Disallow hidden references to mutable static

compiler/rustc_error_codes/src/error_codes/E0796.md

@@ -1,14 +1,14 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 You have created a reference to a mutable static.
 
 Erroneous code example:
 
-```compile_fail,edition2024,E0796
+```
 static mut X: i32 = 23;
-
 fn work() {
   let _val = unsafe { X };
 }
-
 let x_ref = unsafe { &mut X };
 work();
 // The next line has Undefined Behavior!

compiler/rustc_error_codes/src/lib.rs

@@ -679,3 +679,4 @@ E0798: 0798,
 //  E0723, // unstable feature in `const` context
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
 //  E0744, // merged into E0728
+//  E0796, // unused error code. We use `static_mut_refs` lint instead.