Rollup merge of rust-lang#76894 - ecstatic-morse:lint-unused-borrows, r=lcnr

Lint for unused borrows as part of `UNUSED_MUST_USE`

Resolves rust-lang#76264.

This means an `unused_must_use` warning for statements like `&expr;` and `&mut expr;`. `unused_must_use` was chosen because it already lints for logical operators (`&&` and `||`) whose result is unused. Unused borrows actually appear fairly often in `rustc`'s test suite, since we have tests that rely on side-effects of the index operator (panicking). These cannot be written as `expr[..];`, since the result is unsized, but they can be written as `let _ = &expr[..];`, which is what this PR does.

See the linked issue for the motivating example. This lint also found a benign mistake in the derived impl of `HashStable`.

Author: Dylan-DPC

compiler/rustc_lint/src/unused.rs

@@ -154,6 +154,9 @@ impl<'tcx> LateLintPass<'tcx> for UnusedResults {
                 | hir::BinOpKind::Shl
                 | hir::BinOpKind::Shr => Some("bitwise operation"),
             },
+
+            hir::ExprKind::AddrOf(..) => Some("borrow"),
+
             hir::ExprKind::Unary(..) => Some("unary operation"),
             _ => None,
         };

compiler/rustc_macros/src/hash_stable.rs

@@ -54,7 +54,7 @@ pub fn hash_stable_generic_derive(mut s: synstructure::Structure<'_>) -> proc_ma
             quote! {}
         } else if let Some(project) = attrs.project {
             quote! {
-                &#bi.#project.hash_stable(__hcx, __hasher);
+                (&#bi.#project).hash_stable(__hcx, __hasher);
             }
         } else {
             quote! {
@@ -95,7 +95,7 @@ pub fn hash_stable_derive(mut s: synstructure::Structure<'_>) -> proc_macro2::To
             quote! {}
         } else if let Some(project) = attrs.project {
             quote! {
-                &#bi.#project.hash_stable(__hcx, __hasher);
+                (&#bi.#project).hash_stable(__hcx, __hasher);
             }
         } else {
             quote! {

library/alloc/tests/str.rs

@@ -504,7 +504,7 @@ mod slice_index {
     #[test]
     #[should_panic]
     fn test_slice_fail() {
-        &"中华Việt Nam"[0..2];
+        let _ = &"中华Việt Nam"[0..2];
     }
 
     panic_cases! {
@@ -684,13 +684,13 @@ mod slice_index {
     #[test]
     #[should_panic(expected = "byte index 1024 is out of bounds of `Lorem ipsum dolor sit amet")]
     fn test_slice_fail_truncated_1() {
-        &LOREM_PARAGRAPH[..1024];
+        let _ = &LOREM_PARAGRAPH[..1024];
     }
     // check the truncation in the panic message
     #[test]
     #[should_panic(expected = "luctus, im`[...]")]
     fn test_slice_fail_truncated_2() {
-        &LOREM_PARAGRAPH[..1024];
+        let _ = &LOREM_PARAGRAPH[..1024];
     }
 }
 
@@ -705,7 +705,7 @@ fn test_str_slice_rangetoinclusive_ok() {
 #[should_panic]
 fn test_str_slice_rangetoinclusive_notok() {
     let s = "abcαβγ";
-    &s[..=3];
+    let _ = &s[..=3];
 }
 
 #[test]
@@ -721,7 +721,7 @@ fn test_str_slicemut_rangetoinclusive_ok() {
 fn test_str_slicemut_rangetoinclusive_notok() {
     let mut s = "abcαβγ".to_owned();
     let s: &mut str = &mut s;
-    &mut s[..=3];
+    let _ = &mut s[..=3];
 }
 
 #[test]

library/alloc/tests/vec.rs

@@ -541,35 +541,35 @@ fn test_index_out_of_bounds() {
 #[should_panic]
 fn test_slice_out_of_bounds_1() {
     let x = vec![1, 2, 3, 4, 5];
-    &x[!0..];
+    let _ = &x[!0..];
 }
 
 #[test]
 #[should_panic]
 fn test_slice_out_of_bounds_2() {
     let x = vec![1, 2, 3, 4, 5];
-    &x[..6];
+    let _ = &x[..6];
 }
 
 #[test]
 #[should_panic]
 fn test_slice_out_of_bounds_3() {
     let x = vec![1, 2, 3, 4, 5];
-    &x[!0..4];
+    let _ = &x[!0..4];
 }
 
 #[test]
 #[should_panic]
 fn test_slice_out_of_bounds_4() {
     let x = vec![1, 2, 3, 4, 5];
-    &x[1..6];
+    let _ = &x[1..6];
 }
 
 #[test]
 #[should_panic]
 fn test_slice_out_of_bounds_5() {
     let x = vec![1, 2, 3, 4, 5];
-    &x[3..2];
+    let _ = &x[3..2];
 }
 
 #[test]

library/std/src/sys_common/wtf8/tests.rs

@@ -301,7 +301,7 @@ fn wtf8_slice() {
 #[test]
 #[should_panic]
 fn wtf8_slice_not_code_point_boundary() {
-    &Wtf8::from_str("aé 💩")[2..4];
+    let _ = &Wtf8::from_str("aé 💩")[2..4];
 }
 
 #[test]
@@ -312,18 +312,18 @@ fn wtf8_slice_from() {
 #[test]
 #[should_panic]
 fn wtf8_slice_from_not_code_point_boundary() {
-    &Wtf8::from_str("aé 💩")[2..];
+    let _ = &Wtf8::from_str("aé 💩")[2..];
 }
 
 #[test]
 fn wtf8_slice_to() {
-    assert_eq!(&Wtf8::from_str("aé 💩")[..4].bytes, b"a\xC3\xA9 ");
+    let _ = assert_eq!(&Wtf8::from_str("aé 💩")[..4].bytes, b"a\xC3\xA9 ");
 }
 
 #[test]
 #[should_panic]
 fn wtf8_slice_to_not_code_point_boundary() {
-    &Wtf8::from_str("aé 💩")[5..];
+    let _ = &Wtf8::from_str("aé 💩")[5..];
 }
 
 #[test]

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

@@ -8,7 +8,7 @@ fn promote<const N: i32>() {
     // let n = N;
     // &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:23:9
+   |
+LL |         && foo(42);
+   |         ^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+