Account for '_ in suggestions

Author: estebank

src/librustc_resolve/diagnostics.rs

@@ -1521,6 +1521,9 @@ crate fn add_missing_lifetime_specifiers_label(
                         if snippet.starts_with("&") && !snippet.starts_with("&'") {
                             introduce_suggestion
                                 .push((param.span, format!("&'a {}", &snippet[1..])));
+                        } else if snippet.starts_with("&'_ ") {
+                            introduce_suggestion
+                                .push((param.span, format!("&'a {}", &snippet[4..])));
                         }
                     }
                 }

src/test/ui/underscore-lifetime/underscore-lifetime-binders.stderr

@@ -37,8 +37,8 @@ LL | fn foo2(_: &'_ u8, y: &'_ u8) -> &'_ u8 { y }
    |            ^^^^^^     ^^^^^^
 help: consider introducing a named lifetime parameter
    |
-LL | fn foo2<'a>(_: &'_ u8, y: &'_ u8) -> &'a u8 { y }
-   |        ^^^^                           ^^
+LL | fn foo2<'a>(_: &'a u8, y: &'a u8) -> &'a u8 { y }
+   |        ^^^^    ^^^^^^     ^^^^^^      ^^
 
 error: aborting due to 5 previous errors