref(region, t_type, mutability) = self_ty.kind {
- let trait_type = self.tcx.mk_ref(region, ty::TypeAndMut {
- ty: t_type,
- mutbl: mutability.invert(),
- });
- match self.lookup_probe(
- span,
- segment.ident,
- trait_type,
- call_expr,
- ProbeScope::TraitsInScope
- ) {
- Ok(ref new_pick) if *new_pick != pick => {
- needs_mut = true;
+ if result.illegal_sized_bound {
+ let mut needs_mut = false;
+ if let ty::Ref(region, t_type, mutability) = self_ty.kind {
+ let trait_type = self.tcx.mk_ref(region, ty::TypeAndMut {
+ ty: t_type,
+ mutbl: mutability.invert(),
+ });
+ // We probe again to see if there might be a borrow mutability discrepancy.
+ match self.lookup_probe(
+ span,
+ segment.ident,
+ trait_type,
+ call_expr,
+ ProbeScope::TraitsInScope
+ ) {
+ Ok(ref new_pick) if *new_pick != pick => {
+ needs_mut = true;
+ }
+ _ => {}
  }
- _ => {}
  }
- }
 
- if result.illegal_sized_bound {
  // We probe again, taking all traits into account (not only those in scope).
  let candidates = match self.lookup_probe(
  span,