Output a note when lifetimes cannot be elided from functions

src/test/compile-fail/lifetime-elision-return-type-requires-explicit-lifetime.rs

@@ -10,11 +10,13 @@
 
 // Lifetime annotation needed because we have no arguments.
 fn f() -> &int {    //~ ERROR missing lifetime specifier
+//~^ NOTE there is no value for it to be borrowed from
     fail!()
 }
 
 // Lifetime annotation needed because we have two by-reference parameters.
-fn g(_: &int, _: &int) -> &int {    //~ ERROR missing lifetime specifier
+fn g(_x: &int, _y: &int) -> &int {    //~ ERROR missing lifetime specifier
+//~^ NOTE the signature does not say whether it is borrowed from `_x` or `_y`
     fail!()
 }
 
@@ -24,7 +26,8 @@ struct Foo<'a> {
 
 // Lifetime annotation needed because we have two lifetimes: one as a parameter
 // and one on the reference.
-fn h(_: &Foo) -> &int { //~ ERROR missing lifetime specifier
+fn h(_x: &Foo) -> &int { //~ ERROR missing lifetime specifier
+//~^ NOTE the signature does not say which one of `_x`'s 2 elided lifetimes it is borrowed from
     fail!()
 }