errors: introduce DecorateLint

Add a new trait to be generated by diagnostic derives which uses a
`LintDiagnosticBuilder`.

Signed-off-by: David Wood <david.wood@huawei.com>

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 {

compiler/rustc_errors/src/lib.rs

@@ -370,7 +370,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 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, LintDiagnosticBuilder};

compiler/rustc_lint/src/context.rs

@@ -22,7 +22,9 @@
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync;
 use rustc_errors::{add_elided_lifetime_in_path_suggestion, struct_span_err};
-use rustc_errors::{Applicability, LintDiagnosticBuilder, MultiSpan, SuggestionStyle};
+use rustc_errors::{
+    Applicability, DecorateLint, LintDiagnosticBuilder, MultiSpan, SuggestionStyle,
+};
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_hir::def_id::{CrateNum, DefId};
@@ -870,6 +872,17 @@ fn lookup<S: Into<MultiSpan>>(
         decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>),
     );
 
+    /// Emit a lint at `span` from a lint struct (some type that implements `DecorateLint`,
+    /// typically generated by `#[derive(LintDiagnostic)]`).
+    fn emit_spanned_lint<S: Into<MultiSpan>>(
+        &self,
+        lint: &'static Lint,
+        span: S,
+        decorator: impl for<'a> DecorateLint<'a, ()>,
+    ) {
+        self.lookup(lint, Some(span), |diag| decorator.decorate_lint(diag));
+    }
+
     fn struct_span_lint<S: Into<MultiSpan>>(
         &self,
         lint: &'static Lint,
@@ -878,6 +891,13 @@ fn struct_span_lint<S: Into<MultiSpan>>(
     ) {
         self.lookup(lint, Some(span), decorate);
     }
+
+    /// Emit a lint from a lint struct (some type that implements `DecorateLint`, typically
+    /// generated by `#[derive(LintDiagnostic)]`).
+    fn emit_lint(&self, lint: &'static Lint, decorator: impl for<'a> DecorateLint<'a, ()>) {
+        self.lookup(lint, None as Option<Span>, |diag| decorator.decorate_lint(diag));
+    }
+
     /// Emit a lint at the appropriate level, with no associated span.
     fn lint(
         &self,

compiler/rustc_lint/src/internal.rs

@@ -414,7 +414,7 @@ fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
                 let Impl { of_trait: Some(of_trait), .. } = impl_ &&
                 let Some(def_id) = of_trait.trait_def_id() &&
                 let Some(name) = cx.tcx.get_diagnostic_name(def_id) &&
-                matches!(name, sym::SessionDiagnostic | sym::AddSubdiagnostic)
+                matches!(name, sym::SessionDiagnostic | sym::AddSubdiagnostic | sym::DecorateLint)
             {
                 found_impl = true;
                 break;

compiler/rustc_middle/src/ty/context.rs

@@ -34,7 +34,7 @@
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::sync::{self, Lock, Lrc, WorkerLocal};
 use rustc_data_structures::vec_map::VecMap;
-use rustc_errors::{ErrorGuaranteed, LintDiagnosticBuilder, MultiSpan};
+use rustc_errors::{DecorateLint, ErrorGuaranteed, LintDiagnosticBuilder, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LocalDefId, LOCAL_CRATE};
@@ -2787,6 +2787,18 @@ pub fn lint_level_at_node(
         }
     }
 
+    /// Emit a lint at `span` from a lint struct (some type that implements `DecorateLint`,
+    /// typically generated by `#[derive(LintDiagnostic)]`).
+    pub fn emit_spanned_lint(
+        self,
+        lint: &'static Lint,
+        hir_id: HirId,
+        span: impl Into<MultiSpan>,
+        decorator: impl for<'a> DecorateLint<'a, ()>,
+    ) {
+        self.struct_span_lint_hir(lint, hir_id, span, |diag| decorator.decorate_lint(diag))
+    }
+
     pub fn struct_span_lint_hir(
         self,
         lint: &'static Lint,
@@ -2798,6 +2810,17 @@ pub fn struct_span_lint_hir(
         struct_lint_level(self.sess, lint, level, src, Some(span.into()), decorate);
     }
 
+    /// Emit a lint from a lint struct (some type that implements `DecorateLint`, typically
+    /// generated by `#[derive(LintDiagnostic)]`).
+    pub fn emit_lint(
+        self,
+        lint: &'static Lint,
+        id: HirId,
+        decorator: impl for<'a> DecorateLint<'a, ()>,
+    ) {
+        self.struct_lint_node(lint, id, |diag| decorator.decorate_lint(diag))
+    }
+
     pub fn struct_lint_node(
         self,
         lint: &'static Lint,

compiler/rustc_span/src/symbol.rs

@@ -173,6 +173,7 @@
         DebugTuple,
         Decodable,
         Decoder,
+        DecorateLint,
         Default,
         Deref,
         DiagnosticMessage,