Tweak region-related error messages that changed slightly due to coercion

changes, and also stop printing semi-useless inference by-products.

Author: nikomatsakis

src/librustc/middle/ty.rs

@@ -4577,6 +4577,10 @@ pub fn note_and_explain_type_err(cx: &ctxt, err: &type_err) {
                                     "concrete lifetime that was found is ",
                                     conc_region, "");
         }
+        terr_regions_overly_polymorphic(_, ty::ReInfer(ty::ReVar(_))) => {
+            // don't bother to print out the message below for
+            // inference variables, it's not very illuminating.
+        }
         terr_regions_overly_polymorphic(_, conc_region) => {
             note_and_explain_region(cx,
                                     "expected concrete lifetime is ",

src/test/compile-fail/region-lifetime-bounds-on-fns-where-clause.rs

@@ -15,7 +15,7 @@ fn a<'a, 'b>(x: &mut &'a int, y: &mut &'b int) where 'b: 'a {
 
 fn b<'a, 'b>(x: &mut &'a int, y: &mut &'b int) {
     // Illegal now because there is no `'b:'a` declaration.
-    *x = *y; //~ ERROR mismatched types
+    *x = *y; //~ ERROR cannot infer
 }
 
 fn c<'a,'b>(x: &mut &'a int, y: &mut &'b int) {

src/test/compile-fail/region-multiple-lifetime-bounds-on-fns-where-clause.rs

@@ -16,8 +16,8 @@ fn a<'a, 'b, 'c>(x: &mut &'a int, y: &mut &'b int, z: &mut &'c int) where 'b: 'a
 
 fn b<'a, 'b, 'c>(x: &mut &'a int, y: &mut &'b int, z: &mut &'c int) {
     // Illegal now because there is no `'b:'a` declaration.
-    *x = *y; //~ ERROR mismatched types
-    *z = *y; //~ ERROR mismatched types
+    *x = *y; //~ ERROR cannot infer
+    *z = *y; //~ ERROR cannot infer
 }
 
 fn c<'a,'b, 'c>(x: &mut &'a int, y: &mut &'b int, z: &mut &'c int) {