Auto merge of #97027 - cuviper:yesalias-refcell, r=thomcc

Use pointers in `cell::{Ref,RefMut}` to avoid `noalias`

When `Ref` and `RefMut` were based on references, they would get LLVM `noalias` attributes that were incorrect, because that alias guarantee is only true until the guard drops. A `&RefCell` on the same value can get a new borrow that aliases the previous guard, possibly leading to miscompilation. Using `NonNull` pointers in `Ref` and `RefCell` avoids `noalias`.

Fixes the library side of #63787, but we still might want to explore language solutions there.

Author: bors

library/core/src/cell.rs

@@ -194,10 +194,10 @@
 
 use crate::cmp::Ordering;
 use crate::fmt::{self, Debug, Display};
-use crate::marker::Unsize;
+use crate::marker::{PhantomData, Unsize};
 use crate::mem;
 use crate::ops::{CoerceUnsized, Deref, DerefMut};
-use crate::ptr;
+use crate::ptr::{self, NonNull};
 
 /// A mutable memory location.
 ///
@@ -896,7 +896,8 @@ impl<T: ?Sized> RefCell<T> {
 
                 // SAFETY: `BorrowRef` ensures that there is only immutable access
                 // to the value while borrowed.
-                Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b })
+                let value = unsafe { NonNull::new_unchecked(self.value.get()) };
+                Ok(Ref { value, borrow: b })
             }
             None => Err(BorrowError {
                 // If a borrow occurred, then we must already have an outstanding borrow,
@@ -980,8 +981,9 @@ impl<T: ?Sized> RefCell<T> {
                     self.borrowed_at.set(Some(crate::panic::Location::caller()));
                 }
 
-                // SAFETY: `BorrowRef` guarantees unique access.
-                Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b })
+                // SAFETY: `BorrowRefMut` guarantees unique access.
+                let value = unsafe { NonNull::new_unchecked(self.value.get()) };
+                Ok(RefMut { value, borrow: b, marker: PhantomData })
             }
             None => Err(BorrowMutError {
                 // If a borrow occurred, then we must already have an outstanding borrow,
@@ -1314,7 +1316,10 @@ impl Clone for BorrowRef<'_> {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"]
 pub struct Ref<'b, T: ?Sized + 'b> {
-    value: &'b T,
+    // NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a
+    // `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
+    // `NonNull` is also covariant over `T`, just like we would have with `&T`.
+    value: NonNull<T>,
     borrow: BorrowRef<'b>,
 }
 
@@ -1324,7 +1329,8 @@ impl<T: ?Sized> Deref for Ref<'_, T> {
 
     #[inline]
     fn deref(&self) -> &T {
-        self.value
+        // SAFETY: the value is accessible as long as we hold our borrow.
+        unsafe { self.value.as_ref() }
     }
 }
 
@@ -1368,7 +1374,7 @@ impl<'b, T: ?Sized> Ref<'b, T> {
     where
         F: FnOnce(&T) -> &U,
     {
-        Ref { value: f(orig.value), borrow: orig.borrow }
+        Ref { value: NonNull::from(f(&*orig)), borrow: orig.borrow }
     }
 
     /// Makes a new `Ref` for an optional component of the borrowed data. The
@@ -1399,8 +1405,8 @@ impl<'b, T: ?Sized> Ref<'b, T> {
     where
         F: FnOnce(&T) -> Option<&U>,
     {
-        match f(orig.value) {
-            Some(value) => Ok(Ref { value, borrow: orig.borrow }),
+        match f(&*orig) {
+            Some(value) => Ok(Ref { value: NonNull::from(value), borrow: orig.borrow }),
             None => Err(orig),
         }
     }
@@ -1431,9 +1437,12 @@ impl<'b, T: ?Sized> Ref<'b, T> {
     where
         F: FnOnce(&T) -> (&U, &V),
     {
-        let (a, b) = f(orig.value);
+        let (a, b) = f(&*orig);
         let borrow = orig.borrow.clone();
-        (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
+        (
+            Ref { value: NonNull::from(a), borrow },
+            Ref { value: NonNull::from(b), borrow: orig.borrow },
+        )
     }
 
     /// Convert into a reference to the underlying data.
@@ -1467,7 +1476,8 @@ impl<'b, T: ?Sized> Ref<'b, T> {
         // unique reference to the borrowed RefCell. No further mutable references can be created
         // from the original cell.
         mem::forget(orig.borrow);
-        orig.value
+        // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
+        unsafe { orig.value.as_ref() }
     }
 }
 
@@ -1507,13 +1517,12 @@ impl<'b, T: ?Sized> RefMut<'b, T> {
     /// ```
     #[stable(feature = "cell_map", since = "1.8.0")]
     #[inline]
-    pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
+    pub fn map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
     where
         F: FnOnce(&mut T) -> &mut U,
     {
-        // FIXME(nll-rfc#40): fix borrow-check
-        let RefMut { value, borrow } = orig;
-        RefMut { value: f(value), borrow }
+        let value = NonNull::from(f(&mut *orig));
+        RefMut { value, borrow: orig.borrow, marker: PhantomData }
     }
 
     /// Makes a new `RefMut` for an optional component of the borrowed data. The
@@ -1548,23 +1557,19 @@ impl<'b, T: ?Sized> RefMut<'b, T> {
     /// ```
     #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
     #[inline]
-    pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
+    pub fn filter_map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
     where
         F: FnOnce(&mut T) -> Option<&mut U>,
     {
-        // FIXME(nll-rfc#40): fix borrow-check
-        let RefMut { value, borrow } = orig;
-        let value = value as *mut T;
         // SAFETY: function holds onto an exclusive reference for the duration
         // of its call through `orig`, and the pointer is only de-referenced
         // inside of the function call never allowing the exclusive reference to
         // escape.
-        match f(unsafe { &mut *value }) {
-            Some(value) => Ok(RefMut { value, borrow }),
-            None => {
-                // SAFETY: same as above.
-                Err(RefMut { value: unsafe { &mut *value }, borrow })
+        match f(&mut *orig) {
+            Some(value) => {
+                Ok(RefMut { value: NonNull::from(value), borrow: orig.borrow, marker: PhantomData })
             }
+            None => Err(orig),
         }
     }
 
@@ -1596,15 +1601,18 @@ impl<'b, T: ?Sized> RefMut<'b, T> {
     #[stable(feature = "refcell_map_split", since = "1.35.0")]
     #[inline]
     pub fn map_split<U: ?Sized, V: ?Sized, F>(
-        orig: RefMut<'b, T>,
+        mut orig: RefMut<'b, T>,
         f: F,
     ) -> (RefMut<'b, U>, RefMut<'b, V>)
     where
         F: FnOnce(&mut T) -> (&mut U, &mut V),
     {
-        let (a, b) = f(orig.value);
         let borrow = orig.borrow.clone();
-        (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
+        let (a, b) = f(&mut *orig);
+        (
+            RefMut { value: NonNull::from(a), borrow, marker: PhantomData },
+            RefMut { value: NonNull::from(b), borrow: orig.borrow, marker: PhantomData },
+        )
     }
 
     /// Convert into a mutable reference to the underlying data.
@@ -1630,14 +1638,15 @@ impl<'b, T: ?Sized> RefMut<'b, T> {
     /// assert!(cell.try_borrow_mut().is_err());
     /// ```
     #[unstable(feature = "cell_leak", issue = "69099")]
-    pub fn leak(orig: RefMut<'b, T>) -> &'b mut T {
+    pub fn leak(mut orig: RefMut<'b, T>) -> &'b mut T {
         // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
         // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
         // require a unique reference to the borrowed RefCell. No further references can be created
         // from the original cell within that lifetime, making the current borrow the only
         // reference for the remaining lifetime.
         mem::forget(orig.borrow);
-        orig.value
+        // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
+        unsafe { orig.value.as_mut() }
     }
 }
 
@@ -1692,8 +1701,12 @@ impl<'b> BorrowRefMut<'b> {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"]
 pub struct RefMut<'b, T: ?Sized + 'b> {
-    value: &'b mut T,
+    // NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
+    // `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.
+    value: NonNull<T>,
     borrow: BorrowRefMut<'b>,
+    // `NonNull` is covariant over `T`, so we need to reintroduce invariance.
+    marker: PhantomData<&'b mut T>,
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1702,15 +1715,17 @@ impl<T: ?Sized> Deref for RefMut<'_, T> {
 
     #[inline]
     fn deref(&self) -> &T {
-        self.value
+        // SAFETY: the value is accessible as long as we hold our borrow.
+        unsafe { self.value.as_ref() }
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
     #[inline]
     fn deref_mut(&mut self) -> &mut T {
-        self.value
+        // SAFETY: the value is accessible as long as we hold our borrow.
+        unsafe { self.value.as_mut() }
     }
 }
 

src/etc/natvis/libcore.natvis

@@ -7,15 +7,15 @@
     </Expand>
   </Type>
   <Type Name="core::cell::Ref&lt;*&gt;">
-    <DisplayString>{value}</DisplayString>
+    <DisplayString>{value.pointer}</DisplayString>
     <Expand>
-      <ExpandedItem>value</ExpandedItem>
+      <ExpandedItem>value.pointer</ExpandedItem>
     </Expand>
   </Type>
   <Type Name="core::cell::RefMut&lt;*&gt;">
-    <DisplayString>{value}</DisplayString>
+    <DisplayString>{value.pointer}</DisplayString>
     <Expand>
-      <ExpandedItem>value</ExpandedItem>
+      <ExpandedItem>value.pointer</ExpandedItem>
     </Expand>
   </Type>
   <Type Name="core::cell::RefCell&lt;*&gt;">

src/test/codegen/noalias-refcell.rs

@@ -0,0 +1,14 @@
+// compile-flags: -O -C no-prepopulate-passes -Z mutable-noalias=yes
+
+#![crate_type = "lib"]
+
+use std::cell::{Ref, RefCell, RefMut};
+
+// Make sure that none of the arguments get a `noalias` attribute, because
+// the `RefCell` might alias writes after either `Ref`/`RefMut` is dropped.
+
+// CHECK-LABEL: @maybe_aliased(
+// CHECK-NOT: noalias
+// CHECK-SAME: %_refcell
+#[no_mangle]
+pub unsafe fn maybe_aliased(_: Ref<'_, i32>, _: RefMut<'_, i32>, _refcell: &RefCell<i32>) {}

src/test/ui/issues/issue-63787.rs

@@ -0,0 +1,36 @@
+// run-pass
+// compile-flags: -O
+
+// Make sure that `Ref` and `RefMut` do not make false promises about aliasing,
+// because once they drop, their reference/pointer can alias other writes.
+
+// Adapted from comex's proof of concept:
+// https://github.com/rust-lang/rust/issues/63787#issuecomment-523588164
+
+use std::cell::RefCell;
+use std::ops::Deref;
+
+pub fn break_if_r_is_noalias(rc: &RefCell<i32>, r: impl Deref<Target = i32>) -> i32 {
+    let ptr1 = &*r as *const i32;
+    let a = *r;
+    drop(r);
+    *rc.borrow_mut() = 2;
+    let r2 = rc.borrow();
+    let ptr2 = &*r2 as *const i32;
+    if ptr2 != ptr1 {
+        panic!();
+    }
+    // If LLVM knows the pointers are the same, and if `r` was `noalias`,
+    // then it may replace this with `a + a`, ignoring the earlier write.
+    a + *r2
+}
+
+fn main() {
+    let mut rc = RefCell::new(1);
+    let res = break_if_r_is_noalias(&rc, rc.borrow());
+    assert_eq!(res, 3);
+
+    *rc.get_mut() = 1;
+    let res = break_if_r_is_noalias(&rc, rc.borrow_mut());
+    assert_eq!(res, 3);
+}