add test ensuring we detect a static actually having a mutable reference

tests/ui/consts/const-mut-refs/mut_ref_in_final_dynamic_check.rs

@@ -1,8 +1,11 @@
 // normalize-stderr-test "(the raw bytes of the constant) \(size: [0-9]*, align: [0-9]*\)" -> "$1 (size: $$SIZE, align: $$ALIGN)"
-// normalize-stderr-test "([0-9a-f][0-9a-f] |╾─*ALLOC[0-9]+(\+[a-z0-9]+)?(<imm>)?─*╼ )+ *│.*" -> "HEX_DUMP"
+// normalize-stderr-test "( 0x[0-9a-f][0-9a-f] │)? ([0-9a-f][0-9a-f] |__ |╾─*ALLOC[0-9]+(\+[a-z0-9]+)?(<imm>)?─*╼ )+ *│.*" -> " HEX_DUMP"
+// normalize-stderr-test "HEX_DUMP\s*\n\s*HEX_DUMP" -> "HEX_DUMP"
 #![feature(const_mut_refs, const_refs_to_static)]
 #![feature(raw_ref_op)]
 
+use std::sync::Mutex;
+
 // This file checks that our dynamic checks catch things that the static checks miss.
 // We do not have static checks for these, because we do not look into function bodies.
 // We treat all functions as not returning a mutable reference, because there is no way to
@@ -35,4 +38,9 @@ const fn helper_dangling() -> Option<&'static mut i32> { unsafe {
 const DANGLING: Option<&mut i32> = helper_dangling(); //~ ERROR encountered dangling pointer
 static DANGLING_STATIC: Option<&mut i32> = helper_dangling(); //~ ERROR encountered dangling pointer
 
+// Variant of the real-world case in <https://github.com/rust-lang/rust/issues/120450>.
+static mut MUT_ARRAY: &mut [u8] = &mut [42];
+static MUTEX: Mutex<&mut [u8]> = Mutex::new(unsafe { &mut *MUT_ARRAY }); //~ ERROR it is undefined behavior to use this value
+//~^ encountered mutable reference
+
 fn main() {}

tests/ui/consts/const-mut-refs/mut_ref_in_final_dynamic_check.stderr

@@ -1,5 +1,5 @@
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/mut_ref_in_final_dynamic_check.rs:17:1
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:20:1
    |
 LL | const MUT: Option<&mut i32> = helper();
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .<enum-variant(Some)>.0: encountered mutable reference in a `const` or `static`
@@ -10,7 +10,7 @@ LL | const MUT: Option<&mut i32> = helper();
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/mut_ref_in_final_dynamic_check.rs:19:1
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:22:1
    |
 LL | static MUT_STATIC: Option<&mut i32> = helper();
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .<enum-variant(Some)>.0: encountered mutable reference in a `const` or `static`
@@ -21,7 +21,7 @@ LL | static MUT_STATIC: Option<&mut i32> = helper();
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/mut_ref_in_final_dynamic_check.rs:26:1
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:29:1
    |
 LL | const INT2PTR: Option<&mut i32> = helper_int2ptr();
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .<enum-variant(Some)>.0: encountered a dangling reference (0x2a[noalloc] has no provenance)
@@ -32,7 +32,7 @@ LL | const INT2PTR: Option<&mut i32> = helper_int2ptr();
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/mut_ref_in_final_dynamic_check.rs:28:1
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:31:1
    |
 LL | static INT2PTR_STATIC: Option<&mut i32> = helper_int2ptr();
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .<enum-variant(Some)>.0: encountered a dangling reference (0x2a[noalloc] has no provenance)
@@ -43,17 +43,28 @@ LL | static INT2PTR_STATIC: Option<&mut i32> = helper_int2ptr();
            }
 
 error: encountered dangling pointer in final value of constant
-  --> $DIR/mut_ref_in_final_dynamic_check.rs:35:1
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:38:1
    |
 LL | const DANGLING: Option<&mut i32> = helper_dangling();
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: encountered dangling pointer in final value of static
-  --> $DIR/mut_ref_in_final_dynamic_check.rs:36:1
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:39:1
    |
 LL | static DANGLING_STATIC: Option<&mut i32> = helper_dangling();
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 6 previous errors
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:43:1
+   |
+LL | static MUTEX: Mutex<&mut [u8]> = Mutex::new(unsafe { &mut *MUT_ARRAY });
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .data.value: encountered mutable reference in a `const` or `static`
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: $SIZE, align: $ALIGN) {
+               HEX_DUMP
+           }
+
+error: aborting due to 7 previous errors
 
 For more information about this error, try `rustc --explain E0080`.