RawVecInner: add missing unsafe to unsafe fns

- RawVecInner::grow_exact causes UB if called with len and additional arguments such that len + additional is less than the current capacity. Indeed, in that case it calls Allocator::grow with a new_layout that is smaller than old_layout, which violates a safety precondition. - All RawVecInner methods for resizing the buffer cause UB if called with an elem_layout different from the one used to initially allocate the buffer, because in that case Allocator::grow/shrink is called with an old_layout that does not fit the allocated block, which violates a safety precondition. - RawVecInner::current_memory might cause UB if called with an elem_layout different from the one used to initially allocate the buffer, because the unchecked_mul might overflow. - Furthermore, these methods cause UB if called with an elem_layout where the size is not a multiple of the alignment. This is because Layout::repeat is used (in layout_array) to compute the allocation's layout when allocating, which includes padding to ensure alignment of array elements, but simple multiplication is used (in current_memory) to compute the old allocation's layout when resizing or deallocating, which would cause the layout used to resize or deallocate to not fit the allocated block, which violates a safety precondition.
rust-lang · bors · Sep 25, 2025 · Aug 7, 2025 · Sep 18, 2025 · Sep 22, 2025
commit 96f385b20aa252629570cadeca16a9e159e6810c
diff --git a/library/alloc/src/raw_vec/mod.rs b/library/alloc/src/raw_vec/mod.rs
@@ -177,6 +177,8 @@ impl<T, A: Allocator> RawVec<T, A> {
     /// the returned `RawVec`.
     #[inline]
     pub(crate) const fn new_in(alloc: A) -> Self {
+        // Check assumption made in `current_memory`
+        const { assert!(T::LAYOUT.size() % T::LAYOUT.align() == 0) };
         Self { inner: RawVecInner::new_in(alloc, Alignment::of::<T>()), _marker: PhantomData }
     }
 
@@ -328,7 +330,8 @@ impl<T, A: Allocator> RawVec<T, A> {
     #[inline]
     #[track_caller]
     pub(crate) fn reserve(&mut self, len: usize, additional: usize) {
-        self.inner.reserve(len, additional, T::LAYOUT)
+        // SAFETY: All calls on self.inner pass T::LAYOUT as the elem_layout
+        unsafe { self.inner.reserve(len, additional, T::LAYOUT) }
     }
 
     /// A specialized version of `self.reserve(len, 1)` which requires the
@@ -337,7 +340,8 @@ impl<T, A: Allocator> RawVec<T, A> {
     #[inline(never)]
     #[track_caller]
     pub(crate) fn grow_one(&mut self) {
-        self.inner.grow_one(T::LAYOUT)
+        // SAFETY: All calls on self.inner pass T::LAYOUT as the elem_layout
+        unsafe { self.inner.grow_one(T::LAYOUT) }
     }
 
     /// The same as `reserve`, but returns on errors instead of panicking or aborting.
@@ -346,7 +350,8 @@ impl<T, A: Allocator> RawVec<T, A> {
         len: usize,
         additional: usize,
     ) -> Result<(), TryReserveError> {
-        self.inner.try_reserve(len, additional, T::LAYOUT)
+        // SAFETY: All calls on self.inner pass T::LAYOUT as the elem_layout
     }
 
     /// Ensures that the buffer contains at least enough space to hold `len +
@@ -369,7 +374,8 @@ impl<T, A: Allocator> RawVec<T, A> {
     #[cfg(not(no_global_oom_handling))]
     #[track_caller]
     pub(crate) fn reserve_exact(&mut self, len: usize, additional: usize) {
-        self.inner.reserve_exact(len, additional, T::LAYOUT)
+        // SAFETY: All calls on self.inner pass T::LAYOUT as the elem_layout
+        unsafe { self.inner.reserve_exact(len, additional, T::LAYOUT) }
     }
 
     /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
@@ -378,7 +384,8 @@ impl<T, A: Allocator> RawVec<T, A> {
         len: usize,
         additional: usize,
     ) -> Result<(), TryReserveError> {
-        self.inner.try_reserve_exact(len, additional, T::LAYOUT)
+        // SAFETY: All calls on self.inner pass T::LAYOUT as the elem_layout
+        unsafe { self.inner.try_reserve_exact(len, additional, T::LAYOUT) }
     }
 
     /// Shrinks the buffer down to the specified capacity. If the given amount
@@ -395,7 +402,8 @@ impl<T, A: Allocator> RawVec<T, A> {
     #[track_caller]
     #[inline]
     pub(crate) fn shrink_to_fit(&mut self, cap: usize) {
-        self.inner.shrink_to_fit(cap, T::LAYOUT)
+        // SAFETY: All calls on self.inner pass T::LAYOUT as the elem_layout
+        unsafe { self.inner.shrink_to_fit(cap, T::LAYOUT) }
     }
 }
 
@@ -518,8 +526,12 @@ impl<A: Allocator> RawVecInner<A> {
         &self.alloc
     }
 
+    /// # Safety
+    /// - `elem_layout` must be valid for `self`, i.e. it must be the same `elem_layout` used to
+    ///   initially construct `self`
+    /// - `elem_layout`'s size must be a multiple of its alignment
     #[inline]
-    fn current_memory(&self, elem_layout: Layout) -> Option<(NonNull<u8>, Layout)> {
+    unsafe fn current_memory(&self, elem_layout: Layout) -> Option<(NonNull<u8>, Layout)> {
         if elem_layout.size() == 0 || self.cap.as_inner() == 0 {
             None
         } else {
@@ -535,48 +547,67 @@ impl<A: Allocator> RawVecInner<A> {
         }
     }
 
+    /// # Safety
+    /// - `elem_layout` must be valid for `self`, i.e. it must be the same `elem_layout` used to
+    ///   initially construct `self`
+    /// - `elem_layout`'s size must be a multiple of its alignment
     #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[track_caller]
-    fn reserve(&mut self, len: usize, additional: usize, elem_layout: Layout) {
+    unsafe fn reserve(&mut self, len: usize, additional: usize, elem_layout: Layout) {
         // Callers expect this function to be very cheap when there is already sufficient capacity.
         // Therefore, we move all the resizing and error-handling logic from grow_amortized and
         // handle_reserve behind a call, while making sure that this function is likely to be
         // inlined as just a comparison and a call if the comparison fails.
         #[cold]
-        fn do_reserve_and_handle<A: Allocator>(
+        unsafe fn do_reserve_and_handle<A: Allocator>(
             slf: &mut RawVecInner<A>,
             len: usize,
             additional: usize,
             elem_layout: Layout,
         ) {
-            if let Err(err) = slf.grow_amortized(len, additional, elem_layout) {
+            // SAFETY: Precondition passed to caller
+            if let Err(err) = unsafe { slf.grow_amortized(len, additional, elem_layout) } {
                 handle_error(err);
             }
         }
 
         if self.needs_to_grow(len, additional, elem_layout) {
-            do_reserve_and_handle(self, len, additional, elem_layout);
+            unsafe {
+                do_reserve_and_handle(self, len, additional, elem_layout);
+            }
         }
     }
 
+    /// # Safety
+    /// - `elem_layout` must be valid for `self`, i.e. it must be the same `elem_layout` used to
+    ///   initially construct `self`
+    /// - `elem_layout`'s size must be a multiple of its alignment
     #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[track_caller]
-    fn grow_one(&mut self, elem_layout: Layout) {
-        if let Err(err) = self.grow_amortized(self.cap.as_inner(), 1, elem_layout) {
+    unsafe fn grow_one(&mut self, elem_layout: Layout) {
+        // SAFETY: Precondition passed to caller
+        if let Err(err) = unsafe { self.grow_amortized(self.cap.as_inner(), 1, elem_layout) } {
             handle_error(err);
         }
     }
 
-    fn try_reserve(
+    /// # Safety
+    /// - `elem_layout` must be valid for `self`, i.e. it must be the same `elem_layout` used to
+    ///   initially construct `self`
+    /// - `elem_layout`'s size must be a multiple of its alignment
+    unsafe fn try_reserve(
         &mut self,
         len: usize,
         additional: usize,
         elem_layout: Layout,
     ) -> Result<(), TryReserveError> {
         if self.needs_to_grow(len, additional, elem_layout) {
-            self.grow_amortized(len, additional, elem_layout)?;
+            // SAFETY: Precondition passed to caller
+            unsafe {
+                self.grow_amortized(len, additional, elem_layout)?;
+            }
         }
         unsafe {
             // Inform the optimizer that the reservation has succeeded or wasn't needed
@@ -585,22 +616,34 @@ impl<A: Allocator> RawVecInner<A> {
         Ok(())
     }
 
+    /// # Safety
+    /// - `elem_layout` must be valid for `self`, i.e. it must be the same `elem_layout` used to
+    ///   initially construct `self`
+    /// - `elem_layout`'s size must be a multiple of its alignment
     #[cfg(not(no_global_oom_handling))]
     #[track_caller]
-    fn reserve_exact(&mut self, len: usize, additional: usize, elem_layout: Layout) {
-        if let Err(err) = self.try_reserve_exact(len, additional, elem_layout) {
+    unsafe fn reserve_exact(&mut self, len: usize, additional: usize, elem_layout: Layout) {
+        // SAFETY: Precondition passed to caller
+        if let Err(err) = unsafe { self.try_reserve_exact(len, additional, elem_layout) } {
             handle_error(err);
         }
     }
 
-    fn try_reserve_exact(
+    /// # Safety
+    /// - `elem_layout` must be valid for `self`, i.e. it must be the same `elem_layout` used to
+    ///   initially construct `self`
+    /// - `elem_layout`'s size must be a multiple of its alignment
+    unsafe fn try_reserve_exact(
         &mut self,
         len: usize,
         additional: usize,
         elem_layout: Layout,
     ) -> Result<(), TryReserveError> {
         if self.needs_to_grow(len, additional, elem_layout) {
-            self.grow_exact(len, additional, elem_layout)?;
+            // SAFETY: Precondition passed to caller
+            unsafe {
+                self.grow_exact(len, additional, elem_layout)?;
+            }
         }
         unsafe {
             // Inform the optimizer that the reservation has succeeded or wasn't needed
@@ -609,11 +652,16 @@ impl<A: Allocator> RawVecInner<A> {
         Ok(())
     }
 
+    /// # Safety
+    /// - `elem_layout` must be valid for `self`, i.e. it must be the same `elem_layout` used to
+    ///   initially construct `self`
+    /// - `elem_layout`'s size must be a multiple of its alignment
+    /// - `cap` must be less than or equal to `self.capacity(elem_layout.size())`
     #[cfg(not(no_global_oom_handling))]
     #[inline]
     #[track_caller]
-    fn shrink_to_fit(&mut self, cap: usize, elem_layout: Layout) {
-        if let Err(err) = self.shrink(cap, elem_layout) {
+    unsafe fn shrink_to_fit(&mut self, cap: usize, elem_layout: Layout) {
+        if let Err(err) = unsafe { self.shrink(cap, elem_layout) } {
             handle_error(err);
         }
     }
@@ -632,7 +680,13 @@ impl<A: Allocator> RawVecInner<A> {
         self.cap = unsafe { Cap::new_unchecked(cap) };
     }
 
-    fn grow_amortized(
+    /// # Safety
+    /// - `elem_layout` must be valid for `self`, i.e. it must be the same `elem_layout` used to
+    ///   initially construct `self`
+    /// - `elem_layout`'s size must be a multiple of its alignment
+    /// - The sum of `len` and `additional` must be greater than or equal to
+    ///   `self.capacity(elem_layout.size())`
+    unsafe fn grow_amortized(
         &mut self,
         len: usize,
         additional: usize,
@@ -657,14 +711,25 @@ impl<A: Allocator> RawVecInner<A> {
 
         let new_layout = layout_array(cap, elem_layout)?;
 
-        let ptr = finish_grow(new_layout, self.current_memory(elem_layout), &mut self.alloc)?;
-        // SAFETY: layout_array would have resulted in a capacity overflow if we tried to allocate more than `isize::MAX` items
+        // SAFETY:
+        // - For the `current_memory` call: Precondition passed to caller
+        // - For the `finish_grow` call: Precondition passed to caller
+        //   + `current_memory` does the right thing
+        let ptr =
+            unsafe { finish_grow(new_layout, self.current_memory(elem_layout), &mut self.alloc)? };
 
+        // SAFETY: layout_array would have resulted in a capacity overflow if we tried to allocate more than `isize::MAX` items
         unsafe { self.set_ptr_and_cap(ptr, cap) };
         Ok(())
     }
 
-    fn grow_exact(
+    /// # Safety
+    /// - `elem_layout` must be valid for `self`, i.e. it must be the same `elem_layout` used to
+    ///   initially construct `self`
+    /// - `elem_layout`'s size must be a multiple of its alignment
+    /// - The sum of `len` and `additional` must be greater than or equal to
+    ///   `self.capacity(elem_layout.size())`
+    unsafe fn grow_exact(
         &mut self,
         len: usize,
         additional: usize,
@@ -679,17 +744,27 @@ impl<A: Allocator> RawVecInner<A> {
         let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
         let new_layout = layout_array(cap, elem_layout)?;
 
-        let ptr = finish_grow(new_layout, self.current_memory(elem_layout), &mut self.alloc)?;
+        // SAFETY:
+        // - For the `current_memory` call: Precondition passed to caller
+        // - For the `finish_grow` call: Precondition passed to caller
+        //   + `current_memory` does the right thing
+        let ptr =
+            unsafe { finish_grow(new_layout, self.current_memory(elem_layout), &mut self.alloc)? };
         // SAFETY: layout_array would have resulted in a capacity overflow if we tried to allocate more than `isize::MAX` items
         unsafe {
             self.set_ptr_and_cap(ptr, cap);
         }
         Ok(())
     }
 
+    /// # Safety
+    /// - `elem_layout` must be valid for `self`, i.e. it must be the same `elem_layout` used to
+    ///   initially construct `self`
+    /// - `elem_layout`'s size must be a multiple of its alignment
+    /// - `cap` must be less than or equal to `self.capacity(elem_layout.size())`
     #[cfg(not(no_global_oom_handling))]
     #[inline]
-    fn shrink(&mut self, cap: usize, elem_layout: Layout) -> Result<(), TryReserveError> {
+    unsafe fn shrink(&mut self, cap: usize, elem_layout: Layout) -> Result<(), TryReserveError> {
         assert!(cap <= self.capacity(elem_layout.size()), "Tried to shrink to a larger capacity");
         // SAFETY: Just checked this isn't trying to grow
         unsafe { self.shrink_unchecked(cap, elem_layout) }
@@ -711,8 +786,12 @@ impl<A: Allocator> RawVecInner<A> {
         cap: usize,
         elem_layout: Layout,
     ) -> Result<(), TryReserveError> {
-        let (ptr, layout) =
-            if let Some(mem) = self.current_memory(elem_layout) { mem } else { return Ok(()) };
+        // SAFETY: Precondition passed to caller
+        let (ptr, layout) = if let Some(mem) = unsafe { self.current_memory(elem_layout) } {
+            mem
+        } else {
+            return Ok(());
+        };
 
         // If shrinking to 0, deallocate the buffer. We don't reach this point
         // for the T::IS_ZST case since current_memory() will have returned
@@ -748,18 +827,26 @@ impl<A: Allocator> RawVecInner<A> {
     /// Ideally this function would take `self` by move, but it cannot because it exists to be
     /// called from a `Drop` impl.
     unsafe fn deallocate(&mut self, elem_layout: Layout) {
-        if let Some((ptr, layout)) = self.current_memory(elem_layout) {
+        // SAFETY: Precondition passed to caller
+        if let Some((ptr, layout)) = unsafe { self.current_memory(elem_layout) } {
             unsafe {
                 self.alloc.deallocate(ptr, layout);
             }
         }
     }
 }
 
+/// # Safety
+/// If `current_memory` matches `Some((ptr, old_layout))`:
+/// - `ptr` must denote a block of memory *currently allocated* via `alloc`
+/// - `old_layout` must *fit* that block of memory
+/// - `new_layout` must have the same alignment as `old_layout`
+/// - `new_layout.size()` must be greater than or equal to `old_layout.size()`
+/// If `current_memory` is `None`, this function is safe.
 // not marked inline(never) since we want optimizers to be able to observe the specifics of this
 // function, see tests/codegen-llvm/vec-reserve-extend.rs.
 #[cold]
-fn finish_grow<A>(
+unsafe fn finish_grow<A>(
     new_layout: Layout,
     current_memory: Option<(NonNull<u8>, Layout)>,
     alloc: &mut A,

diff --git a/library/alloc/src/raw_vec/tests.rs b/library/alloc/src/raw_vec/tests.rs
@@ -85,7 +85,7 @@ struct ZST;
 fn zst_sanity<T>(v: &RawVec<T>) {
     assert_eq!(v.capacity(), usize::MAX);
     assert_eq!(v.ptr(), core::ptr::Unique::<T>::dangling().as_ptr());
-    assert_eq!(v.inner.current_memory(T::LAYOUT), None);
+    assert_eq!(unsafe { v.inner.current_memory(T::LAYOUT) }, None);
 }
 
 #[test]
@@ -126,12 +126,12 @@ fn zst() {
     assert_eq!(v.try_reserve_exact(101, usize::MAX - 100), cap_err);
     zst_sanity(&v);
 
-    assert_eq!(v.inner.grow_amortized(100, usize::MAX - 100, ZST::LAYOUT), cap_err);
-    assert_eq!(v.inner.grow_amortized(101, usize::MAX - 100, ZST::LAYOUT), cap_err);
+    assert_eq!(unsafe { v.inner.grow_amortized(100, usize::MAX - 100, ZST::LAYOUT) }, cap_err);
+    assert_eq!(unsafe { v.inner.grow_amortized(101, usize::MAX - 100, ZST::LAYOUT) }, cap_err);
     zst_sanity(&v);
 
-    assert_eq!(v.inner.grow_exact(100, usize::MAX - 100, ZST::LAYOUT), cap_err);
-    assert_eq!(v.inner.grow_exact(101, usize::MAX - 100, ZST::LAYOUT), cap_err);
+    assert_eq!(unsafe { v.inner.grow_exact(100, usize::MAX - 100, ZST::LAYOUT) }, cap_err);
+    assert_eq!(unsafe { v.inner.grow_exact(101, usize::MAX - 100, ZST::LAYOUT) }, cap_err);
     zst_sanity(&v);
 }