Rollup merge of #98884 - davidtwco:translation-on-lints-derive, r=oli-obk

macros: `LintDiagnostic` derive

- Move `LintDiagnosticBuilder` into `rustc_errors` so that a diagnostic derive can refer to it.
- Introduce a `DecorateLint` trait, which is equivalent to `SessionDiagnostic` or `AddToDiagnostic` but for lints. Necessary without making more changes to the lint infrastructure as `DecorateLint` takes a `LintDiagnosticBuilder` and re-uses all of the existing logic for determining what type of diagnostic a lint should be emitted as (e.g. error/warning).
- Various refactorings of the diagnostic derive machinery (extracting `build_field_mapping` helper and moving `sess` field out of the `DiagnosticDeriveBuilder`).
- Introduce a `LintDiagnostic` derive macro that works almost exactly like the `SessionDiagnostic` derive macro  except that it derives a `DecorateLint` implementation instead. A new derive is necessary for this because `SessionDiagnostic` is intended for when the generated code creates the diagnostic. `AddToDiagnostic` could have been used but it would have required more changes to the lint machinery.

~~At time of opening this pull request, ignore all of the commits from #98624, it's just the last few commits that are new.~~

r? `@oli-obk`

3 files changed, 35 insertions, 4 deletions

diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index c15dc024736..0d1d017d874 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -1,7 +1,7 @@
 use crate::snippet::Style;
 use crate::{
-    CodeSuggestion, DiagnosticMessage, Level, MultiSpan, SubdiagnosticMessage, Substitution,
-    SubstitutionPart, SuggestionStyle,
+    CodeSuggestion, DiagnosticMessage, EmissionGuarantee, Level, LintDiagnosticBuilder, MultiSpan,
+    SubdiagnosticMessage, Substitution, SubstitutionPart, SuggestionStyle,
 };
 use rustc_data_structures::stable_map::FxHashMap;
 use rustc_error_messages::FluentValue;
@@ -168,6 +168,14 @@ pub trait AddSubdiagnostic {
     fn add_to_diagnostic(self, diag: &mut Diagnostic);
 }
 
+/// Trait implemented by lint types. This should not be implemented manually. Instead, use
+/// `#[derive(LintDiagnostic)]` -- see [rustc_macros::LintDiagnostic].
+#[rustc_diagnostic_item = "DecorateLint"]
+pub trait DecorateLint<'a, G: EmissionGuarantee> {
+    /// Decorate and emit a lint.
+    fn decorate_lint(self, diag: LintDiagnosticBuilder<'a, G>);
+}
+
 #[must_use]
 #[derive(Clone, Debug, Encodable, Decodable)]
 pub struct Diagnostic {
diff --git a/compiler/rustc_errors/src/diagnostic_builder.rs b/compiler/rustc_errors/src/diagnostic_builder.rs
index 1ad33ef25b7..c3341fd68f4 100644
--- a/compiler/rustc_errors/src/diagnostic_builder.rs
+++ b/compiler/rustc_errors/src/diagnostic_builder.rs
@@ -589,3 +589,26 @@ macro_rules! struct_span_err {
 macro_rules! error_code {
     ($code:ident) => {{ $crate::DiagnosticId::Error(stringify!($code).to_owned()) }};
 }
+
+/// Wrapper around a `DiagnosticBuilder` for creating lints.
+pub struct LintDiagnosticBuilder<'a, G: EmissionGuarantee>(DiagnosticBuilder<'a, G>);
+
+impl<'a, G: EmissionGuarantee> LintDiagnosticBuilder<'a, G> {
+    /// Return the inner `DiagnosticBuilder`, first setting the primary message to `msg`.
+    pub fn build(mut self, msg: impl Into<DiagnosticMessage>) -> DiagnosticBuilder<'a, G> {
+        self.0.set_primary_message(msg);
+        self.0.set_is_lint();
+        self.0
+    }
+
+    /// Create a `LintDiagnosticBuilder` from some existing `DiagnosticBuilder`.
+    pub fn new(err: DiagnosticBuilder<'a, G>) -> LintDiagnosticBuilder<'a, G> {
+        LintDiagnosticBuilder(err)
+    }
+}
+
+impl<'a> LintDiagnosticBuilder<'a, ErrorGuaranteed> {
+    pub fn forget_guarantee(self) -> LintDiagnosticBuilder<'a, ()> {
+        LintDiagnosticBuilder(self.0.forget_guarantee())
+    }
+}
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index b084e128483..ffe4ecebb2e 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -370,10 +370,10 @@ impl fmt::Display for ExplicitBug {
 impl error::Error for ExplicitBug {}
 
 pub use diagnostic::{
-    AddSubdiagnostic, Diagnostic, DiagnosticArg, DiagnosticArgValue, DiagnosticId,
+    AddSubdiagnostic, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgValue, DiagnosticId,
     DiagnosticStyledString, IntoDiagnosticArg, SubDiagnostic,
 };
-pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee};
+pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee, LintDiagnosticBuilder};
 use std::backtrace::Backtrace;
 
 /// A handler deals with errors and other compiler output.