phase 1

Author: dallasmarlow

rust/flatbuffers/src/builder.rs

@@ -22,7 +22,7 @@ use core::fmt::{Debug, Display};
 use core::iter::{DoubleEndedIterator, ExactSizeIterator};
 use core::marker::PhantomData;
 use core::ops::{Add, AddAssign, Deref, DerefMut, Index, IndexMut, Sub, SubAssign};
-use core::ptr::write_bytes;
+use core::ptr::{write_bytes, NonNull};
 
 use crate::endian_scalar::emplace_scalar;
 use crate::primitives::*;
@@ -55,6 +55,29 @@ pub unsafe trait Allocator: DerefMut<Target = [u8]> {
     fn len(&self) -> usize;
 }
 
+/// A trait for supplying memory to a FlatBufferBuilder from an external source.
+///
+/// This trait represents a more traditional allocator model, separating the
+/// concerns of memory allocation from buffer management. It is designed to be
+/// easily implemented by modern, high-performance arena allocators like `bumpalo`.
+pub unsafe trait ExternalAllocator {
+    /// Reallocates a buffer to a new size.
+    ///
+    /// The implementer is responsible for allocating a new block of memory
+    /// of at least `new_size` bytes, copying the contents of the `old_buf`,
+    /// and deallocating the old buffer.
+    ///
+    /// The returned pointer must point to the new block of memory.
+    fn reallocate(&mut self, old_buf: NonNull<u8>, old_size: usize, new_size: usize) -> NonNull<u8>;
+
+    /// Deallocates a buffer.
+    ///
+    /// This method is called when the `FlatBufferBuilder` is dropped, giving the
+    /// allocator a chance to free the memory. For arena allocators, this is
+    /// typically a no-op.
+    fn deallocate(&mut self, buf: NonNull<u8>, size: usize);
+}
+
 /// Default [`FlatBufferBuilder`] allocator backed by a [`Vec<u8>`].
 #[derive(Default)]
 pub struct DefaultAllocator(Vec<u8>);