Implement Serialize on generated rust types (#7022)

* fix for rust build

* Rust: Implement Serialize on generated types

For debugging convenience it is really handy to be able to dump out
types as another format (ie: json). For example, if we are logging a
type to a structured logging system, or even printing it out in a
structured way to the console.

Right now we handle this by shelling out to `flatc` which is not ideal;
by implementing Serialize on the generated types we can use any of the
Serializer-implementing packages for our structured debug output.

* clang-format

* Make the flatbuffers Rust crate only have an optional dependency on the `serde` packages.

* fix warning

* fix rust test build

* Oh yeah this needs to be initialized

* fix toml syntax

* code review feedback

* rebuild test data

tests/monster_test_serialize/my_game/example/test_generated.rs

@@ -0,0 +1,170 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+extern crate flatbuffers;
+use std::mem;
+use std::cmp::Ordering;
+extern crate serde;
+use self::serde::ser::{Serialize, Serializer, SerializeStruct};
+use self::flatbuffers::{EndianScalar, Follow};
+use super::*;
+// struct Test, aligned to 2
+#[repr(transparent)]
+#[derive(Clone, Copy, PartialEq)]
+pub struct Test(pub [u8; 4]);
+impl Default for Test { 
+  fn default() -> Self { 
+    Self([0; 4])
+  }
+}
+impl std::fmt::Debug for Test {
+  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+    f.debug_struct("Test")
+      .field("a", &self.a())
+      .field("b", &self.b())
+      .finish()
+  }
+}
+
+impl flatbuffers::SimpleToVerifyInSlice for Test {}
+impl flatbuffers::SafeSliceAccess for Test {}
+impl<'a> flatbuffers::Follow<'a> for Test {
+  type Inner = &'a Test;
+  #[inline]
+  fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {
+    <&'a Test>::follow(buf, loc)
+  }
+}
+impl<'a> flatbuffers::Follow<'a> for &'a Test {
+  type Inner = &'a Test;
+  #[inline]
+  fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {
+    flatbuffers::follow_cast_ref::<Test>(buf, loc)
+  }
+}
+impl<'b> flatbuffers::Push for Test {
+    type Output = Test;
+    #[inline]
+    fn push(&self, dst: &mut [u8], _rest: &[u8]) {
+        let src = unsafe {
+            ::std::slice::from_raw_parts(self as *const Test as *const u8, Self::size())
+        };
+        dst.copy_from_slice(src);
+    }
+}
+impl<'b> flatbuffers::Push for &'b Test {
+    type Output = Test;
+
+    #[inline]
+    fn push(&self, dst: &mut [u8], _rest: &[u8]) {
+        let src = unsafe {
+            ::std::slice::from_raw_parts(*self as *const Test as *const u8, Self::size())
+        };
+        dst.copy_from_slice(src);
+    }
+}
+
+impl<'a> flatbuffers::Verifiable for Test {
+  #[inline]
+  fn run_verifier(
+    v: &mut flatbuffers::Verifier, pos: usize
+  ) -> Result<(), flatbuffers::InvalidFlatbuffer> {
+    use self::flatbuffers::Verifiable;
+    v.in_buffer::<Self>(pos)
+  }
+}
+
+impl Serialize for Test {
+  fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+  where
+    S: Serializer,
+  {
+    let mut s = serializer.serialize_struct("Test", 2)?;
+      s.serialize_field("a", &self.a())?;
+      s.serialize_field("b", &self.b())?;
+    s.end()
+  }
+}
+
+impl<'a> Test {
+  #[allow(clippy::too_many_arguments)]
+  pub fn new(
+    a: i16,
+    b: i8,
+  ) -> Self {
+    let mut s = Self([0; 4]);
+    s.set_a(a);
+    s.set_b(b);
+    s
+  }
+
+  pub const fn get_fully_qualified_name() -> &'static str {
+    "MyGame.Example.Test"
+  }
+
+  pub fn a(&self) -> i16 {
+    let mut mem = core::mem::MaybeUninit::<i16>::uninit();
+    unsafe {
+      core::ptr::copy_nonoverlapping(
+        self.0[0..].as_ptr(),
+        mem.as_mut_ptr() as *mut u8,
+        core::mem::size_of::<i16>(),
+      );
+      mem.assume_init()
+    }.from_little_endian()
+  }
+
+  pub fn set_a(&mut self, x: i16) {
+    let x_le = x.to_little_endian();
+    unsafe {
+      core::ptr::copy_nonoverlapping(
+        &x_le as *const i16 as *const u8,
+        self.0[0..].as_mut_ptr(),
+        core::mem::size_of::<i16>(),
+      );
+    }
+  }
+
+  pub fn b(&self) -> i8 {
+    let mut mem = core::mem::MaybeUninit::<i8>::uninit();
+    unsafe {
+      core::ptr::copy_nonoverlapping(
+        self.0[2..].as_ptr(),
+        mem.as_mut_ptr() as *mut u8,
+        core::mem::size_of::<i8>(),
+      );
+      mem.assume_init()
+    }.from_little_endian()
+  }
+
+  pub fn set_b(&mut self, x: i8) {
+    let x_le = x.to_little_endian();
+    unsafe {
+      core::ptr::copy_nonoverlapping(
+        &x_le as *const i8 as *const u8,
+        self.0[2..].as_mut_ptr(),
+        core::mem::size_of::<i8>(),
+      );
+    }
+  }
+
+  pub fn unpack(&self) -> TestT {
+    TestT {
+      a: self.a(),
+      b: self.b(),
+    }
+  }
+}
+
+#[derive(Debug, Clone, PartialEq, Default)]
+pub struct TestT {
+  pub a: i16,
+  pub b: i8,
+}
+impl TestT {
+  pub fn pack(&self) -> Test {
+    Test::new(
+      self.a,
+      self.b,
+    )
+  }
+}
+