From 2619bd2cec9fc07a8ad4af3bfa3416ea36e1ff81 Mon Sep 17 00:00:00 2001
From: Jesung Yang <y.j3ms.n@gmail.com>
Date: Sun, 14 Dec 2025 03:25:08 +0000
Subject: [PATCH] fix: respect rustc's lint attribute application order

Reverse the order of returned lint attributes for a `SyntaxNode` to
match rustc's behavior.

When multiple lint attributes are present, rustc overrides earlier ones
with the last defined attribute. The previous iteration order was
incorrect, causing earlier attributes to override the later ones.
---
 crates/hir/src/semantics.rs                   |  2 +-
 .../src/handlers/incorrect_case.rs            |  3 +-
 .../src/handlers/unused_variables.rs          | 55 +++++++++++++++++++
 crates/ide-diagnostics/src/lib.rs             | 10 +---
 4 files changed, 60 insertions(+), 10 deletions(-)

diff --git a/crates/hir/src/semantics.rs b/crates/hir/src/semantics.rs
index ffb518b1e66f..b15e642daae7 100644
--- a/crates/hir/src/semantics.rs
+++ b/crates/hir/src/semantics.rs
@@ -267,7 +267,7 @@ impl<DB: HirDatabase + ?Sized> Semantics<'_, DB> {
         &self,
         krate: Crate,
         item: ast::AnyHasAttrs,
-    ) -> impl Iterator<Item = (LintAttr, SmolStr)> {
+    ) -> impl DoubleEndedIterator<Item = (LintAttr, SmolStr)> {
         let mut cfg_options = None;
         let cfg_options = || *cfg_options.get_or_insert_with(|| krate.id.cfg_options(self.db));
         let mut result = Vec::new();
diff --git a/crates/ide-diagnostics/src/handlers/incorrect_case.rs b/crates/ide-diagnostics/src/handlers/incorrect_case.rs
index fdc426c32c7b..4a12c5a26d45 100644
--- a/crates/ide-diagnostics/src/handlers/incorrect_case.rs
+++ b/crates/ide-diagnostics/src/handlers/incorrect_case.rs
@@ -1000,7 +1000,8 @@ mod OtherBadCase;
  // ^^^^^^^^^^^^ 💡 error: Module `OtherBadCase` should have snake_case name, e.g. `other_bad_case`
 
 //- /BAD_CASE/OtherBadCase.rs
-#![deny(non_snake_case)]
+#![allow(non_snake_case)]
+#![deny(non_snake_case)] // The lint level has been overridden.
 
 fn FOO() {}
 // ^^^ 💡 error: Function `FOO` should have snake_case name, e.g. `foo`
diff --git a/crates/ide-diagnostics/src/handlers/unused_variables.rs b/crates/ide-diagnostics/src/handlers/unused_variables.rs
index 84e63acbc04f..b7ec8fa53fa7 100644
--- a/crates/ide-diagnostics/src/handlers/unused_variables.rs
+++ b/crates/ide-diagnostics/src/handlers/unused_variables.rs
@@ -182,6 +182,61 @@ fn main2() {
         );
     }
 
+    #[test]
+    fn apply_last_lint_attribute_when_multiple_are_present() {
+        check_diagnostics(
+            r#"
+#![allow(unused_variables)]
+#![warn(unused_variables)]
+#![deny(unused_variables)]
+
+fn main() {
+    let x = 2;
+      //^ 💡 error: unused variable
+
+    #[deny(unused_variables)]
+    #[warn(unused_variables)]
+    #[allow(unused_variables)]
+    let y = 0;
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn prefer_closest_ancestor_lint_attribute() {
+        check_diagnostics(
+            r#"
+#![allow(unused_variables)]
+
+fn main() {
+    #![warn(unused_variables)]
+
+    #[deny(unused_variables)]
+    let x = 2;
+      //^ 💡 error: unused variable
+}
+
+#[warn(unused_variables)]
+fn main2() {
+    #[deny(unused_variables)]
+    let x = 2;
+      //^ 💡 error: unused variable
+}
+
+#[warn(unused_variables)]
+fn main3() {
+    let x = 2;
+      //^ 💡 warn: unused variable
+}
+
+fn main4() {
+    let x = 2;
+}
+"#,
+        );
+    }
+
     #[test]
     fn fix_unused_variable() {
         check_fix(
diff --git a/crates/ide-diagnostics/src/lib.rs b/crates/ide-diagnostics/src/lib.rs
index fe04bd175c96..343c28e8f9a4 100644
--- a/crates/ide-diagnostics/src/lib.rs
+++ b/crates/ide-diagnostics/src/lib.rs
@@ -643,19 +643,13 @@ fn find_outline_mod_lint_severity(
 
     let mod_def = sema.to_module_def(&mod_node)?;
     let module_source_file = sema.module_definition_node(mod_def);
-    let mut result = None;
     let lint_groups = lint_groups(&diag.code, edition);
     lint_attrs(
         sema,
         krate,
         ast::AnyHasAttrs::cast(module_source_file.value).expect("SourceFile always has attrs"),
     )
-    .for_each(|(lint, severity)| {
-        if lint_groups.contains(&lint) {
-            result = Some(severity);
-        }
-    });
-    result
+    .find_map(|(lint, severity)| lint_groups.contains(&lint).then_some(severity))
 }
 
 fn lint_severity_at(
@@ -682,7 +676,7 @@ fn lint_attrs(
     krate: hir::Crate,
     ancestor: ast::AnyHasAttrs,
 ) -> impl Iterator<Item = (SmolStr, Severity)> {
-    sema.lint_attrs(krate, ancestor).map(|(lint_attr, lint)| {
+    sema.lint_attrs(krate, ancestor).rev().map(|(lint_attr, lint)| {
         let severity = match lint_attr {
             hir::LintAttr::Allow | hir::LintAttr::Expect => Severity::Allow,
             hir::LintAttr::Warn => Severity::Warning,