Add a note if the function is behind a cfg

clippy_lints/src/needless_pass_by_ref_mut.rs

@@ -12,11 +12,11 @@
 use rustc_middle::hir::map::associated_body;
 use rustc_middle::hir::nested_filter::OnlyBodies;
 use rustc_middle::mir::FakeReadCause;
-use rustc_middle::ty::{self, Ty, UpvarId, UpvarPath};
+use rustc_middle::ty::{self, Ty, TyCtxt, UpvarId, UpvarPath};
 use rustc_session::{declare_tool_lint, impl_lint_pass};
-use rustc_span::def_id::LocalDefId;
+use rustc_span::def_id::{LocalDefId, CRATE_DEF_ID};
 use rustc_span::symbol::kw;
-use rustc_span::Span;
+use rustc_span::{sym, Span};
 use rustc_target::spec::abi::Abi;
 
 declare_clippy_lint! {
@@ -93,6 +93,16 @@ fn should_skip<'tcx>(
     is_from_proc_macro(cx, &input)
 }
 
+fn inherits_cfg(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
+    if def_id == CRATE_DEF_ID {
+        false
+    } else if tcx.has_attr(def_id, sym::cfg) {
+        true
+    } else {
+        inherits_cfg(tcx, tcx.parent_module_from_def_id(def_id))
+    }
+}
+
 impl<'tcx> LateLintPass<'tcx> for NeedlessPassByRefMut<'tcx> {
     fn check_fn(
         &mut self,
@@ -192,10 +202,12 @@ fn check_crate_post(&mut self, cx: &LateContext<'tcx>) {
             let show_semver_warning =
                 self.avoid_breaking_exported_api && cx.effective_visibilities.is_exported(*fn_def_id);
 
+            let mut is_cfged = None;
             for input in unused {
                 // If the argument is never used mutably, we emit the warning.
                 let sp = input.span;
                 if let rustc_hir::TyKind::Ref(_, inner_ty) = input.kind {
+                    let is_cfged = is_cfged.get_or_insert_with(|| inherits_cfg(cx.tcx, *fn_def_id));
                     span_lint_hir_and_then(
                         cx,
                         NEEDLESS_PASS_BY_REF_MUT,
@@ -212,6 +224,9 @@ fn check_crate_post(&mut self, cx: &LateContext<'tcx>) {
                             if show_semver_warning {
                                 diag.warn("changing this function will impact semver compatibility");
                             }
+                            if *is_cfged {
+                                diag.note("this is cfg-gated and may require further changes");
+                            }
                         },
                     );
                 }

tests/ui/needless_pass_by_ref_mut.rs

@@ -184,6 +184,16 @@ fn used_as_path(s: &mut u32) {}
 #[expect(clippy::needless_pass_by_ref_mut)]
 fn lint_attr(s: &mut u32) {}
 
+#[cfg(not(feature = "a"))]
+// Should warn with note.
+fn cfg_warn(s: &mut u32) {}
+
+#[cfg(not(feature = "a"))]
+mod foo {
+    // Should warn with note.
+    fn cfg_warn(s: &mut u32) {}
+}
+
 fn main() {
     let mut u = 0;
     let mut v = vec![0];

tests/ui/needless_pass_by_ref_mut.stderr

@@ -78,5 +78,21 @@ error: this argument is a mutable reference, but not used mutably
 LL | async fn a8(x: i32, a: &mut i32, y: i32, z: &mut i32) {
    |                                             ^^^^^^^^ help: consider changing to: `&i32`
 
-error: aborting due to 13 previous errors
+error: this argument is a mutable reference, but not used mutably
+  --> $DIR/needless_pass_by_ref_mut.rs:189:16
+   |
+LL | fn cfg_warn(s: &mut u32) {}
+   |                ^^^^^^^^ help: consider changing to: `&u32`
+   |
+   = note: this is cfg-gated and may require further changes
+
+error: this argument is a mutable reference, but not used mutably
+  --> $DIR/needless_pass_by_ref_mut.rs:194:20
+   |
+LL |     fn cfg_warn(s: &mut u32) {}
+   |                    ^^^^^^^^ help: consider changing to: `&u32`
+   |
+   = note: this is cfg-gated and may require further changes
+
+error: aborting due to 15 previous errors