---

yaml
---
r: 160131
b: refs/heads/issue-18208-method-dispatch-3-quick-reject
c: 2009f85
h: refs/heads/master
i:
  160129: 10f496f
  160127: 3989d9c
v: v3

Author: nikomatsakis

[refs]

@@ -28,4 +28,4 @@ refs/tags/0.11.0: e1247cb1d0d681be034adb4b558b5a0c0d5720f9
 refs/tags/0.12.0: f0c419429ef30723ceaf6b42f9b5a2aeb5d2e2d1
 refs/heads/issue-18208-method-dispatch-2: 9e1eae4fb9b6527315b4441cf8a0f5ca911d1671
 refs/heads/automation-fail: 1bf06495443584539b958873e04cc2f864ab10e4
-refs/heads/issue-18208-method-dispatch-3-quick-reject: 7c2c232c55f8ca63f5308ea88c279fc70f4b8773
+refs/heads/issue-18208-method-dispatch-3-quick-reject: 2009f85b9f99dedcec4404418eda9ddba90258a2

branches/issue-18208-method-dispatch-3-quick-reject/src/librustc/middle/traits/select.rs

@@ -21,6 +21,7 @@ use super::{VtableBuiltin, VtableImpl, VtableParam, VtableUnboxedClosure};
 use super::{VtableImplData, VtableParamData, VtableBuiltinData};
 use super::{util};
 
+use middle::fast_reject;
 use middle::mem_categorization::Typer;
 use middle::subst::{Subst, Substs, VecPerParamSpace};
 use middle::ty;
@@ -1762,12 +1763,20 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                   obligation: &Obligation)
                   -> Result<Substs, ()>
     {
+        let impl_trait_ref = ty::impl_trait_ref(self.tcx(),
+                                                impl_def_id).unwrap();
+
+        // Before we create the substitutions and everything, first
+        // consider a "quick reject". This avoids creating more types
+        // and so forth that we need to.
+        if self.fast_reject_trait_refs(obligation, &*impl_trait_ref) {
+            return Err(());
+        }
+
         let impl_substs = util::fresh_substs_for_impl(self.infcx,
                                                       obligation.cause.span,
                                                       impl_def_id);
 
-        let impl_trait_ref = ty::impl_trait_ref(self.tcx(),
-                                                impl_def_id).unwrap();
         let impl_trait_ref = impl_trait_ref.subst(self.tcx(),
                                                   &impl_substs);
 
@@ -1777,6 +1786,29 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
+    fn fast_reject_trait_refs(&mut self,
+                              obligation: &Obligation,
+                              impl_trait_ref: &ty::TraitRef)
+                              -> bool
+    {
+        // We can avoid creating type variables and doing the full
+        // substitution if we find that any of the input types, when
+        // simplified, do not match.
+
+        obligation.trait_ref.input_types().iter()
+            .zip(impl_trait_ref.input_types().iter())
+            .any(|(&obligation_ty, &impl_ty)| {
+                let simplified_obligation_ty =
+                    fast_reject::simplify_type(self.tcx(), obligation_ty, true);
+                let simplified_impl_ty =
+                    fast_reject::simplify_type(self.tcx(), impl_ty, false);
+
+                simplified_obligation_ty.is_some() &&
+                    simplified_impl_ty.is_some() &&
+                    simplified_obligation_ty != simplified_impl_ty
+            })
+    }
+
     fn match_trait_refs(&mut self,
                         obligation: &Obligation,
                         trait_ref: Rc<ty::TraitRef>)