Emit missing_const_for_fn for CONST_MUT_REFS (#13839)

1.83 stabilized `CONST_MUT_REFS`, also allowing for `const fn` to mutate
through mutable references. This ensures `missing_const_for_fn` is
emitted for those cases.

changelog: [`missing_const_for_fn`]: Now suggests marking some functions
taking mutable references `const`

clippy_utils/src/msrvs.rs

@@ -18,7 +18,7 @@ macro_rules! msrv_aliases {
 
 // names may refer to stabilized feature flags or library items
 msrv_aliases! {
-    1,83,0 { CONST_EXTERN_FN, CONST_FLOAT_BITS_CONV, CONST_FLOAT_CLASSIFY, CONST_UNWRAP }
+    1,83,0 { CONST_EXTERN_FN, CONST_FLOAT_BITS_CONV, CONST_FLOAT_CLASSIFY, CONST_MUT_REFS, CONST_UNWRAP }
     1,82,0 { IS_NONE_OR, REPEAT_N, RAW_REF_OP }
     1,81,0 { LINT_REASONS_STABILIZATION, ERROR_IN_CORE, EXPLICIT_SELF_TYPE_ELISION }
     1,80,0 { BOX_INTO_ITER }

clippy_utils/src/qualify_min_const_fn.rs

@@ -29,13 +29,14 @@ pub fn is_min_const_fn<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, msrv: &Msrv)
     let def_id = body.source.def_id();
 
     for local in &body.local_decls {
-        check_ty(tcx, local.ty, local.source_info.span)?;
+        check_ty(tcx, local.ty, local.source_info.span, msrv)?;
     }
     // impl trait is gone in MIR, so check the return type manually
     check_ty(
         tcx,
         tcx.fn_sig(def_id).instantiate_identity().output().skip_binder(),
         body.local_decls.iter().next().unwrap().source_info.span,
+        msrv,
     )?;
 
     for bb in &*body.basic_blocks {
@@ -51,7 +52,7 @@ pub fn is_min_const_fn<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, msrv: &Msrv)
     Ok(())
 }
 
-fn check_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span) -> McfResult {
+fn check_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span, msrv: &Msrv) -> McfResult {
     for arg in ty.walk() {
         let ty = match arg.unpack() {
             GenericArgKind::Type(ty) => ty,
@@ -62,7 +63,7 @@ fn check_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, span: Span) -> McfResult {
         };
 
         match ty.kind() {
-            ty::Ref(_, _, hir::Mutability::Mut) => {
+            ty::Ref(_, _, hir::Mutability::Mut) if !msrv.meets(msrvs::CONST_MUT_REFS) => {
                 return Err((span, "mutable references in const fn are unstable".into()));
             },
             ty::Alias(ty::Opaque, ..) => return Err((span, "`impl Trait` in const fn is unstable".into())),

tests/ui/missing_const_for_fn/cant_be_const.rs

@@ -220,3 +220,9 @@ mod with_ty_alias {
         let _: Foo = 1;
     }
 }
+
+// Do not lint because mutable references in const functions are unstable in 1.82
+#[clippy::msrv = "1.82"]
+fn mut_add(x: &mut i32) {
+    *x += 1;
+}

tests/ui/missing_const_for_fn/could_be_const.fixed

@@ -212,3 +212,8 @@ mod extern_fn {
     const extern "system-unwind" fn system_unwind() {}
     //~^ ERROR: this could be a `const fn`
 }
+
+const fn mut_add(x: &mut i32) {
+    //~^ ERROR: this could be a `const fn`
+    *x += 1;
+}

tests/ui/missing_const_for_fn/could_be_const.rs

@@ -212,3 +212,8 @@ mod extern_fn {
     extern "system-unwind" fn system_unwind() {}
     //~^ ERROR: this could be a `const fn`
 }
+
+fn mut_add(x: &mut i32) {
+    //~^ ERROR: this could be a `const fn`
+    *x += 1;
+}

tests/ui/missing_const_for_fn/could_be_const.stderr

@@ -316,5 +316,19 @@ help: make the function `const`
 LL |     const extern "system-unwind" fn system_unwind() {}
    |     +++++
 
-error: aborting due to 24 previous errors
+error: this could be a `const fn`
+  --> tests/ui/missing_const_for_fn/could_be_const.rs:216:1
+   |
+LL | / fn mut_add(x: &mut i32) {
+LL | |
+LL | |     *x += 1;
+LL | | }
+   | |_^
+   |
+help: make the function `const`
+   |
+LL | const fn mut_add(x: &mut i32) {
+   | +++++
+
+error: aborting due to 25 previous errors