Auto merge of #119922 - nnethercote:fix-Diag-code-is_lint, r=oli-obk

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.

r? `@oli-obk`

compiler/rustc_session/src/parse.rs

@@ -15,8 +15,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::{AppendOnlyVec, Lock, Lrc};
 use rustc_errors::{emitter::SilentEmitter, DiagCtxt};
 use rustc_errors::{
-    fallback_fluent_bundle, Diagnostic, DiagnosticBuilder, DiagnosticId, DiagnosticMessage,
-    MultiSpan, StashKey,
+    fallback_fluent_bundle, Diagnostic, DiagnosticBuilder, DiagnosticMessage, MultiSpan, StashKey,
 };
 use rustc_feature::{find_feature_issue, GateIssue, UnstableFeatures};
 use rustc_span::edition::Edition;
@@ -148,7 +147,7 @@ pub fn feature_warn_issue(
     // Decorate this as a future-incompatibility lint as in rustc_middle::lint::struct_lint_level
     let lint = UNSTABLE_SYNTAX_PRE_EXPANSION;
     let future_incompatible = lint.future_incompatible.as_ref().unwrap();
-    err.code(DiagnosticId::Lint { name: lint.name_lower(), has_future_breakage: false });
+    err.is_lint(lint.name_lower(), /* has_future_breakage */ false);
     err.warn(lint.desc);
     err.note(format!("for more information, see {}", future_incompatible.reference));
 

compiler/rustc_session/src/session.rs

@@ -20,9 +20,8 @@ use rustc_errors::emitter::{DynEmitter, HumanEmitter, HumanReadableErrorType};
 use rustc_errors::json::JsonEmitter;
 use rustc_errors::registry::Registry;
 use rustc_errors::{
-    error_code, fallback_fluent_bundle, DiagCtxt, DiagnosticBuilder, DiagnosticId,
-    DiagnosticMessage, ErrorGuaranteed, FatalAbort, FluentBundle, IntoDiagnostic,
-    LazyFallbackBundle, TerminalUrl,
+    error_code, fallback_fluent_bundle, DiagCtxt, DiagnosticBuilder, DiagnosticMessage,
+    ErrorGuaranteed, FatalAbort, FluentBundle, IntoDiagnostic, LazyFallbackBundle, TerminalUrl,
 };
 use rustc_macros::HashStable_Generic;
 pub use rustc_span::def_id::StableCrateId;
@@ -906,7 +905,7 @@ impl Session {
         CodegenUnits::Default(16)
     }
 
-    pub fn teach(&self, code: &DiagnosticId) -> bool {
+    pub fn teach(&self, code: &str) -> bool {
         self.opts.unstable_opts.teach && self.dcx().must_teach(code)
     }