Auto merge of #86426 - hi-rustin:rustin-patch-lint-warn, r=Aaron1011

Lint for unused borrows as part of UNUSED_MUST_USE

close https://github.com/rust-lang/rust/issues/76264

base on https://github.com/rust-lang/rust/pull/76894

r? `@RalfJung`

src/test/ui/array-slice-vec/slice-panic-1.rs

@@ -17,7 +17,7 @@ impl Drop for Foo {
 
 fn foo() {
     let x: &[_] = &[Foo, Foo];
-    &x[3..4];
+    let _ = &x[3..4];
 }
 
 fn main() {

src/test/ui/array-slice-vec/slice-panic-2.rs

@@ -21,7 +21,7 @@ fn bar() -> usize {
 
 fn foo() {
     let x: &[_] = &[Foo, Foo];
-    &x[3..bar()];
+    let _ = &x[3..bar()];
 }
 
 fn main() {

src/test/ui/array-slice-vec/slice.rs

@@ -67,14 +67,14 @@ impl IndexMut<RangeFull> for Foo {
 
 fn main() {
     let mut x = Foo;
-    &x[..];
-    &x[Foo..];
-    &x[..Foo];
-    &x[Foo..Foo];
-    &mut x[..];
-    &mut x[Foo..];
-    &mut x[..Foo];
-    &mut x[Foo..Foo];
+    let _ = &x[..];
+    let _ = &x[Foo..];
+    let _ = &x[..Foo];
+    let _ = &x[Foo..Foo];
+    let _ = &mut x[..];
+    let _ = &mut x[Foo..];
+    let _ = &mut x[..Foo];
+    let _ = &mut x[Foo..Foo];
     unsafe {
         assert_eq!(COUNT, 8);
     }

src/test/ui/const-generics/issues/issue-61432.rs

@@ -6,9 +6,9 @@ fn promote<const N: i32>() {
     // works:
     //
     // let n = N;
-    // &n;
+    // let _ = &n;
 
-    &N;
+    let _ = &N;
 }
 
 fn main() {

src/test/ui/dynamically-sized-types/dst-index.rs

@@ -29,6 +29,6 @@ impl Index<usize> for T {
 
 fn main() {
     assert_eq!(&S[0], "hello");
-    &T[0];
+    let _ = &T[0];
     // let x = &x as &Debug;
 }

src/test/ui/generator/too-live-local-in-immovable-gen.rs

@@ -15,7 +15,7 @@ fn main() {
                 yield ();
                 4i32
             };
-            &a;
+            let _ = &a;
         };
     }
 }

src/test/ui/generator/too-live-local-in-immovable-gen.stderr

@@ -6,7 +6,7 @@ LL | |             // Tests that the generator transformation finds out that `a`
 LL | |             // during the yield expression. Type checking will also compute liveness
 LL | |             // and it should also find out that `a` is not live.
 ...  |
-LL | |             &a;
+LL | |             let _ = &a;
 LL | |         };
    | |__________^
    |

src/test/ui/generator/yield-in-initializer.rs

@@ -11,7 +11,7 @@ fn main() {
                 yield;
                 true
             };
-            &opt;
+            let _ = &opt;
         }
     };
 }

src/test/ui/issues/issue-43205.rs

@@ -1,5 +1,5 @@
 // run-pass
 fn main() {
-   &&[()][0];
+   let _ = &&[()][0];
    println!("{:?}", &[(),()][1]);
 }

src/test/ui/issues/issue-5280.rs

@@ -9,7 +9,7 @@ trait FontTableTagConversions {
 
 impl FontTableTagConversions for FontTableTag {
   fn tag_to_string(self) {
-    &self;
+      let _ = &self;
   }
 }
 

src/test/ui/issues/issue-54696.rs

@@ -2,7 +2,7 @@
 
 fn main() {
     // We shouldn't promote this
-    &(main as fn() == main as fn());
+    let _ = &(main as fn() == main as fn());
     // Also check nested case
-    &(&(main as fn()) == &(main as fn()));
+    let _ = &(&(main as fn()) == &(main as fn()));
 }

src/test/ui/lint/unused-borrows.rs

@@ -0,0 +1,33 @@
+#![deny(unused_must_use)]
+
+fn foo(_: i32) -> bool { todo!() }
+
+fn bar() -> &'static i32 {
+    &42;
+    //~^ unused
+
+    &mut foo(42);
+    //~^ unused
+
+    &&42;
+    //~^ unused
+
+    &&mut 42;
+    //~^ unused
+
+    &mut &42;
+    //~^ unused
+
+    let _result = foo(4)
+        && foo(2); // Misplaced semi-colon (perhaps due to reordering of lines)
+    && foo(42);
+    //~^ unused
+
+    let _ = &42; // ok
+
+    &42 // ok
+}
+
+fn main() {
+    let _ = bar();
+}

src/test/ui/lint/unused-borrows.stderr

@@ -0,0 +1,44 @@
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:6:5
+   |
+LL |     &42;
+   |     ^^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-borrows.rs:1:9
+   |
+LL | #![deny(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^
+
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:9:5
+   |
+LL |     &mut foo(42);
+   |     ^^^^^^^^^^^^
+
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:12:5
+   |
+LL |     &&42;
+   |     ^^^^
+
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:15:5
+   |
+LL |     &&mut 42;
+   |     ^^^^^^^^
+
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:18:5
+   |
+LL |     &mut &42;
+   |     ^^^^^^^^
+
+error: unused borrow that must be used
+  --> $DIR/unused-borrows.rs:23:5
+   |
+LL |     && foo(42);
+   |     ^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+