Added ability to report on generic argument mismatch better

Needs some checking over and some tests have altered that need sanity checking, but overall this is starting to get somewhere now.

compiler/rustc_errors/src/lib.rs

@@ -50,7 +50,7 @@ pub use rustc_error_messages::{
     fallback_fluent_bundle, fluent_bundle, DelayDm, DiagMessage, FluentBundle, LanguageIdentifier,
     LazyFallbackBundle, MultiSpan, SpanLabel, SubdiagMessage,
 };
-pub use rustc_lint_defs::{pluralize, Applicability};
+pub use rustc_lint_defs::{a_or_an, display_list_with_comma_and, pluralize, Applicability};
 pub use rustc_span::fatal_error::{FatalError, FatalErrorMarker};
 pub use rustc_span::ErrorGuaranteed;
 pub use snippet::Style;