Account for cases where we can find the type arg name, but the local name is _

Author: estebank

src/librustc/infer/error_reporting/need_type_info.rs

@@ -152,6 +152,11 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         //   |         the type parameter `E` is specified
         // ```
         let (ty_msg, suffix) = match &local_visitor.found_ty {
+            Some(ty) if &ty.to_string() != "_" && name == "_" => {
+                let ty = ty_to_string(ty);
+                (format!(" for `{}`", ty),
+                 format!("the explicit type `{}`, with the type parameters specified", ty))
+            }
             Some(ty) if &ty.to_string() != "_" && ty.to_string() != name => {
                 let ty = ty_to_string(ty);
                 (format!(" for `{}`", ty),

src/test/ui/issues/issue-12187-1.stderr

@@ -5,7 +5,7 @@ LL |     let &v = new();
    |         -^
    |         ||
    |         |cannot infer type
-   |         consider giving this pattern the explicit type `&T`, where the type parameter `_` is specified
+   |         consider giving this pattern the explicit type `&T`, with the type parameters specified
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-12187-2.stderr

@@ -5,7 +5,7 @@ LL |     let &v = new();
    |         -^
    |         ||
    |         |cannot infer type
-   |         consider giving this pattern the explicit type `&T`, where the type parameter `_` is specified
+   |         consider giving this pattern the explicit type `&T`, with the type parameters specified
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-7813.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed for `&[_; 0]`
 LL |     let v = &[];
    |         -   ^^^ cannot infer type
    |         |
-   |         consider giving `v` the explicit type `&[_; 0]`, where the type parameter `_` is specified
+   |         consider giving `v` the explicit type `&[_; 0]`, with the type parameters specified
 
 error: aborting due to previous error
 

src/test/ui/type/type-check/cannot_infer_local_or_array.stderr

@@ -4,7 +4,7 @@ error[E0282]: type annotations needed for `[_; 0]`
 LL |     let x = [];
    |         -   ^^ cannot infer type
    |         |
-   |         consider giving `x` the explicit type `[_; 0]`, where the type parameter `_` is specified
+   |         consider giving `x` the explicit type `[_; 0]`, with the type parameters specified
 
 error: aborting due to previous error
 

src/test/ui/unboxed-closures/unboxed-closures-failed-recursive-fn-2.stderr

@@ -2,7 +2,7 @@ error[E0282]: type annotations needed for `std::option::Option<T>`
   --> $DIR/unboxed-closures-failed-recursive-fn-2.rs:16:32
    |
 LL |     let mut closure0 = None;
-   |         ------------ consider giving `closure0` the explicit type `std::option::Option<T>`, where the type parameter `_` is specified
+   |         ------------ consider giving `closure0` the explicit type `std::option::Option<T>`, with the type parameters specified
 ...
 LL |                         return c();
    |                                ^^^ cannot infer type