Add test for static_mut_ref lint

tests/ui/borrowck/borrowck-unsafe-static-mutable-borrows.rs

@@ -2,17 +2,22 @@
 
 // Test file taken from issue 45129 (https://github.com/rust-lang/rust/issues/45129)
 
-struct Foo { x: [usize; 2] }
+struct Foo {
+    x: [usize; 2],
+}
 
 static mut SFOO: Foo = Foo { x: [23, 32] };
 
 impl Foo {
-    fn x(&mut self) -> &mut usize { &mut self.x[0] }
+    fn x(&mut self) -> &mut usize {
+        &mut self.x[0]
+    }
 }
 
 fn main() {
     unsafe {
         let sfoo: *mut Foo = &mut SFOO;
+        //~^ WARN use of mutable static [static_mut_ref]
         let x = (*sfoo).x();
         (*sfoo).x[1] += 1;
         *x += 1;

tests/ui/borrowck/borrowck-unsafe-static-mutable-borrows.stderr

@@ -0,0 +1,11 @@
+warning: use of mutable static
+  --> $DIR/borrowck-unsafe-static-mutable-borrows.rs:19:30
+   |
+LL |         let sfoo: *mut Foo = &mut SFOO;
+   |                              ^^^^^^^^^ use of mutable static
+   |
+   = note: use of mutable static is a hard error in 2024 edition
+   = note: `#[warn(static_mut_ref)]` on by default
+
+warning: 1 warning emitted
+

tests/ui/lint/lint_static_mut_ref.rs

@@ -0,0 +1,15 @@
+#![deny(static_mut_ref)]
+
+fn main() {
+    static mut X: i32 = 1;
+    unsafe {
+        let _y = &X;
+        //~^ ERROR use of mutable static
+    }
+}
+
+unsafe fn _foo() {
+    static mut X: i32 = 1;
+    let _y = &X;
+    //~^ ERROR use of mutable static
+}

tests/ui/lint/lint_static_mut_ref.stderr

@@ -0,0 +1,23 @@
+error: use of mutable static
+  --> $DIR/lint_static_mut_ref.rs:6:18
+   |
+LL |         let _y = &X;
+   |                  ^^ use of mutable static
+   |
+   = note: use of mutable static is a hard error in 2024 edition
+note: the lint level is defined here
+  --> $DIR/lint_static_mut_ref.rs:1:9
+   |
+LL | #![deny(static_mut_ref)]
+   |         ^^^^^^^^^^^^^^
+
+error: use of mutable static
+  --> $DIR/lint_static_mut_ref.rs:13:14
+   |
+LL |     let _y = &X;
+   |              ^^ use of mutable static
+   |
+   = note: use of mutable static is a hard error in 2024 edition
+
+error: aborting due to 2 previous errors
+