handle Box with non-ZST allocator

Author: RalfJung

src/stacked_borrows.rs

@@ -1,18 +1,19 @@
 //! Implements "Stacked Borrows".  See <https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md>
 //! for further information.
 
-use log::trace;
 use std::cell::RefCell;
 use std::cmp;
 use std::fmt;
 use std::num::NonZeroU64;
 
+use log::trace;
+
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::Mutability;
 use rustc_middle::mir::RetagKind;
 use rustc_middle::ty::{
     self,
-    layout::{HasParamEnv, LayoutOf},
+    layout::{HasParamEnv, LayoutOf, TyAndLayout},
 };
 use rustc_span::DUMMY_SP;
 use rustc_target::abi::Size;
@@ -1086,8 +1087,8 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
         // Determine mutability and whether to add a protector.
         // Cannot use `builtin_deref` because that reports *immutable* for `Box`,
         // making it useless.
-        fn qualify(ty: ty::Ty<'_>, kind: RetagKind) -> Option<(RefKind, bool)> {
-            match ty.kind() {
+        let qualify = |layout: TyAndLayout<'tcx>, kind: RetagKind| -> Option<(RefKind, bool)> {
+            match layout.ty.kind() {
                 // References are simple.
                 ty::Ref(_, _, Mutability::Mut) =>
                     Some((
@@ -1101,15 +1102,18 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                     Some((RefKind::Raw { mutable: tym.mutbl == Mutability::Mut }, false)),
                 // Boxes do not get a protector: protectors reflect that references outlive the call
                 // they were passed in to; that's just not the case for boxes.
-                ty::Adt(..) if ty.is_box() => Some((RefKind::Unique { two_phase: false }, false)),
+                // HACK: We only treat boxes with ZST allocators as 'noalias'.
+                // See https://github.com/rust-lang/rust/issues/95453.
+                ty::Adt(..) if layout.ty.is_box() && layout.field(this, 1).is_zst() =>
+                    Some((RefKind::Unique { two_phase: false }, false)),
                 _ => None,
             }
-        }
+        };
 
         // We only reborrow "bare" references/boxes.
         // Not traversing into fields helps with <https://github.com/rust-lang/unsafe-code-guidelines/issues/125>,
         // but might also cost us optimization and analyses. We will have to experiment more with this.
-        if let Some((mutbl, protector)) = qualify(place.layout.ty, kind) {
+        if let Some((mutbl, protector)) = qualify(place.layout, kind) {
             // Fast path.
             let val = this.read_immediate(&this.place_to_op(place)?)?;
             let val = this.retag_reference(&val, mutbl, protector)?;

tests/pass/issue-95453.rs

@@ -0,0 +1,78 @@
+#![feature(allocator_api)]
+
+use std::alloc::{AllocError, Allocator};
+use std::alloc::Layout;
+use std::cell::Cell;
+use std::mem::MaybeUninit;
+use std::ptr::{self, NonNull};
+
+struct OnceAlloc<'a> {
+    space: Cell<&'a mut [MaybeUninit<u8>]>,
+}
+
+unsafe impl<'shared, 'a: 'shared> Allocator for &'shared OnceAlloc<'a> {
+    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
+        let space = self.space.replace(&mut []);
+
+        let (ptr, len) = (space.as_mut_ptr(), space.len());
+
+        if ptr.align_offset(layout.align()) != 0 || len < layout.size() {
+            return Err(AllocError);
+        }
+
+        let slice_ptr = ptr::slice_from_raw_parts_mut(ptr as *mut u8, len);
+        unsafe { Ok(NonNull::new_unchecked(slice_ptr)) }
+    }
+
+    unsafe fn deallocate(&self, _ptr: NonNull<u8>, _layout: Layout) {}
+}
+
+trait MyTrait {
+    fn hello(&self) -> u8;
+}
+
+impl MyTrait for [u8; 1] {
+    fn hello(&self) -> u8 {
+        self[0]
+    }
+}
+
+/// `Box<T, G>` is a `ScalarPair` where the 2nd component is the allocator.
+fn test1() {
+    let mut space = vec![MaybeUninit::new(0); 1];
+    let once_alloc = OnceAlloc {
+        space: Cell::new(&mut space[..]),
+    };
+
+    let boxed = Box::new_in([42u8; 1], &once_alloc);
+    let _val = *boxed;
+}
+
+// Make the allocator itself so big that the Box is not even a ScalarPair any more.
+struct OnceAllocRef<'s, 'a>(&'s OnceAlloc<'a>, u64);
+
+unsafe impl<'shared, 'a: 'shared> Allocator for OnceAllocRef<'shared, 'a> {
+    fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
+        self.0.allocate(layout)
+    }
+
+    unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
+        self.0.deallocate(ptr, layout)
+    }
+}
+
+/// `Box<T, G>` is an `Aggregate`.
+fn test2() {
+    let mut space = vec![MaybeUninit::new(0); 1];
+    let once_alloc = OnceAlloc {
+        space: Cell::new(&mut space[..]),
+    };
+
+    let boxed = Box::new_in([0u8; 1], OnceAllocRef(&once_alloc, 0));
+    let _val = *boxed;
+}
+
+fn main() {
+    test1();
+    test2();
+}