From 924748717bfc90d3a34bed407d75eb6c988f9fc9 Mon Sep 17 00:00:00 2001
From: Guillaume Gomez <guillaume1.gomez@gmail.com>
Date: Tue, 3 Mar 2026 17:21:16 +0100
Subject: [PATCH] Implement `Diagnostic` trait for
 `rustc_errors::DecorateDiagCompat`

---
 compiler/rustc_errors/src/decorate_diag.rs | 11 +++++------
 compiler/rustc_lint/src/early.rs           | 18 +++++++++++++++---
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/compiler/rustc_errors/src/decorate_diag.rs b/compiler/rustc_errors/src/decorate_diag.rs
index 5aef26ccf973..7e53b8195d5b 100644
--- a/compiler/rustc_errors/src/decorate_diag.rs
+++ b/compiler/rustc_errors/src/decorate_diag.rs
@@ -1,15 +1,16 @@
 /// This module provides types and traits for buffering lints until later in compilation.
 use rustc_ast::node_id::NodeId;
 use rustc_data_structures::fx::FxIndexMap;
+use rustc_data_structures::sync::DynSend;
 use rustc_error_messages::MultiSpan;
 use rustc_lint_defs::{BuiltinLintDiag, Lint, LintId};
 
-use crate::{DynSend, LintDiagnostic, LintDiagnosticBox};
+use crate::{Diag, DiagCtxtHandle, Diagnostic, Level};
 
 /// We can't implement `LintDiagnostic` for `BuiltinLintDiag`, because decorating some of its
 /// variants requires types we don't have yet. So, handle that case separately.
 pub enum DecorateDiagCompat {
-    Dynamic(Box<dyn for<'a> LintDiagnosticBox<'a, ()> + DynSend + 'static>),
+    Dynamic(Box<dyn for<'a> FnOnce(DiagCtxtHandle<'a>, Level) -> Diag<'a, ()> + DynSend + 'static>),
     Builtin(BuiltinLintDiag),
 }
 
@@ -19,12 +20,10 @@ fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
     }
 }
 
-impl !LintDiagnostic<'_, ()> for BuiltinLintDiag {}
-
-impl<D: for<'a> LintDiagnostic<'a, ()> + DynSend + 'static> From<D> for DecorateDiagCompat {
+impl<D: for<'a> Diagnostic<'a, ()> + DynSend + 'static> From<D> for DecorateDiagCompat {
     #[inline]
     fn from(d: D) -> Self {
-        Self::Dynamic(Box::new(d))
+        Self::Dynamic(Box::new(|dcx, level| d.into_diag(dcx, level)))
     }
 }
 
diff --git a/compiler/rustc_lint/src/early.rs b/compiler/rustc_lint/src/early.rs
index a2ee7936685b..34381d2c5d48 100644
--- a/compiler/rustc_lint/src/early.rs
+++ b/compiler/rustc_lint/src/early.rs
@@ -7,7 +7,10 @@
 use rustc_ast::visit::{self as ast_visit, Visitor, walk_list};
 use rustc_ast::{self as ast, AttrVec, HasAttrs};
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_errors::{BufferedEarlyLint, DecorateDiagCompat, LintBuffer};
+use rustc_data_structures::sync::DynSend;
+use rustc_errors::{
+    BufferedEarlyLint, DecorateDiagCompat, Diag, DiagCtxtHandle, Diagnostic, Level, LintBuffer,
+};
 use rustc_feature::Features;
 use rustc_middle::ty::{RegisteredTools, TyCtxt};
 use rustc_session::Session;
@@ -35,6 +38,16 @@ pub struct EarlyContextAndPass<'ecx, 'tcx, T: EarlyLintPass> {
 
 impl<'ecx, 'tcx, T: EarlyLintPass> EarlyContextAndPass<'ecx, 'tcx, T> {
     fn check_id(&mut self, id: ast::NodeId) {
+        struct BoxDiag(
+            Box<dyn for<'a> FnOnce(DiagCtxtHandle<'a>, Level) -> Diag<'a, ()> + DynSend + 'static>,
+        );
+
+        impl<'a> Diagnostic<'a, ()> for BoxDiag {
+            fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, ()> {
+                (self.0)(dcx, level)
+            }
+        }
+
         for early_lint in self.context.buffered.take(id) {
             let BufferedEarlyLint { span, node_id: _, lint_id, diagnostic } = early_lint;
             match diagnostic {
@@ -50,8 +63,7 @@ fn check_id(&mut self, id: ast::NodeId) {
                     );
                 }
                 DecorateDiagCompat::Dynamic(d) => {
-                    self.context
-                        .opt_span_lint(lint_id.lint, span, |diag| d.decorate_lint_box(diag));
+                    self.context.opt_span_diag_lint(lint_id.lint, span, BoxDiag(d));
                 }
             }
         }