From 56d636a762505bef848152581f9f144e2154a3de Mon Sep 17 00:00:00 2001
From: Guillaume Gomez <guillaume1.gomez@gmail.com>
Date: Tue, 3 Mar 2026 22:14:53 +0100
Subject: [PATCH] Implement `Diagnostic` directly on `Box<FnOnce...>`

---
 compiler/rustc_errors/src/diagnostic.rs |  9 +++++++++
 compiler/rustc_lint/src/early.rs        | 17 ++---------------
 2 files changed, 11 insertions(+), 15 deletions(-)

diff --git a/compiler/rustc_errors/src/diagnostic.rs b/compiler/rustc_errors/src/diagnostic.rs
index 4dec167b982b..888009bf9d4c 100644
--- a/compiler/rustc_errors/src/diagnostic.rs
+++ b/compiler/rustc_errors/src/diagnostic.rs
@@ -7,6 +7,7 @@
 use std::path::PathBuf;
 use std::thread::panicking;
 
+use rustc_data_structures::sync::DynSend;
 use rustc_error_messages::{DiagArgMap, DiagArgName, DiagArgValue, IntoDiagArg};
 use rustc_lint_defs::{Applicability, LintExpectationId};
 use rustc_macros::{Decodable, Encodable};
@@ -118,6 +119,14 @@ fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, G> {
     }
 }
 
+impl<'a> Diagnostic<'a, ()>
+    for Box<dyn for<'b> FnOnce(DiagCtxtHandle<'b>, Level) -> Diag<'b, ()> + DynSend + 'static>
+{
+    fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, ()> {
+        self(dcx, level)
+    }
+}
+
 /// Trait implemented by error types. This should not be implemented manually. Instead, use
 /// `#[derive(Subdiagnostic)]` -- see [rustc_macros::Subdiagnostic].
 #[rustc_diagnostic_item = "Subdiagnostic"]
diff --git a/compiler/rustc_lint/src/early.rs b/compiler/rustc_lint/src/early.rs
index 34381d2c5d48..92232c7d230b 100644
--- a/compiler/rustc_lint/src/early.rs
+++ b/compiler/rustc_lint/src/early.rs
@@ -7,10 +7,7 @@
 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_data_structures::sync::DynSend;
-use rustc_errors::{
-    BufferedEarlyLint, DecorateDiagCompat, Diag, DiagCtxtHandle, Diagnostic, Level, LintBuffer,
-};
+use rustc_errors::{BufferedEarlyLint, DecorateDiagCompat, LintBuffer};
 use rustc_feature::Features;
 use rustc_middle::ty::{RegisteredTools, TyCtxt};
 use rustc_session::Session;
@@ -38,16 +35,6 @@ 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 {
@@ -63,7 +50,7 @@ fn into_diag(self, dcx: DiagCtxtHandle<'a>, level: Level) -> Diag<'a, ()> {
                     );
                 }
                 DecorateDiagCompat::Dynamic(d) => {
-                    self.context.opt_span_diag_lint(lint_id.lint, span, BoxDiag(d));
+                    self.context.opt_span_diag_lint(lint_id.lint, span, d);
                 }
             }
         }