unnecessary_mut_passed: add structured suggestion

clippy_lints/src/mut_reference.rs

@@ -1,4 +1,6 @@
-use clippy_utils::diagnostics::span_lint;
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::sugg::Sugg;
+use rustc_errors::Applicability;
 use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty::{self, Ty};
@@ -83,13 +85,18 @@ fn check_arguments<'tcx>(
         let parameters = type_definition.fn_sig(cx.tcx).skip_binder().inputs();
         for (argument, parameter) in iter::zip(arguments, parameters) {
             if let ty::Ref(_, _, Mutability::Not) | ty::RawPtr(_, Mutability::Not) = parameter.kind()
-                && let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, _) = argument.kind
+                && let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Mut, arg) = argument.kind
             {
-                span_lint(
+                let mut applicability = Applicability::MachineApplicable;
+                let sugg = Sugg::hir_with_applicability(cx, arg, "_", &mut applicability).addr();
+                span_lint_and_sugg(
                     cx,
                     UNNECESSARY_MUT_PASSED,
                     argument.span,
                     format!("the {fn_kind} `{name}` doesn't need a mutable reference"),
+                    "remove the `mut`",
+                    sugg.to_string(),
+                    applicability,
                 );
             }
         }

tests/ui/mut_reference.fixed

@@ -0,0 +1,59 @@
+#![allow(unused_variables, dead_code)]
+fn takes_an_immutable_reference(a: &i32) {}
+fn takes_a_mutable_reference(a: &mut i32) {}
+
+mod issue11268 {
+    macro_rules! x {
+        ($f:expr) => {
+            $f(&mut 1);
+        };
+    }
+
+    fn f() {
+        x!(super::takes_an_immutable_reference);
+        x!(super::takes_a_mutable_reference);
+    }
+}
+
+struct MyStruct;
+
+impl MyStruct {
+    fn takes_an_immutable_reference(&self, a: &i32) {}
+
+    fn takes_a_mutable_reference(&self, a: &mut i32) {}
+}
+
+#[warn(clippy::unnecessary_mut_passed)]
+fn main() {
+    // Functions
+    takes_an_immutable_reference(&42);
+    //~^ unnecessary_mut_passed
+
+    let as_ptr: fn(&i32) = takes_an_immutable_reference;
+    as_ptr(&42);
+    //~^ unnecessary_mut_passed
+
+    // Methods
+    let my_struct = MyStruct;
+    my_struct.takes_an_immutable_reference(&42);
+    //~^ unnecessary_mut_passed
+
+    // No error
+
+    // Functions
+    takes_an_immutable_reference(&42);
+    let as_ptr: fn(&i32) = takes_an_immutable_reference;
+    as_ptr(&42);
+
+    takes_a_mutable_reference(&mut 42);
+    let as_ptr: fn(&mut i32) = takes_a_mutable_reference;
+    as_ptr(&mut 42);
+
+    let a = &mut 42;
+    takes_an_immutable_reference(a);
+
+    // Methods
+    my_struct.takes_an_immutable_reference(&42);
+    my_struct.takes_a_mutable_reference(&mut 42);
+    my_struct.takes_an_immutable_reference(a);
+}

tests/ui/mut_reference.rs

@@ -1,5 +1,4 @@
 #![allow(unused_variables, dead_code)]
-//@no-rustfix
 fn takes_an_immutable_reference(a: &i32) {}
 fn takes_a_mutable_reference(a: &mut i32) {}
 

tests/ui/mut_reference.stderr

@@ -1,23 +1,23 @@
 error: the function `takes_an_immutable_reference` doesn't need a mutable reference
-  --> tests/ui/mut_reference.rs:30:34
+  --> tests/ui/mut_reference.rs:29:34
    |
 LL |     takes_an_immutable_reference(&mut 42);
-   |                                  ^^^^^^^
+   |                                  ^^^^^^^ help: remove the `mut`: `&42`
    |
    = note: `-D clippy::unnecessary-mut-passed` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::unnecessary_mut_passed)]`
 
 error: the function `as_ptr` doesn't need a mutable reference
-  --> tests/ui/mut_reference.rs:34:12
+  --> tests/ui/mut_reference.rs:33:12
    |
 LL |     as_ptr(&mut 42);
-   |            ^^^^^^^
+   |            ^^^^^^^ help: remove the `mut`: `&42`
 
 error: the method `takes_an_immutable_reference` doesn't need a mutable reference
-  --> tests/ui/mut_reference.rs:39:44
+  --> tests/ui/mut_reference.rs:38:44
    |
 LL |     my_struct.takes_an_immutable_reference(&mut 42);
-   |                                            ^^^^^^^
+   |                                            ^^^^^^^ help: remove the `mut`: `&42`
 
 error: aborting due to 3 previous errors