[Rust] Add support for fixed size arrays (#6548)

* Add support for fixed size arrays

* clang-format

* Update rust image to 1.51 to support const generics

* Handle correctly big endian

* Add fuzz tests and clean code

* Add struct fuzz test and optimize struct arrays for api

* Bump flatbuffers crate version

rust/flatbuffers/src/array.rs

@@ -0,0 +1,136 @@
+/*
+ * Copyright 2021 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+use crate::follow::Follow;
+use crate::{
+    vector::{SafeSliceAccess, VectorIter},
+    EndianScalar,
+};
+use std::fmt::{Debug, Formatter, Result};
+use std::marker::PhantomData;
+use std::mem::size_of;
+use std::slice::from_raw_parts;
+
+#[derive(Copy, Clone)]
+pub struct Array<'a, T: 'a, const N: usize>(&'a [u8], PhantomData<T>);
+
+impl<'a, T: 'a, const N: usize> Debug for Array<'a, T, N>
+where
+    T: 'a + Follow<'a>,
+    <T as Follow<'a>>::Inner: Debug,
+{
+    fn fmt(&self, f: &mut Formatter) -> Result {
+        f.debug_list().entries(self.iter()).finish()
+    }
+}
+
+impl<'a, T: 'a, const N: usize> Array<'a, T, N> {
+    #[inline(always)]
+    pub fn new(buf: &'a [u8]) -> Self {
+        debug_assert!(size_of::<T>() * N == buf.len());
+
+        Array {
+            0: buf,
+            1: PhantomData,
+        }
+    }
+
+    #[inline(always)]
+    pub fn len(&self) -> usize {
+        N
+    }
+}
+
+impl<'a, T: Follow<'a> + 'a, const N: usize> Array<'a, T, N> {
+    #[inline(always)]
+    pub fn get(&self, idx: usize) -> T::Inner {
+        debug_assert!(idx < N);
+        let sz = size_of::<T>();
+        T::follow(self.0, sz * idx)
+    }
+
+    #[inline(always)]
+    pub fn iter(&self) -> VectorIter<'a, T> {
+        VectorIter::from_slice(self.0, self.len())
+    }
+}
+
+impl<'a, T: Follow<'a> + Debug, const N: usize> Into<[T::Inner; N]> for Array<'a, T, N> {
+    #[inline(always)]
+    fn into(self) -> [T::Inner; N] {
+        array_init(|i| self.get(i))
+    }
+}
+
+impl<'a, T: SafeSliceAccess + 'a, const N: usize> Array<'a, T, N> {
+    pub fn safe_slice(self) -> &'a [T] {
+        let sz = size_of::<T>();
+        debug_assert!(sz > 0);
+        let ptr = self.0.as_ptr() as *const T;
+        unsafe { from_raw_parts(ptr, N) }
+    }
+}
+
+/// Implement Follow for all possible Arrays that have Follow-able elements.
+impl<'a, T: Follow<'a> + 'a, const N: usize> Follow<'a> for Array<'a, T, N> {
+    type Inner = Array<'a, T, N>;
+    #[inline(always)]
+    fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {
+        Array::new(&buf[loc..loc + N * size_of::<T>()])
+    }
+}
+
+pub fn emplace_scalar_array<T: EndianScalar, const N: usize>(
+    buf: &mut [u8],
+    loc: usize,
+    src: &[T; N],
+) {
+    let mut buf_ptr = buf[loc..].as_mut_ptr() as *mut T;
+    for item in src.iter() {
+        let item_le = item.to_little_endian();
+        unsafe {
+            buf_ptr.write(item_le);
+            buf_ptr = buf_ptr.add(1);
+        }
+    }
+}
+
+impl<'a, T: Follow<'a> + 'a, const N: usize> IntoIterator for Array<'a, T, N> {
+    type Item = T::Inner;
+    type IntoIter = VectorIter<'a, T>;
+    #[inline]
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+#[inline]
+pub fn array_init<F, T, const N: usize>(mut initializer: F) -> [T; N]
+where
+    F: FnMut(usize) -> T,
+{
+    let mut array: core::mem::MaybeUninit<[T; N]> = core::mem::MaybeUninit::uninit();
+    let mut ptr_i = array.as_mut_ptr() as *mut T;
+
+    unsafe {
+        for i in 0..N {
+            let value_i = initializer(i);
+            ptr_i.write(value_i);
+            ptr_i = ptr_i.add(1);
+        }
+        array.assume_init()
+    }
+}