make &mut !Unpin not dereferenceable

See rust-lang/unsafe-code-guidelines#381 for discussion.

compiler/rustc_ty_utils/src/abi.rs

@@ -256,13 +256,16 @@ fn adjust_for_rust_scalar<'tcx>(
 
             // `Box` are not necessarily dereferenceable for the entire duration of the function as
             // they can be deallocated at any time. Same for non-frozen shared references (see
-            // <https://github.com/rust-lang/rust/pull/98017>). If LLVM had a way to say
-            // "dereferenceable on entry" we could use it here.
+            // <https://github.com/rust-lang/rust/pull/98017>), and for mutable references to
+            // potentially self-referential types (see
+            // <https://github.com/rust-lang/unsafe-code-guidelines/issues/381>). If LLVM had a way
+            // to say "dereferenceable on entry" we could use it here.
             attrs.pointee_size = match kind {
-                PointerKind::Box | PointerKind::SharedRef { frozen: false } => Size::ZERO,
-                PointerKind::SharedRef { frozen: true } | PointerKind::MutableRef { .. } => {
-                    pointee.size
-                }
+                PointerKind::Box
+                | PointerKind::SharedRef { frozen: false }
+                | PointerKind::MutableRef { unpin: false } => Size::ZERO,
+                PointerKind::SharedRef { frozen: true }
+                | PointerKind::MutableRef { unpin: true } => pointee.size,
             };
 
             // The aliasing rules for `Box<T>` are still not decided, but currently we emit

src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs

@@ -81,21 +81,18 @@ impl NewPermission {
                         protector: None,
                     }
                 } else if pointee.is_unpin(*cx.tcx, cx.param_env()) {
-                    // A regular full mutable reference.
+                    // A regular full mutable reference. On `FnEntry` this is `noalias` and `dereferenceable`.
                     NewPermission::Uniform {
                         perm: Permission::Unique,
                         access: Some(AccessKind::Write),
                         protector,
                     }
                 } else {
+                    // `!Unpin` dereferences do not get `noalias` nor `dereferenceable`.
                     NewPermission::Uniform {
                         perm: Permission::SharedReadWrite,
-                        // FIXME: We emit `dereferenceable` for `!Unpin` mutable references, so we
-                        // should do fake accesses here. But then we run into
-                        // <https://github.com/rust-lang/unsafe-code-guidelines/issues/381>, so for now
-                        // we don't do that.
                         access: None,
-                        protector,
+                        protector: None,
                     }
                 }
             }
@@ -109,6 +106,7 @@ impl NewPermission {
                 }
             }
             ty::Ref(_, _pointee, Mutability::Not) => {
+                // Shared references. If frozen, these get `noalias` and `dereferenceable`; otherwise neither.
                 NewPermission::FreezeSensitive {
                     freeze_perm: Permission::SharedReadOnly,
                     freeze_access: Some(AccessKind::Read),

src/tools/miri/tests/pass/stacked-borrows/stacked-borrows.rs

@@ -19,6 +19,7 @@ fn main() {
     array_casts();
     mut_below_shr();
     wide_raw_ptr_in_tuple();
+    not_unpin_not_protected();
 }
 
 // Make sure that reading from an `&mut` does, like reborrowing to `&`,
@@ -219,3 +220,22 @@ fn wide_raw_ptr_in_tuple() {
     // Make sure the fn ptr part of the vtable is still fine.
     r.type_id();
 }
+
+fn not_unpin_not_protected() {
+    // `&mut !Unpin`, at least for now, does not get `noalias` nor `dereferenceable`, so we also
+    // don't add protectors. (We could, but until we have a better idea for where we want to go with
+    // the self-referntial-generator situation, it does not seem worth the potential trouble.)
+    use std::marker::PhantomPinned;
+
+    pub struct NotUnpin(i32, PhantomPinned);
+
+    fn inner(x: &mut NotUnpin, f: fn(&mut NotUnpin)) {
+        // `f` may mutate, but it may not deallocate!
+        f(x)
+    }
+
+    inner(Box::leak(Box::new(NotUnpin(0, PhantomPinned))), |x| {
+        let raw = x as *mut _;
+        drop(unsafe { Box::from_raw(raw) });
+    });
+}

tests/codegen/function-arguments.rs

@@ -85,6 +85,12 @@ pub fn option_nonzero_int(x: Option<NonZeroU64>) -> Option<NonZeroU64> {
 pub fn readonly_borrow(_: &i32) {
 }
 
+// CHECK: noundef align 4 dereferenceable(4) {{i32\*|ptr}} @readonly_borrow_ret()
+#[no_mangle]
+pub fn readonly_borrow_ret() -> &'static i32 {
+  loop {}
+}
+
 // CHECK: @static_borrow({{i32\*|ptr}} noalias noundef readonly align 4 dereferenceable(4) %_1)
 // static borrow may be captured
 #[no_mangle]
@@ -115,9 +121,17 @@ pub fn mutable_unsafe_borrow(_: &mut UnsafeInner) {
 pub fn mutable_borrow(_: &mut i32) {
 }
 
+// CHECK: noundef align 4 dereferenceable(4) {{i32\*|ptr}} @mutable_borrow_ret()
+#[no_mangle]
+pub fn mutable_borrow_ret() -> &'static mut i32 {
+  loop {}
+}
+
 #[no_mangle]
-// CHECK: @mutable_notunpin_borrow({{i32\*|ptr}} noundef align 4 dereferenceable(4) %_1)
+// CHECK: @mutable_notunpin_borrow({{i32\*|ptr}} noundef nonnull align 4 %_1)
 // This one is *not* `noalias` because it might be self-referential.
+// It is also not `dereferenceable` due to
+// <https://github.com/rust-lang/unsafe-code-guidelines/issues/381>.
 pub fn mutable_notunpin_borrow(_: &mut NotUnpin) {
 }