Rework how diagnostic lints are stored.

`Diagnostic::code` has the type `DiagnosticId`, which has `Error` and
`Lint` variants. Plus `Diagnostic::is_lint` is a bool, which should be
redundant w.r.t. `Diagnostic::code`.

Seems simple. Except it's possible for a lint to have an error code, in
which case its `code` field is recorded as `Error`, and `is_lint` is
required to indicate that it's a lint. This is what happens with
`derive(LintDiagnostic)` lints. Which means those lints don't have a
lint name or a `has_future_breakage` field because those are stored in
the `DiagnosticId::Lint`.

It's all a bit messy and confused and seems unintentional.

This commit:
- removes `DiagnosticId`;
- changes `Diagnostic::code` to `Option<String>`, which means both
  errors and lints can straightforwardly have an error code;
- changes `Diagnostic::is_lint` to `Option<IsLint>`, where `IsLint` is a
  new type containing a lint name and a `has_future_breakage` bool, so
  all lints can have those, error code or not.

compiler/rustc_hir_analysis/src/structured_errors/missing_cast_for_variadic_arg.rs

@@ -1,5 +1,5 @@
 use crate::{errors, structured_errors::StructuredDiagnostic};
-use rustc_errors::{DiagnosticBuilder, DiagnosticId};
+use rustc_errors::DiagnosticBuilder;
 use rustc_middle::ty::{Ty, TypeVisitableExt};
 use rustc_session::Session;
 use rustc_span::Span;
@@ -16,7 +16,7 @@ impl<'tcx> StructuredDiagnostic<'tcx> for MissingCastForVariadicArg<'tcx, '_> {
         self.sess
     }
 
-    fn code(&self) -> DiagnosticId {
+    fn code(&self) -> String {
         rustc_errors::error_code!(E0617)
     }
 

compiler/rustc_hir_analysis/src/structured_errors/sized_unsized_cast.rs

@@ -1,5 +1,5 @@
 use crate::{errors, structured_errors::StructuredDiagnostic};
-use rustc_errors::{DiagnosticBuilder, DiagnosticId};
+use rustc_errors::DiagnosticBuilder;
 use rustc_middle::ty::{Ty, TypeVisitableExt};
 use rustc_session::Session;
 use rustc_span::Span;
@@ -16,7 +16,7 @@ impl<'tcx> StructuredDiagnostic<'tcx> for SizedUnsizedCast<'tcx> {
         self.sess
     }
 
-    fn code(&self) -> DiagnosticId {
+    fn code(&self) -> String {
         rustc_errors::error_code!(E0607)
     }
 

compiler/rustc_hir_analysis/src/structured_errors/wrong_number_of_generic_args.rs

@@ -1,7 +1,5 @@
 use crate::structured_errors::StructuredDiagnostic;
-use rustc_errors::{
-    pluralize, Applicability, Diagnostic, DiagnosticBuilder, DiagnosticId, MultiSpan,
-};
+use rustc_errors::{pluralize, Applicability, Diagnostic, DiagnosticBuilder, MultiSpan};
 use rustc_hir as hir;
 use rustc_middle::ty::{self as ty, AssocItems, AssocKind, TyCtxt};
 use rustc_session::Session;
@@ -1107,7 +1105,7 @@ impl<'tcx> StructuredDiagnostic<'tcx> for WrongNumberOfGenericArgs<'_, 'tcx> {
         self.tcx.sess
     }
 
-    fn code(&self) -> DiagnosticId {
+    fn code(&self) -> String {
         rustc_errors::error_code!(E0107)
     }