rust: alloc: add `RawVec::try_with_capacity_in()` constructor

Add the `RawVec::try_with_capacity_in()` constructor as the fallible
version of `RawVec::with_capacity_in()`.

The implementation follows the original.

The infallible constructor is implemented in terms of the private
`RawVec::allocate_in()` constructor, thus also add the private
`RawVec::try_allocate_in()` constructor following the other.

It will be used to implement `Vec::try_with_capacity{,_in}()` in
the next patch.
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/alloc/raw_vec.rs

@@ -20,11 +20,11 @@
 #[cfg(test)]
 mod tests;
 
-#[cfg(not(no_global_oom_handling))]
 enum AllocInit {
     /// The contents of the new memory are uninitialized.
     Uninitialized,
     /// The new memory is guaranteed to be zeroed.
+    #[allow(dead_code)]
     Zeroed,
 }
 
@@ -133,6 +133,14 @@ pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
         Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
     }
 
+    /// Like `try_with_capacity`, but parameterized over the choice of
+    /// allocator for the returned `RawVec`.
+    #[allow(dead_code)]
+    #[inline]
+    pub fn try_with_capacity_in(capacity: usize, alloc: A) -> Result<Self, TryReserveError> {
+        Self::try_allocate_in(capacity, AllocInit::Uninitialized, alloc)
+    }
+
     /// Like `with_capacity_zeroed`, but parameterized over the choice
     /// of allocator for the returned `RawVec`.
     #[cfg(not(no_global_oom_handling))]
@@ -203,6 +211,30 @@ fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
         }
     }
 
+    fn try_allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Result<Self, TryReserveError> {
+        // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
+        if mem::size_of::<T>() == 0 || capacity == 0 {
+            return Ok(Self::new_in(alloc));
+        }
+
+        let layout = Layout::array::<T>(capacity).map_err(|_| CapacityOverflow)?;
+        alloc_guard(layout.size())?;
+        let result = match init {
+            AllocInit::Uninitialized => alloc.allocate(layout),
+            AllocInit::Zeroed => alloc.allocate_zeroed(layout),
+        };
+        let ptr = result.map_err(|_| AllocError { layout, non_exhaustive: () })?;
+
+        // Allocators currently return a `NonNull<[u8]>` whose length
+        // matches the size requested. If that ever changes, the capacity
+        // here should change to `ptr.len() / mem::size_of::<T>()`.
+        Ok(Self {
+            ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
+            cap: capacity,
+            alloc,
+        })
+    }
+
     /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
     ///
     /// # Safety