[rust] Genericize flexbuffer reader (#6450)

* feature/rust-tokio-bytes added feature name for tokio-bytes

* Added flexbuffer implementation, TODO: typecast to avoid recurse

* Converted codebase to utilize FlexBuffer implementation, need to resolve deserialization issues

* Added todo for lifetime issue, may use &'de [u8] for deserializer instead of current method

* Added proper &[u8] implementation

* Removed unused struct

* Added experimental fix to get_slice

* Added experimental fix to get_slice

* Avoided lifetime issues via ref structs, need to check if this hurts peformance

* Updated deserializer implementation to allow for borrowed data from Reader struct

* Fixed bug with str

* Removed unnecessary generic parameter

* Added unsafe to avoid lifetime complaints, current tests pass, need to review alternatives to unsafe

* Opinionated: Removed bytes crate as this implementation could be done in a separate crate

* Cleaned up flatbuffer

* Fixed sample / example

* Resolved PR feedback, need to resolve issues with tests

* Cleaned up FlexBuffer trait to be an auto impl

* Removed TODO

* Reverted Deserializer to only support &'de [u8]

* Cleaned up / renamed function for clarification

* Renamed FlexBuffer -> InternalBuffer for clarification on it's purpose

* Fixed issue with key bytes

* resolved issues with broken tests, confirming this is a breaking change

* Removed FIXME that's solved by splitting String and Key variants

* Implemented associated types approach

* Fixed backward slice logic

* Fixed MapReader compile error

* Added from_buffer for deserialization, removed  function since it's only needed for deserialization

* Removed dead code

* Cleaned up buffer, removed AsRef in favor of Deref

* Renamed Buffer::as_str -> Buffer::buffer_str

* Minor cleanup

* Updated documentation, need to fix tests

* Removed unnecessary &

* Removed unused lifetime

* removed unnecessary as_ref

* Minor optimization wrap-up

* resolved issue with Clone

* Added test to verify no deep-copy

* Added  for optimization

* Updated to use empty fn instead of default

* Updated comments / test name - plus the 0.3.0 version bump

* comment

tests/rust_usage_test/tests/flexbuffers_tests/other_api.rs

@@ -20,7 +20,7 @@ use quickcheck::QuickCheck;
 #[cfg(not(miri))]  // slow.
 fn qc_reader_no_crash() {
     fn no_crash(xs: Vec<u8>) -> bool {
-        let r = Reader::get_root(&xs);
+        let r = Reader::get_root(xs.as_ref());
         r.is_err() || r.is_ok()
     }
     QuickCheck::new()
@@ -133,7 +133,7 @@ fn string_as_num() {
 }
 #[test]
 fn null_reader() {
-    let n = Reader::default();
+    let n = Reader::<&[u8]>::default();
     assert_eq!(n.as_i8(), 0);
     assert_eq!(n.as_i16(), 0);
     assert_eq!(n.as_i32(), 0);
@@ -159,7 +159,7 @@ fn get_root_deref_oob() {
         (FlexBufferType::Vector as u8) << 2 | BitWidth::W8 as u8,
         1,
     ];
-    assert!(Reader::get_root(s).is_err());
+    assert!(Reader::get_root(s.as_ref()).is_err());
 }
 #[test]
 fn get_root_deref_u64() {
@@ -170,7 +170,24 @@ fn get_root_deref_u64() {
         1,
     ];
     // The risk of crashing is reading 8 bytes from index 0.
-    assert_eq!(Reader::get_root(s).unwrap().as_u64(), 0);
+    assert_eq!(Reader::get_root(s.as_ref()).unwrap().as_u64(), 0);
+}
+
+/// Verifies that the clone operation is shallow / zero copy.
+#[test]
+fn clone_is_shallow() {
+    let mut fxb = Builder::default();
+    let mut m = fxb.start_map();
+    m.push("a", &[-1i8, -2, -3, -4]);
+    m.push("b", 250i64);
+    m.push("c", 5000u16);
+    m.end_map();
+
+    let r = Reader::get_root(fxb.view()).unwrap();
+
+    let r2 = r.clone();
+
+    assert_eq!(r.buffer().as_ptr(), r2.buffer().as_ptr());
 }
 
 #[test]