allow statics pointing to mutable statics

compiler/rustc_const_eval/messages.ftl

@@ -453,7 +453,6 @@ const_eval_validation_invalid_fn_ptr = {$front_matter}: encountered {$value}, bu
 const_eval_validation_invalid_ref_meta = {$front_matter}: encountered invalid reference metadata: total size is bigger than largest supported object
 const_eval_validation_invalid_ref_slice_meta = {$front_matter}: encountered invalid reference metadata: slice is bigger than largest supported object
 const_eval_validation_invalid_vtable_ptr = {$front_matter}: encountered {$value}, but expected a vtable pointer
-const_eval_validation_mutable_ref_in_const_or_static = {$front_matter}: encountered mutable reference in a `const` or `static`
 const_eval_validation_mutable_ref_to_immutable = {$front_matter}: encountered mutable reference or box pointing to read-only memory
 const_eval_validation_never_val = {$front_matter}: encountered a value of the never type `!`
 const_eval_validation_null_box = {$front_matter}: encountered a null box

compiler/rustc_const_eval/src/errors.rs

@@ -614,7 +614,6 @@ impl<'tcx> ReportErrorExt for ValidationErrorInfo<'tcx> {
             PartialPointer => const_eval_validation_partial_pointer,
             ConstRefToMutable => const_eval_validation_const_ref_to_mutable,
             ConstRefToExtern => const_eval_validation_const_ref_to_extern,
-            MutableRefInConstOrStatic => const_eval_validation_mutable_ref_in_const_or_static,
             MutableRefToImmutable => const_eval_validation_mutable_ref_to_immutable,
             NullFnPtr => const_eval_validation_null_fn_ptr,
             NeverVal => const_eval_validation_never_val,
@@ -768,7 +767,6 @@ impl<'tcx> ReportErrorExt for ValidationErrorInfo<'tcx> {
             }
             NullPtr { .. }
             | PtrToStatic { .. }
-            | MutableRefInConstOrStatic
             | ConstRefToMutable
             | ConstRefToExtern
             | MutableRefToImmutable

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -148,14 +148,6 @@ impl CtfeValidationMode {
             }
         }
     }
-
-    fn may_contain_mutable_ref(self) -> bool {
-        match self {
-            CtfeValidationMode::Static { mutbl } => mutbl == Mutability::Mut,
-            CtfeValidationMode::Promoted { .. } => false,
-            CtfeValidationMode::Const { .. } => false,
-        }
-    }
 }
 
 /// State for tracking recursive validation of references
@@ -511,20 +503,19 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 // If this allocation has size zero, there is no actual mutability here.
                 let (size, _align, _alloc_kind) = self.ecx.get_alloc_info(alloc_id);
                 if size != Size::ZERO {
+                    // Mutable pointer to immutable memory is no good.
                     if ptr_expected_mutbl == Mutability::Mut
                         && alloc_actual_mutbl == Mutability::Not
                     {
                         throw_validation_failure!(self.path, MutableRefToImmutable);
                     }
-                    if ptr_expected_mutbl == Mutability::Mut
-                        && self.ctfe_mode.is_some_and(|c| !c.may_contain_mutable_ref())
-                    {
-                        throw_validation_failure!(self.path, MutableRefInConstOrStatic);
-                    }
-                    if alloc_actual_mutbl == Mutability::Mut
-                        && matches!(self.ctfe_mode, Some(CtfeValidationMode::Const { .. }))
-                    {
-                        throw_validation_failure!(self.path, ConstRefToMutable);
+                    // In a const, everything must be completely immutable.
+                    if matches!(self.ctfe_mode, Some(CtfeValidationMode::Const { .. })) {
+                        if ptr_expected_mutbl == Mutability::Mut
+                            || alloc_actual_mutbl == Mutability::Mut
+                        {
+                            throw_validation_failure!(self.path, ConstRefToMutable);
+                        }
                     }
                 }
                 // Potentially skip recursive check.

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -420,7 +420,6 @@ pub enum ValidationErrorKind<'tcx> {
     PartialPointer,
     PtrToUninhabited { ptr_kind: PointerKind, ty: Ty<'tcx> },
     PtrToStatic { ptr_kind: PointerKind },
-    MutableRefInConstOrStatic,
     ConstRefToMutable,
     ConstRefToExtern,
     MutableRefToImmutable,

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

@@ -33,6 +33,11 @@ const fn bazz(foo: &mut Foo) -> usize {
 // Empty slices get promoted so this passes the static checks.
 // Make sure it also passes the dynamic checks.
 static MUTABLE_REFERENCE_HOLDER: Mutex<&mut [u8]> = Mutex::new(&mut []);
+// This variant with a non-empty slice also seems entirely reasonable.
+static MUTABLE_REFERENCE_HOLDER2: Mutex<&mut [u8]> = unsafe {
+    static mut FOO: [u8; 1] = [42]; // a private static that we are sure nobody else will reference
+    Mutex::new(&mut *std::ptr::addr_of_mut!(FOO))
+};
 
 fn main() {
     let _: [(); foo().bar()] = [(); 1];

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

@@ -18,9 +18,7 @@ const fn helper() -> Option<&'static mut i32> { unsafe {
     Some(&mut *std::ptr::addr_of_mut!(BUFFER))
 } }
 const MUT: Option<&mut i32> = helper(); //~ ERROR it is undefined behavior to use this value
-//~^ encountered mutable reference
-static MUT_STATIC: Option<&mut i32> = helper(); //~ ERROR it is undefined behavior to use this value
-//~^ encountered mutable reference
+//~^ encountered reference to mutable
 
 const fn helper_int2ptr() -> Option<&'static mut i32> { unsafe {
     // Undefined behaviour (integer as pointer), who doesn't love tests like this.
@@ -38,11 +36,9 @@ 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>.
-// Maybe we should allow this in the future (then the rest should move to `const_mut_refs.rs`),
-// but for now we reject it.
+// These are fine! Just statics pointing to mutable statics, nothing fundamentally wrong with this.
+static MUT_STATIC: Option<&mut i32> = helper();
 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
+static MUTEX: Mutex<&mut [u8]> = Mutex::new(unsafe { &mut *MUT_ARRAY });
 
 fn main() {}

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

@@ -2,26 +2,15 @@ error[E0080]: it is undefined behavior to use this value
   --> $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`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .<enum-variant(Some)>.0: encountered reference to mutable memory in `const`
    |
    = 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[E0080]: it is undefined behavior to use this value
-  --> $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`
-   |
-   = 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[E0080]: it is undefined behavior to use this value
-  --> $DIR/mut_ref_in_final_dynamic_check.rs:29:1
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:27:1
    |
 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 +21,7 @@ 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:31:1
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:29:1
    |
 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)
@@ -43,28 +32,17 @@ 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:38:1
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:36:1
    |
 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:39:1
+  --> $DIR/mut_ref_in_final_dynamic_check.rs:37:1
    |
 LL | static DANGLING_STATIC: Option<&mut i32> = helper_dangling();
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error[E0080]: it is undefined behavior to use this value
-  --> $DIR/mut_ref_in_final_dynamic_check.rs:45:1
-   |
-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
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0080`.

tests/ui/consts/miri_unleashed/mutable_references_err.rs

@@ -1,5 +1,6 @@
-//@ stderr-per-bitwidth
 //@ compile-flags: -Zunleash-the-miri-inside-of-you
+//@ 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"
 #![allow(invalid_reference_casting, static_mut_refs)]
 
 use std::cell::UnsafeCell;