Auto merge of rust-lang#7657 - dswij:needless-borrow-mut, r=llogiq

`needless_borrow` checks for mutable borrow

closes rust-lang#7635

changelog: [`needless_borrow`] now checks for needless mutable borrow

Author: bors

clippy_lints/src/needless_borrow.rs

@@ -104,7 +104,7 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessBorrow {
         if e.span.from_expansion() {
             return;
         }
-        if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, inner) = e.kind {
+        if let ExprKind::AddrOf(BorrowKind::Ref, mutability, inner) = e.kind {
             if let ty::Ref(_, ty, _) = cx.typeck_results().expr_ty(inner).kind() {
                 for adj3 in cx.typeck_results().expr_adjustments(e).windows(3) {
                     if let [Adjustment {
@@ -116,14 +116,20 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessBorrow {
                         ..
                     }] = *adj3
                     {
+                        let help_msg_ty = if matches!(mutability, Mutability::Not) {
+                            format!("&{}", ty)
+                        } else {
+                            format!("&mut {}", ty)
+                        };
+
                         span_lint_and_then(
                             cx,
                             NEEDLESS_BORROW,
                             e.span,
                             &format!(
-                                "this expression borrows a reference (`&{}`) that is immediately dereferenced \
+                                "this expression borrows a reference (`{}`) that is immediately dereferenced \
                              by the compiler",
-                                ty
+                                help_msg_ty
                             ),
                             |diag| {
                                 if let Some(snippet) = snippet_opt(cx, inner.span) {

tests/ui/needless_borrow.fixed

@@ -1,17 +1,16 @@
 // run-rustfix
 
-#![allow(clippy::needless_borrowed_reference)]
-
-fn x(y: &i32) -> i32 {
-    *y
-}
-
 #[warn(clippy::all, clippy::needless_borrow)]
 #[allow(unused_variables)]
 fn main() {
     let a = 5;
-    let b = x(&a);
-    let c = x(&a);
+    let _ = x(&a); // no warning
+    let _ = x(&a); // warn
+
+    let mut b = 5;
+    mut_ref(&mut b); // no warning
+    mut_ref(&mut b); // warn
+
     let s = &String::from("hi");
     let s_ident = f(&s); // should not error, because `&String` implements Copy, but `String` does not
     let g_val = g(&Vec::new()); // should not error, because `&Vec<T>` derefs to `&[T]`
@@ -29,6 +28,15 @@ fn main() {
     };
 }
 
+#[allow(clippy::needless_borrowed_reference)]
+fn x(y: &i32) -> i32 {
+    *y
+}
+
+fn mut_ref(y: &mut i32) {
+    *y = 5;
+}
+
 fn f<T: Copy>(y: &T) -> T {
     *y
 }

tests/ui/needless_borrow.rs

@@ -1,17 +1,16 @@
 // run-rustfix
 
-#![allow(clippy::needless_borrowed_reference)]
-
-fn x(y: &i32) -> i32 {
-    *y
-}
-
 #[warn(clippy::all, clippy::needless_borrow)]
 #[allow(unused_variables)]
 fn main() {
     let a = 5;
-    let b = x(&a);
-    let c = x(&&a);
+    let _ = x(&a); // no warning
+    let _ = x(&&a); // warn
+
+    let mut b = 5;
+    mut_ref(&mut b); // no warning
+    mut_ref(&mut &mut b); // warn
+
     let s = &String::from("hi");
     let s_ident = f(&s); // should not error, because `&String` implements Copy, but `String` does not
     let g_val = g(&Vec::new()); // should not error, because `&Vec<T>` derefs to `&[T]`
@@ -29,6 +28,15 @@ fn main() {
     };
 }
 
+#[allow(clippy::needless_borrowed_reference)]
+fn x(y: &i32) -> i32 {
+    *y
+}
+
+fn mut_ref(y: &mut i32) {
+    *y = 5;
+}
+
 fn f<T: Copy>(y: &T) -> T {
     *y
 }

tests/ui/needless_borrow.stderr

@@ -1,16 +1,22 @@
 error: this expression borrows a reference (`&i32`) that is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:14:15
+  --> $DIR/needless_borrow.rs:8:15
    |
-LL |     let c = x(&&a);
+LL |     let _ = x(&&a); // warn
    |               ^^^ help: change this to: `&a`
    |
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
 
+error: this expression borrows a reference (`&mut i32`) that is immediately dereferenced by the compiler
+  --> $DIR/needless_borrow.rs:12:13
+   |
+LL |     mut_ref(&mut &mut b); // warn
+   |             ^^^^^^^^^^^ help: change this to: `&mut b`
+
 error: this expression borrows a reference (`&i32`) that is immediately dereferenced by the compiler
-  --> $DIR/needless_borrow.rs:27:15
+  --> $DIR/needless_borrow.rs:26:15
    |
 LL |         46 => &&a,
    |               ^^^ help: change this to: `&a`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors