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.

1 files changed, 7 insertions, 4 deletions

diff --git a/compiler/rustc_errors/src/diagnostic_builder.rs b/compiler/rustc_errors/src/diagnostic_builder.rs
index 93b2dd61b05..87e2d295c7f 100644
--- a/compiler/rustc_errors/src/diagnostic_builder.rs
+++ b/compiler/rustc_errors/src/diagnostic_builder.rs
@@ -1,8 +1,8 @@
 use crate::diagnostic::IntoDiagnosticArg;
 use crate::{DiagCtxt, Level, MultiSpan, StashKey};
 use crate::{
-    Diagnostic, DiagnosticId, DiagnosticMessage, DiagnosticStyledString, ErrorGuaranteed,
-    ExplicitBug, SubdiagnosticMessage,
+    Diagnostic, DiagnosticMessage, DiagnosticStyledString, ErrorGuaranteed, ExplicitBug,
+    SubdiagnosticMessage,
 };
 use rustc_lint_defs::Applicability;
 use rustc_span::source_map::Spanned;
@@ -395,8 +395,11 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
     forward!((span, with_span)(
         sp: impl Into<MultiSpan>,
     ));
+    forward!((is_lint, with_is_lint)(
+        name: String, has_future_breakage: bool,
+    ));
     forward!((code, with_code)(
-        s: DiagnosticId,
+        s: String,
     ));
     forward!((arg, with_arg)(
         name: impl Into<Cow<'static, str>>, arg: impl IntoDiagnosticArg,
@@ -443,5 +446,5 @@ macro_rules! struct_span_code_err {
 
 #[macro_export]
 macro_rules! error_code {
-    ($code:ident) => {{ $crate::DiagnosticId::Error(stringify!($code).to_owned()) }};
+    ($code:ident) => {{ stringify!($code).to_owned() }};
 }