Combine try and commit_if_ok and make some details of inference

context private.

src/test/compile-fail/object-lifetime-default-elision.rs

@@ -81,8 +81,8 @@ fn load3<'a,'b>(ss: &'a SomeTrait) -> &'b SomeTrait {
     // which fails to type check.
 
     ss
-        //~^ ERROR cannot infer
-        //~| ERROR mismatched types
+        //~^ ERROR lifetime of the source pointer does not outlive lifetime bound
+        //~| ERROR cannot infer
 }
 
 fn main() {

src/test/compile-fail/object-lifetime-default-from-box-error.rs

@@ -25,7 +25,7 @@ fn load(ss: &mut SomeStruct) -> Box<SomeTrait> {
     // `Box<SomeTrait>` defaults to a `'static` bound, so this return
     // is illegal.
 
-    ss.r //~ ERROR mismatched types
+    ss.r //~ ERROR lifetime of the source pointer does not outlive lifetime bound
 }
 
 fn store(ss: &mut SomeStruct, b: Box<SomeTrait>) {
@@ -38,7 +38,7 @@ fn store(ss: &mut SomeStruct, b: Box<SomeTrait>) {
 fn store1<'b>(ss: &mut SomeStruct, b: Box<SomeTrait+'b>) {
     // Here we override the lifetimes explicitly, and so naturally we get an error.
 
-    ss.r = b; //~ ERROR mismatched types
+    ss.r = b; //~ ERROR lifetime of the source pointer does not outlive lifetime bound
 }
 
 fn main() {

src/test/compile-fail/regions-close-over-type-parameter-multiple.rs

@@ -27,7 +27,7 @@ fn make_object_good2<'a,'b,A:SomeTrait+'a+'b>(v: A) -> Box<SomeTrait+'b> {
 
 fn make_object_bad<'a,'b,'c,A:SomeTrait+'a+'b>(v: A) -> Box<SomeTrait+'c> {
     // A outlives 'a AND 'b...but not 'c.
-    box v as Box<SomeTrait+'a> //~ ERROR mismatched types
+    box v as Box<SomeTrait+'a> //~ ERROR lifetime of the source pointer does not outlive
 }
 
 fn main() {

src/test/compile-fail/regions-trait-object-subtyping.rs

@@ -22,7 +22,7 @@ fn foo2<'a:'b,'b>(x: &'b mut (Dummy+'a)) -> &'b mut (Dummy+'b) {
 
 fn foo3<'a,'b>(x: &'a mut Dummy) -> &'b mut Dummy {
     // Without knowing 'a:'b, we can't coerce
-    x //~ ERROR mismatched types
+    x //~ ERROR lifetime of the source pointer does not outlive
      //~^ ERROR cannot infer
 }