static mut: allow reference to arbitrary types, not just slices and arrays

Author: RalfJung

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -471,35 +471,21 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 }
             }
 
-            Rvalue::Ref(_, BorrowKind::Mut { .. }, place) => {
-                let ty = place.ty(self.body, self.tcx).ty;
-                let is_allowed = match ty.kind() {
-                    // Inside a `static mut`, `&mut [...]` is allowed.
-                    ty::Array(..) | ty::Slice(_)
-                        if self.const_kind() == hir::ConstContext::Static(hir::Mutability::Mut) =>
-                    {
-                        true
-                    }
-
-                    // FIXME(ecstaticmorse): We could allow `&mut []` inside a const context given
-                    // that this is merely a ZST and it is already eligible for promotion.
-                    // This may require an RFC?
-                    /*
-                    ty::Array(_, len) if len.try_eval_target_usize(cx.tcx, cx.param_env) == Some(0)
-                        => true,
-                    */
-                    _ => false,
-                };
+            Rvalue::Ref(_, BorrowKind::Mut { .. }, place) | Rvalue::AddressOf(Mutability::Mut, place) => {
+                // Inside mutable statics, we allow arbitrary mutable references.
+                // We've allowed `static mut FOO = &mut [elements];` for a long time (the exact
+                // reasons why are lost to history), and there is no reason to restrict that to
+                // arrays and slices.
+                let is_allowed = self.const_kind() == hir::ConstContext::Static(hir::Mutability::Mut);
 
                 if !is_allowed {
-                    self.check_mut_borrow(place.local, hir::BorrowKind::Ref)
+                    self.check_mut_borrow(
+                        place.local,
+                        if matches!(rvalue, Rvalue::Ref(..)) { hir::BorrowKind::Ref } else {hir::BorrowKind::Raw}
+                    );
                 }
             }
 
-            Rvalue::AddressOf(Mutability::Mut, place) => {
-                self.check_mut_borrow(place.local, hir::BorrowKind::Raw)
-            }
-
             Rvalue::Ref(_, BorrowKind::Shared | BorrowKind::Shallow, place)
             | Rvalue::AddressOf(Mutability::Not, place) => {
                 let borrowed_place_has_mut_interior = qualifs::in_place::<HasMutInterior, _>(

tests/ui/array-slice-vec/check-static-mut-slices.rs

@@ -0,0 +1,24 @@
+// run-pass
+#![allow(dead_code)]
+
+// Checks that mutable static items can have mutable slices and other references
+
+
+static mut TEST: &'static mut [isize] = &mut [1];
+static mut EMPTY: &'static mut [isize] = &mut [];
+static mut INT: &'static mut isize = &mut 1;
+
+// And the same for raw pointers.
+
+static mut TEST_RAW: *mut [isize] = &mut [1isize] as *mut _;
+static mut EMPTY_RAW: *mut [isize] = &mut [] as *mut _;
+static mut INT_RAW: *mut isize = &mut 1isize as *mut _;
+
+pub fn main() {
+    unsafe {
+        TEST[0] += 1;
+        assert_eq!(TEST[0], 2);
+        *INT_RAW += 1;
+        assert_eq!(*INT_RAW, 2);
+    }
+}

tests/ui/consts/static-mut-refs.rs

@@ -6,6 +6,6 @@ static mut STDERR_BUFFER_SPACE: u8 = 0;
 
 pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
 //[mut_refs]~^ ERROR could not evaluate static initializer
-//[stock]~^^ ERROR mutable references are not allowed in statics
+//[stock]~^^ ERROR mutation through a reference is not allowed in statics
 
 fn main() {}

tests/ui/consts/static_mut_containing_mut_ref2.rs

@@ -1,8 +1,8 @@
-error[E0658]: mutable references are not allowed in statics
-  --> $DIR/static_mut_containing_mut_ref2.rs:7:46
+error[E0658]: mutation through a reference is not allowed in statics
+  --> $DIR/static_mut_containing_mut_ref2.rs:7:45
    |
 LL | pub static mut STDERR_BUFFER: () = unsafe { *(&mut STDERR_BUFFER_SPACE) = 42; };
-   |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable