Rust Flexbuffers (#5669)

* Cargo clippy lints

* more lints

* more lints

* Restored a doc comment

* Comment on float eps-eq and adjusted casting

* Rust Flexbuffers

* more serde tests, removed some unsafe

* Redid serde to be map-like and Reader is Display

* Moved iter from Reader to VectorReader

* Serious quickcheck + bugs

* wvo api

* Made types smaller for a reasonable speedup

* redid reading in a way that's a bit faster.

Profiling shows the rust slowdown as building +10%, reading +20%

* src/bin are developer binaries in rust

* Root and Map width are not packed

* key null check is debug only + doc changes

* BuilderOptions

* Documentation

* Documentation

* Moved tests to rust_usage_test

* Moved rust flexbuffers samples to Flatbuffers/samples

* Fixed RustTest

* Fixed for Rust 1.37.0

* Upgraded to rust 1_40_0

* fixed a little-endian-only feature in a test

* 1.40.0

* fixed some benchmarks for bigendian

* Updated .bat file

* misspelling

* Gold Flexbuffer test.

* Serialize,Deserialize, std::error::Error for Errors.

* Undo rustfmt in integration_test.rs

* from_slice instead of from_vec

* Added comments to unsafe blocks

* expanded on comment

* bump

Co-authored-by: CasperN <cneo@google.com>

tests/rust_usage_test/bin/flexbuffers_alloc_check.rs

@@ -0,0 +1,138 @@
+// Copyright 2019 Google LLC
+//
+// 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
+//
+//     https://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.
+
+extern crate flexbuffers;
+
+use flexbuffers::*;
+use std::alloc::{GlobalAlloc, Layout, System};
+
+/// We take over the Rust allocator to count allocations. This is super not thread safe.
+static mut NUM_ALLOCS: usize = 0;
+fn current_allocs() -> usize {
+    unsafe { NUM_ALLOCS }
+}
+struct TrackingAllocator;
+unsafe impl GlobalAlloc for TrackingAllocator {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        NUM_ALLOCS += 1;
+        System.alloc(layout)
+    }
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        System.dealloc(ptr, layout)
+    }
+}
+#[global_allocator]
+static T: TrackingAllocator = TrackingAllocator;
+
+/// Make some example data
+fn make_monster(mut monster: MapBuilder) {
+    monster.push("type", "great orc");
+    monster.push("age", 100u8);
+    monster.push("name", "Mr. Orc");
+    monster.push("coins", &[1, 25, 50, 100, 250]);
+    monster.push("color", &[255u8, 0, 0, 0]);
+    {
+        let mut weapons = monster.start_vector("weapons");
+        {
+            let mut hammer = weapons.start_map();
+            hammer.push("name", "hammer");
+            hammer.push("damage type", "crush");
+            hammer.push("damage", 20);
+        }
+        {
+            let mut axe = weapons.start_map();
+            axe.push("name", "Great Axe");
+            axe.push("damage type", "slash");
+            axe.push("damage", 30);
+        }
+    }
+    {
+        let mut sounds = monster.start_vector("sounds");
+        sounds.push("grr");
+        sounds.push("rawr");
+        sounds.push("muahaha");
+    }
+    // TODO(cneo): Directly pushing string slices has alloc.
+}
+
+// Read back the data from make_monster.
+fn validate_monster(flexbuffer: &[u8]) {
+    let r = Reader::get_root(flexbuffer).unwrap().as_map();
+
+    assert_eq!(r.idx("type").as_str(), "great orc");
+    assert_eq!(r.idx("age").as_u8(), 100);
+    assert_eq!(r.idx("name").as_str(), "Mr. Orc");
+
+    let coins = r.idx("coins").as_vector();
+    for (i, &c) in [1, 25, 50, 100, 250].iter().enumerate() {
+        assert_eq!(coins.idx(i).as_u16(), c);
+    }
+    let color = r.idx("color").as_vector();
+    for (i, &c) in [255, 0, 0, 0].iter().enumerate() {
+        assert_eq!(color.idx(i).as_i32(), c);
+    }
+    let weapons = r.idx("weapons").as_vector();
+    assert_eq!(weapons.len(), 2);
+
+    let hammer = weapons.idx(0).as_map();
+    assert_eq!(hammer.idx("name").as_str(), "hammer");
+    assert_eq!(hammer.idx("damage type").as_str(), "crush");
+    assert_eq!(hammer.idx("damage").as_u64(), 20);
+
+    let axe = weapons.idx(1).as_map();
+    assert_eq!(axe.idx("name").as_str(), "Great Axe");
+    assert_eq!(axe.idx("damage type").as_str(), "slash");
+    assert_eq!(axe.idx("damage").as_u64(), 30);
+
+    let sounds = r.idx("sounds").as_vector();
+    for (i, &s) in ["grr", "rawr", "muahaha"].iter().enumerate() {
+        assert_eq!(sounds.idx(i).as_str(), s);
+    }
+}
+
+// This is in a separate binary than tests because taking over the global allocator is not
+// hermetic and not thread safe.
+fn main() {
+    let start_up = current_allocs();
+
+    // Let's build a flexbuffer from a new (cold) flexbuffer builder.
+    let mut builder = Builder::default();
+    make_monster(builder.start_map());
+    let after_warmup = current_allocs();
+
+    // The builder makes some allocations while warming up.
+    assert!(after_warmup > start_up);
+    assert!(after_warmup < start_up + 20);
+
+    // A warm builder should make no allocations.
+    make_monster(builder.start_map());
+    assert_eq!(after_warmup, current_allocs());
+
+    // Nor should a reader.
+    validate_monster(builder.view());
+    assert_eq!(after_warmup, current_allocs());
+
+    // Do it again just for kicks.
+    make_monster(builder.start_map());
+    validate_monster(builder.view());
+    assert_eq!(after_warmup, current_allocs());
+
+    let final_allocs = current_allocs(); // dbg! does allocate.
+    dbg!(start_up, after_warmup, final_allocs);
+}
+
+#[test]
+fn no_extra_allocations() {
+    main()
+}