Add check_self_items setting for needless_pass_by_ref_mut lint

Author: GuillaumeGomez

clippy_config/src/conf.rs

@@ -589,6 +589,10 @@ define_Conf! {
     /// 2. Paths with any segment that containing the word 'prelude'
     /// are already allowed by default.
     (allowed_wildcard_imports: FxHashSet<String> = FxHashSet::default()),
+    /// Lint: NEEDLESS_PASS_BY_REF_MUT.
+    ///
+    /// Lint on `self`.
+    (check_self_items: bool = false),
 }
 
 /// Search for the configuration file.

clippy_lints/src/lib.rs

@@ -597,6 +597,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
 
         blacklisted_names: _,
         cyclomatic_complexity_threshold: _,
+        check_self_items,
     } = *conf;
     let msrv = || msrv.clone();
 
@@ -1070,6 +1071,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(move |_| {
         Box::new(needless_pass_by_ref_mut::NeedlessPassByRefMut::new(
             avoid_breaking_exported_api,
+            check_self_items,
         ))
     });
     store.register_late_pass(|_| Box::new(non_canonical_impls::NonCanonicalImpls));

clippy_lints/src/needless_pass_by_ref_mut.rs

@@ -30,6 +30,9 @@ declare_clippy_lint! {
     /// Be careful if the function is publicly reexported as it would break compatibility with
     /// users of this function.
     ///
+    /// By default, `&mut self` is ignored. If you want it to be taken into account, set the
+    /// `check_self_items` clippy setting to `true`.
+    ///
     /// ### Why is this bad?
     /// Less `mut` means less fights with the borrow checker. It can also lead to more
     /// opportunities for parallelization.
@@ -57,14 +60,16 @@ pub struct NeedlessPassByRefMut<'tcx> {
     avoid_breaking_exported_api: bool,
     used_fn_def_ids: FxHashSet<LocalDefId>,
     fn_def_ids_to_maybe_unused_mut: FxIndexMap<LocalDefId, Vec<rustc_hir::Ty<'tcx>>>,
+    check_self_items: bool,
 }
 
 impl NeedlessPassByRefMut<'_> {
-    pub fn new(avoid_breaking_exported_api: bool) -> Self {
+    pub fn new(avoid_breaking_exported_api: bool, check_self_items: bool) -> Self {
         Self {
             avoid_breaking_exported_api,
             used_fn_def_ids: FxHashSet::default(),
             fn_def_ids_to_maybe_unused_mut: FxIndexMap::default(),
+            check_self_items,
         }
     }
 }
@@ -76,13 +81,14 @@ fn should_skip<'tcx>(
     input: rustc_hir::Ty<'tcx>,
     ty: Ty<'_>,
     arg: &rustc_hir::Param<'_>,
+    check_self_items: bool,
 ) -> bool {
     // We check if this a `&mut`. `ref_mutability` returns `None` if it's not a reference.
     if !matches!(ty.ref_mutability(), Some(Mutability::Mut)) {
         return true;
     }
 
-    if is_self(arg) {
+    if !check_self_items && is_self(arg) {
         return true;
     }
 
@@ -172,13 +178,15 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessPassByRefMut<'tcx> {
         let fn_sig = cx.tcx.fn_sig(fn_def_id).instantiate_identity();
         let fn_sig = cx.tcx.liberate_late_bound_regions(fn_def_id.to_def_id(), fn_sig);
 
+        let check_self_items = self.check_self_items;
+
         // If there are no `&mut` argument, no need to go any further.
         let mut it = decl
             .inputs
             .iter()
             .zip(fn_sig.inputs())
             .zip(body.params)
-            .filter(|((&input, &ty), arg)| !should_skip(cx, input, ty, arg))
+            .filter(|((&input, &ty), arg)| !should_skip(cx, input, ty, arg, check_self_items))
             .peekable();
         if it.peek().is_none() {
             return;