fix: respect rustc's lint attribute application order

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();

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`

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(

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,