Disallow use of static mut inside unsafe fn

```rust
unsafe fn foo() {
static mut X: i32 = 23;
let y = &X;
}
```

This is the idea for the 2024 edition.

Author: obeis

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -68,7 +68,7 @@ pub mod dropck;
 mod entry;
 pub mod intrinsic;
 pub mod intrinsicck;
-mod region;
+pub mod region;
 pub mod wfcheck;
 
 pub use check::check_abi;

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -25,6 +25,7 @@ use rustc_span::symbol::{sym, Ident};
 use rustc_span::{Span, DUMMY_SP};
 use std::fmt;
 
+use crate::check::region::static_mut_ref;
 use crate::errors;
 
 trait RegionExt {
@@ -843,6 +844,14 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
         _: Span,
         _: LocalDefId,
     ) {
+        if let hir::intravisit::FnKind::ItemFn(_, _, h) = fk
+            && matches!(h.unsafety, hir::Unsafety::Unsafe)
+            && let body = self.tcx.hir().body(body_id)
+            && let hir::ExprKind::Block(block, _) = body.value.kind
+        {
+            static_mut_ref(self.tcx, block.stmts);
+        }
+
         let output = match fd.output {
             hir::FnRetTy::DefaultReturn(_) => None,
             hir::FnRetTy::Return(ty) => Some(ty),