make trait matching smarter with projections

also, use the right caching logic for type_moves_by_default (this was
broken by @jroesch).

before:
593.10user 5.21system 7:51.41elapsed 126%CPU (0avgtext+0avgdata 1150016maxresident)k
llvm: 427.045

after:
577.76user 4.27system 7:36.13elapsed 127%CPU (0avgtext+0avgdata 1141608maxresident)k
llvm: 431.001

Author: Ariel Ben-Yehuda

src/librustc/middle/fast_reject.rs

@@ -85,11 +85,13 @@ pub fn simplify_type(tcx: &ty::ctxt,
         ty::TyBareFn(_, ref f) => {
             Some(FunctionSimplifiedType(f.sig.0.inputs.len()))
         }
-        ty::TyProjection(_) => {
-            None
-        }
-        ty::TyParam(_) => {
+        ty::TyProjection(_) | ty::TyParam(_) => {
             if can_simplify_params {
+                // In normalized types, projections don't unify with
+                // anything. when lazy normalization happens, this
+                // will change. It would still be nice to have a way
+                // to deal with known-not-to-unify-with-anything
+                // projections (e.g. the likes of <__S as Encoder>::Error).
                 Some(ParameterSimplifiedType)
             } else {
                 None

src/librustc/middle/infer/mod.rs

@@ -1456,9 +1456,15 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
     pub fn type_moves_by_default(&self, ty: Ty<'tcx>, span: Span) -> bool {
         let ty = self.resolve_type_vars_if_possible(&ty);
-        !traits::type_known_to_meet_builtin_bound(self, ty, ty::BoundCopy, span)
-        // FIXME(@jroesch): should be able to use:
-        // ty.moves_by_default(&self.parameter_environment, span)
+        if ty.needs_infer() {
+            // this can get called from typeck (by euv), and moves_by_default
+            // rightly refuses to work with inference variables, but
+            // moves_by_default has a cache, which we want to use in other
+            // cases.
+            !traits::type_known_to_meet_builtin_bound(self, ty, ty::BoundCopy, span)
+        } else {
+            ty.moves_by_default(&self.parameter_environment, span)
+        }
     }
 
     pub fn node_method_ty(&self, method_call: ty::MethodCall)

src/librustc/middle/ty.rs

@@ -3192,6 +3192,8 @@ impl<'tcx> TraitDef<'tcx> {
         }
     }
 
+    /// Iterate over every impl that could possibly match the
+    /// self-type `self_ty`.
     pub fn for_each_relevant_impl<F: FnMut(DefId)>(&self,
                                                    tcx: &ctxt<'tcx>,
                                                    self_ty: Ty<'tcx>,
@@ -3203,7 +3205,19 @@ impl<'tcx> TraitDef<'tcx> {
             f(impl_def_id);
         }
 
-        if let Some(simp) = fast_reject::simplify_type(tcx, self_ty, false) {
+        // simplify_type(.., false) basically replaces type parameters and
+        // projections with infer-variables. This is, of course, done on
+        // the impl trait-ref when it is instantiated, but not on the
+        // predicate trait-ref which is passed here.
+        //
+        // for example, if we match `S: Copy` against an impl like
+        // `impl<T:Copy> Copy for Option<T>`, we replace the type variable
+        // in `Option<T>` with an infer variable, to `Option<_>` (this
+        // doesn't actually change fast_reject output), but we don't
+        // replace `S` with anything - this impl of course can't be
+        // selected, and as there are hundreds of similar impls,
+        // considering them would significantly harm performance.
+        if let Some(simp) = fast_reject::simplify_type(tcx, self_ty, true) {
             if let Some(impls) = self.nonblanket_impls.borrow().get(&simp) {
                 for &impl_def_id in impls {
                     f(impl_def_id);