Rollup merge of rust-lang#120932 - RalfJung:mut-ptr-to-static, r=oli-obk

const_mut_refs: allow mutable pointers to statics

Fixes rust-lang#118447

Writing this PR became a bit messy, see [Zulip](https://rust-lang.zulipchat.com/#narrow/stream/146212-t-compiler.2Fconst-eval/topic/Statics.20pointing.20to.20interior.20mutable.20statics) for some of my journey.^^ Turns out there was a long-standing bug in our qualif logic that led to us incorrectly classifying certain places as "no interior mut" when they actually had interior mut. Due to that the `const_refs_to_cell` feature gate was required far less often than it otherwise would, e.g. in [this code](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=9e0c042c451b3d11d64dd6263679a164). Fixing this however would be a massive breaking change all over libcore and likely the wider ecosystem. So I also changed the const-checking logic to just not require the feature gate for the affected cases. While doing so I discovered a bunch of logic that is not explained and that I could not explain. However I think stabilizing some const-eval feature will make most of that logic inconsequential so I just added some FIXMEs and left it at that.

r? `@oli-obk`

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

@@ -344,7 +344,7 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
         visitor.visit_ty(ty);
     }
 
-    fn check_mut_borrow(&mut self, local: Local, kind: hir::BorrowKind) {
+    fn check_mut_borrow(&mut self, place: &Place<'_>, kind: hir::BorrowKind) {
         match self.const_kind() {
             // In a const fn all borrows are transient or point to the places given via
             // references in the arguments (so we already checked them with
@@ -355,10 +355,19 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
             // to mutable memory.
             hir::ConstContext::ConstFn => self.check_op(ops::TransientMutBorrow(kind)),
             _ => {
+                // For indirect places, we are not creating a new permanent borrow, it's just as
+                // transient as the already existing one. For reborrowing references this is handled
+                // at the top of `visit_rvalue`, but for raw pointers we handle it here.
+                // Pointers/references to `static mut` and cases where the `*` is not the first
+                // projection also end up here.
                 // Locals with StorageDead do not live beyond the evaluation and can
                 // thus safely be borrowed without being able to be leaked to the final
                 // value of the constant.
-                if self.local_has_storage_dead(local) {
+                // Note: This is only sound if every local that has a `StorageDead` has a
+                // `StorageDead` in every control flow path leading to a `return` terminator.
+                // The good news is that interning will detect if any unexpected mutable
+                // pointer slips through.
+                if place.is_indirect() || self.local_has_storage_dead(place.local) {
                     self.check_op(ops::TransientMutBorrow(kind));
                 } else {
                     self.check_op(ops::MutBorrow(kind));
@@ -390,6 +399,11 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
         trace!("visit_rvalue: rvalue={:?} location={:?}", rvalue, location);
 
         // Special-case reborrows to be more like a copy of a reference.
+        // FIXME: this does not actually handle all reborrows. It only detects cases where `*` is the outermost
+        // projection of the borrowed place, it skips deref'ing raw pointers and it skips `static`.
+        // All those cases are handled below with shared/mutable borrows.
+        // Once `const_mut_refs` is stable, we should be able to entirely remove this special case.
+        // (`const_refs_to_cell` is not needed, we already allow all borrows of indirect places anyway.)
         match *rvalue {
             Rvalue::Ref(_, kind, place) => {
                 if let Some(reborrowed_place_ref) = place_as_reborrow(self.tcx, self.body, place) {
@@ -460,7 +474,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
 
                 if !is_allowed {
                     self.check_mut_borrow(
-                        place.local,
+                        place,
                         if matches!(rvalue, Rvalue::Ref(..)) {
                             hir::BorrowKind::Ref
                         } else {
@@ -478,7 +492,14 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     place.as_ref(),
                 );
 
-                if borrowed_place_has_mut_interior {
+                // If the place is indirect, this is basically a reborrow. We have a reborrow
+                // special case above, but for raw pointers and pointers/references to `static` and
+                // when the `*` is not the first projection, `place_as_reborrow` does not recognize
+                // them as such, so we end up here. This should probably be considered a
+                // `TransientCellBorrow` (we consider the equivalent mutable case a
+                // `TransientMutBorrow`), but such reborrows got accidentally stabilized already and
+                // it is too much of a breaking change to take back.
+                if borrowed_place_has_mut_interior && !place.is_indirect() {
                     match self.const_kind() {
                         // In a const fn all borrows are transient or point to the places given via
                         // references in the arguments (so we already checked them with
@@ -495,6 +516,8 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                             // final value.
                             // Note: This is only sound if every local that has a `StorageDead` has a
                             // `StorageDead` in every control flow path leading to a `return` terminator.
+                            // The good news is that interning will detect if any unexpected mutable
+                            // pointer slips through.
                             if self.local_has_storage_dead(place.local) {
                                 self.check_op(ops::TransientCellBorrow);
                             } else {
@@ -948,6 +971,12 @@ fn place_as_reborrow<'tcx>(
 ) -> Option<PlaceRef<'tcx>> {
     match place.as_ref().last_projection() {
         Some((place_base, ProjectionElem::Deref)) => {
+            // FIXME: why do statics and raw pointers get excluded here? This makes
+            // some code involving mutable pointers unstable, but it is unclear
+            // why that code is treated differently from mutable references.
+            // Once TransientMutBorrow and TransientCellBorrow are stable,
+            // this can probably be cleaned up without any behavioral changes.
+
             // A borrow of a `static` also looks like `&(*_1)` in the MIR, but `_1` is a `const`
             // that points to the allocation for the static. Don't treat these as reborrows.
             if body.local_decls[place_base.local].is_ref_to_static() {

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

@@ -74,6 +74,13 @@ pub trait Qualif {
         adt: AdtDef<'tcx>,
         args: GenericArgsRef<'tcx>,
     ) -> bool;
+
+    /// Returns `true` if this `Qualif` behaves sructurally for pointers and references:
+    /// the pointer/reference qualifies if and only if the pointee qualifies.
+    ///
+    /// (This is currently `false` for all our instances, but that may change in the future. Also,
+    /// by keeping it abstract, the handling of `Deref` in `in_place` becomes more clear.)
+    fn deref_structural<'tcx>(cx: &ConstCx<'_, 'tcx>) -> bool;
 }
 
 /// Constant containing interior mutability (`UnsafeCell<T>`).
@@ -103,6 +110,10 @@ impl Qualif for HasMutInterior {
         // It arises structurally for all other types.
         adt.is_unsafe_cell()
     }
+
+    fn deref_structural<'tcx>(_cx: &ConstCx<'_, 'tcx>) -> bool {
+        false
+    }
 }
 
 /// Constant containing an ADT that implements `Drop`.
@@ -131,6 +142,10 @@ impl Qualif for NeedsDrop {
     ) -> bool {
         adt.has_dtor(cx.tcx)
     }
+
+    fn deref_structural<'tcx>(_cx: &ConstCx<'_, 'tcx>) -> bool {
+        false
+    }
 }
 
 /// Constant containing an ADT that implements non-const `Drop`.
@@ -210,6 +225,10 @@ impl Qualif for NeedsNonConstDrop {
     ) -> bool {
         adt.has_non_const_dtor(cx.tcx)
     }
+
+    fn deref_structural<'tcx>(_cx: &ConstCx<'_, 'tcx>) -> bool {
+        false
+    }
 }
 
 // FIXME: Use `mir::visit::Visitor` for the `in_*` functions if/when it supports early return.
@@ -303,6 +322,11 @@ where
             return false;
         }
 
+        if matches!(elem, ProjectionElem::Deref) && !Q::deref_structural(cx) {
+            // We have to assume that this qualifies.
+            return true;
+        }
+
         place = place_base;
     }
 

tests/ui/consts/const-ref-to-static-linux-vtable.rs

@@ -0,0 +1,43 @@
+// check-pass
+//! This is the reduced version of the "Linux kernel vtable" use-case.
+#![feature(const_mut_refs, const_refs_to_static)]
+use std::ptr::addr_of_mut;
+
+#[repr(C)]
+struct ThisModule(i32);
+
+trait Module {
+    const THIS_MODULE_PTR: *mut ThisModule;
+}
+
+struct MyModule;
+
+// Generated by a macro.
+extern "C" {
+    static mut THIS_MODULE: ThisModule;
+}
+
+// Generated by a macro.
+impl Module for MyModule {
+    const THIS_MODULE_PTR: *mut ThisModule = unsafe { addr_of_mut!(THIS_MODULE) };
+}
+
+struct Vtable {
+    module: *mut ThisModule,
+    foo_fn: fn(*mut ()) -> i32,
+}
+
+trait Foo {
+    type Mod: Module;
+
+    fn foo(&mut self) -> i32;
+}
+
+fn generate_vtable<T: Foo>() -> &'static Vtable {
+    &Vtable {
+        module: T::Mod::THIS_MODULE_PTR,
+        foo_fn: |ptr| unsafe { &mut *ptr.cast::<T>() }.foo(),
+    }
+}
+
+fn main() {}

tests/ui/consts/issue-17718-const-bad-values.rs

@@ -6,5 +6,6 @@ const C1: &'static mut [usize] = &mut [];
 static mut S: usize = 3;
 const C2: &'static mut usize = unsafe { &mut S };
 //~^ ERROR: referencing statics in constants
+//~| ERROR: mutable references are not allowed
 
 fn main() {}

tests/ui/consts/issue-17718-const-bad-values.stderr

@@ -16,7 +16,17 @@ LL | const C2: &'static mut usize = unsafe { &mut S };
    = note: `static` and `const` variables can refer to other `const` variables. A `const` variable, however, cannot refer to a `static` variable.
    = help: to fix this, the value can be extracted to a `const` and then used.
 
-error: aborting due to 2 previous errors
+error[E0658]: mutable references are not allowed in constants
+  --> $DIR/issue-17718-const-bad-values.rs:7:41
+   |
+LL | const C2: &'static mut usize = unsafe { &mut S };
+   |                                         ^^^^^^
+   |
+   = 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
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0658, E0764.
 For more information about an error, try `rustc --explain E0658`.

tests/ui/consts/miri_unleashed/box.stderr

@@ -16,7 +16,7 @@ help: skipping check for `const_mut_refs` feature
    |
 LL |     &mut *(Box::new(0))
    |     ^^^^^^^^^^^^^^^^^^^
-help: skipping check that does not even have a feature gate
+help: skipping check for `const_mut_refs` feature
   --> $DIR/box.rs:8:5
    |
 LL |     &mut *(Box::new(0))

tests/ui/consts/miri_unleashed/mutable_references_err.32bit.stderr

@@ -114,7 +114,7 @@ help: skipping check for `const_refs_to_static` feature
    |
 LL | const SUBTLE: &mut i32 = unsafe { &mut FOO };
    |                                        ^^^
-help: skipping check that does not even have a feature gate
+help: skipping check for `const_mut_refs` feature
   --> $DIR/mutable_references_err.rs:32:35
    |
 LL | const SUBTLE: &mut i32 = unsafe { &mut FOO };

tests/ui/consts/miri_unleashed/mutable_references_err.64bit.stderr

@@ -114,7 +114,7 @@ help: skipping check for `const_refs_to_static` feature
    |
 LL | const SUBTLE: &mut i32 = unsafe { &mut FOO };
    |                                        ^^^
-help: skipping check that does not even have a feature gate
+help: skipping check for `const_mut_refs` feature
   --> $DIR/mutable_references_err.rs:32:35
    |
 LL | const SUBTLE: &mut i32 = unsafe { &mut FOO };

tests/ui/consts/mut-ptr-to-static.rs

@@ -0,0 +1,40 @@
+// run-pass
+#![feature(const_mut_refs)]
+#![feature(sync_unsafe_cell)]
+
+use std::cell::SyncUnsafeCell;
+use std::ptr;
+
+#[repr(C)]
+struct SyncPtr {
+    foo: *mut u32,
+}
+unsafe impl Sync for SyncPtr {}
+
+static mut STATIC: u32 = 42;
+
+static INTERIOR_MUTABLE_STATIC: SyncUnsafeCell<u32> = SyncUnsafeCell::new(42);
+
+// A static that mutably points to STATIC.
+static PTR: SyncPtr = SyncPtr {
+    foo: unsafe { ptr::addr_of_mut!(STATIC) },
+};
+static INTERIOR_MUTABLE_PTR: SyncPtr = SyncPtr {
+    foo: ptr::addr_of!(INTERIOR_MUTABLE_STATIC) as *mut u32,
+};
+
+fn main() {
+    let ptr = PTR.foo;
+    unsafe {
+        assert_eq!(*ptr, 42);
+        *ptr = 0;
+        assert_eq!(*PTR.foo, 0);
+    }
+
+    let ptr = INTERIOR_MUTABLE_PTR.foo;
+    unsafe {
+        assert_eq!(*ptr, 42);
+        *ptr = 0;
+        assert_eq!(*INTERIOR_MUTABLE_PTR.foo, 0);
+    }
+}

tests/ui/error-codes/E0017.rs

@@ -1,18 +1,18 @@
+#![feature(const_mut_refs)]
+
 static X: i32 = 1;
 const C: i32 = 2;
 static mut M: i32 = 3;
 
 const CR: &'static mut i32 = &mut C; //~ ERROR mutable references are not allowed
 //~| WARN taking a mutable
 
-static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0658
-//~| ERROR cannot borrow
-//~| ERROR mutable references are not allowed
+static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR cannot borrow immutable static item `X` as mutable
 
 static CONST_REF: &'static mut i32 = &mut C; //~ ERROR mutable references are not allowed
 //~| WARN taking a mutable
 
-static STATIC_MUT_REF: &'static mut i32 = unsafe { &mut M }; //~ ERROR mutable references are not
+static STATIC_MUT_REF: &'static mut i32 = unsafe { &mut M };
 //~^ WARN mutable reference of mutable static is discouraged [static_mut_ref]
 
 fn main() {}

tests/ui/error-codes/E0017.stderr

@@ -14,44 +14,28 @@ LL | static STATIC_MUT_REF: &'static mut i32 = unsafe { addr_of_mut!(M) };
    |                                                    ~~~~~~~~~~~~~~~
 
 warning: taking a mutable reference to a `const` item
-  --> $DIR/E0017.rs:5:30
+  --> $DIR/E0017.rs:7:30
    |
 LL | const CR: &'static mut i32 = &mut C;
    |                              ^^^^^^
    |
    = note: each usage of a `const` item creates a new temporary
    = note: the mutable reference will refer to this temporary, not the original `const` item
 note: `const` item defined here
-  --> $DIR/E0017.rs:2:1
+  --> $DIR/E0017.rs:4:1
    |
 LL | const C: i32 = 2;
    | ^^^^^^^^^^^^
    = note: `#[warn(const_item_mutation)]` on by default
 
 error[E0764]: mutable references are not allowed in the final value of constants
-  --> $DIR/E0017.rs:5:30
+  --> $DIR/E0017.rs:7:30
    |
 LL | const CR: &'static mut i32 = &mut C;
    |                              ^^^^^^
 
-error[E0658]: mutation through a reference is not allowed in statics
-  --> $DIR/E0017.rs:8:39
-   |
-LL | static STATIC_REF: &'static mut i32 = &mut X;
-   |                                       ^^^^^^
-   |
-   = 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
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-
-error[E0764]: mutable references are not allowed in the final value of statics
-  --> $DIR/E0017.rs:8:39
-   |
-LL | static STATIC_REF: &'static mut i32 = &mut X;
-   |                                       ^^^^^^
-
 error[E0596]: cannot borrow immutable static item `X` as mutable
-  --> $DIR/E0017.rs:8:39
+  --> $DIR/E0017.rs:10:39
    |
 LL | static STATIC_REF: &'static mut i32 = &mut X;
    |                                       ^^^^^^ cannot borrow as mutable
@@ -65,7 +49,7 @@ LL | static CONST_REF: &'static mut i32 = &mut C;
    = note: each usage of a `const` item creates a new temporary
    = note: the mutable reference will refer to this temporary, not the original `const` item
 note: `const` item defined here
-  --> $DIR/E0017.rs:2:1
+  --> $DIR/E0017.rs:4:1
    |
 LL | const C: i32 = 2;
    | ^^^^^^^^^^^^
@@ -76,13 +60,7 @@ error[E0764]: mutable references are not allowed in the final value of statics
 LL | static CONST_REF: &'static mut i32 = &mut C;
    |                                      ^^^^^^
 
-error[E0764]: mutable references are not allowed in the final value of statics
-  --> $DIR/E0017.rs:15:52
-   |
-LL | static STATIC_MUT_REF: &'static mut i32 = unsafe { &mut M };
-   |                                                    ^^^^^^
-
-error: aborting due to 6 previous errors; 3 warnings emitted
+error: aborting due to 3 previous errors; 3 warnings emitted
 
-Some errors have detailed explanations: E0596, E0658, E0764.
+Some errors have detailed explanations: E0596, E0764.
 For more information about an error, try `rustc --explain E0596`.

tests/ui/error-codes/E0388.rs

@@ -3,9 +3,7 @@ const C: i32 = 2;
 
 const CR: &'static mut i32 = &mut C; //~ ERROR mutable references are not allowed
                                      //~| WARN taking a mutable
-static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR cannot borrow
-                                              //~| ERROR E0658
-                                              //~| ERROR mutable references are not allowed
+static STATIC_REF: &'static mut i32 = &mut X; //~ ERROR E0658
 
 static CONST_REF: &'static mut i32 = &mut C; //~ ERROR mutable references are not allowed
                                              //~| WARN taking a mutable