Don't call instantiate_type_scheme during method probing

It can introduce obligations to the fulfillment context, which would
incorrectly still remain after the probe finished.

Fixes #25679.

Author: arielb1

src/librustc_typeck/check/method/probe.rs

@@ -398,11 +398,19 @@ impl<'a,'tcx> ProbeContext<'a,'tcx> {
         }
 
         let (impl_ty, impl_substs) = self.impl_ty_and_substs(impl_def_id);
-        let impl_ty = self.fcx.instantiate_type_scheme(self.span, &impl_substs, &impl_ty);
+
+        // We can't use instantiate_type_scheme here as it will pollute
+        // the fcx's fulfillment context if this probe is rolled back.
+        let cause = traits::ObligationCause::misc(self.span, self.fcx.body_id);
+        let mut selcx = &mut traits::SelectionContext::new(self.fcx.infcx(), self.fcx);
+        let traits::Normalized { value: impl_ty, .. } =
+            traits::normalize(selcx, cause, &impl_ty.subst(self.tcx(), &impl_substs));
 
         // Determine the receiver type that the method itself expects.
         let xform_self_ty =
             self.xform_self_ty(&item, impl_ty, &impl_substs);
+        debug!("assemble_inherent_impl_probe: self ty = {:?}",
+               xform_self_ty.repr(self.tcx()));
 
         self.inherent_candidates.push(Candidate {
             xform_self_ty: xform_self_ty,

src/test/run-pass/issue-25679.rs

@@ -0,0 +1,28 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+trait Device {
+    type Resources;
+}
+struct Foo<D, R>(D, R);
+
+impl<D: Device, S> Foo<D, S> {
+    fn present(&self) {}
+}
+
+struct Res;
+struct Dev;
+
+impl Device for Dev { type Resources = Res; }
+
+fn main() {
+    let foo = Foo(Dev, Res);
+    foo.present();
+}