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/table_a_generated.rs

@@ -0,0 +1,154 @@
+// 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::*;
+pub enum TableAOffset {}
+#[derive(Copy, Clone, PartialEq)]
+
+pub struct TableA<'a> {
+  pub _tab: flatbuffers::Table<'a>,
+}
+
+impl<'a> flatbuffers::Follow<'a> for TableA<'a> {
+  type Inner = TableA<'a>;
+  #[inline]
+  fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {
+    Self { _tab: flatbuffers::Table { buf, loc } }
+  }
+}
+
+impl<'a> TableA<'a> {
+  pub const VT_B: flatbuffers::VOffsetT = 4;
+
+  pub const fn get_fully_qualified_name() -> &'static str {
+    "TableA"
+  }
+
+  #[inline]
+  pub fn init_from_table(table: flatbuffers::Table<'a>) -> Self {
+    TableA { _tab: table }
+  }
+  #[allow(unused_mut)]
+  pub fn create<'bldr: 'args, 'args: 'mut_bldr, 'mut_bldr>(
+    _fbb: &'mut_bldr mut flatbuffers::FlatBufferBuilder<'bldr>,
+    args: &'args TableAArgs<'args>
+  ) -> flatbuffers::WIPOffset<TableA<'bldr>> {
+    let mut builder = TableABuilder::new(_fbb);
+    if let Some(x) = args.b { builder.add_b(x); }
+    builder.finish()
+  }
+
+  pub fn unpack(&self) -> TableAT {
+    let b = self.b().map(|x| {
+      Box::new(x.unpack())
+    });
+    TableAT {
+      b,
+    }
+  }
+
+  #[inline]
+  pub fn b(&self) -> Option<my_game::other_name_space::TableB<'a>> {
+    self._tab.get::<flatbuffers::ForwardsUOffset<my_game::other_name_space::TableB>>(TableA::VT_B, None)
+  }
+}
+
+impl flatbuffers::Verifiable for TableA<'_> {
+  #[inline]
+  fn run_verifier(
+    v: &mut flatbuffers::Verifier, pos: usize
+  ) -> Result<(), flatbuffers::InvalidFlatbuffer> {
+    use self::flatbuffers::Verifiable;
+    v.visit_table(pos)?
+     .visit_field::<flatbuffers::ForwardsUOffset<my_game::other_name_space::TableB>>("b", Self::VT_B, false)?
+     .finish();
+    Ok(())
+  }
+}
+pub struct TableAArgs<'a> {
+    pub b: Option<flatbuffers::WIPOffset<my_game::other_name_space::TableB<'a>>>,
+}
+impl<'a> Default for TableAArgs<'a> {
+  #[inline]
+  fn default() -> Self {
+    TableAArgs {
+      b: None,
+    }
+  }
+}
+
+impl Serialize for TableA<'_> {
+  fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+  where
+    S: Serializer,
+  {
+    let mut s = serializer.serialize_struct("TableA", 1)?;
+      if let Some(f) = self.b() {
+        s.serialize_field("b", &f)?;
+      } else {
+        s.skip_field("b")?;
+      }
+    s.end()
+  }
+}
+
+pub struct TableABuilder<'a: 'b, 'b> {
+  fbb_: &'b mut flatbuffers::FlatBufferBuilder<'a>,
+  start_: flatbuffers::WIPOffset<flatbuffers::TableUnfinishedWIPOffset>,
+}
+impl<'a: 'b, 'b> TableABuilder<'a, 'b> {
+  #[inline]
+  pub fn add_b(&mut self, b: flatbuffers::WIPOffset<my_game::other_name_space::TableB<'b >>) {
+    self.fbb_.push_slot_always::<flatbuffers::WIPOffset<my_game::other_name_space::TableB>>(TableA::VT_B, b);
+  }
+  #[inline]
+  pub fn new(_fbb: &'b mut flatbuffers::FlatBufferBuilder<'a>) -> TableABuilder<'a, 'b> {
+    let start = _fbb.start_table();
+    TableABuilder {
+      fbb_: _fbb,
+      start_: start,
+    }
+  }
+  #[inline]
+  pub fn finish(self) -> flatbuffers::WIPOffset<TableA<'a>> {
+    let o = self.fbb_.end_table(self.start_);
+    flatbuffers::WIPOffset::new(o.value())
+  }
+}
+
+impl std::fmt::Debug for TableA<'_> {
+  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    let mut ds = f.debug_struct("TableA");
+      ds.field("b", &self.b());
+      ds.finish()
+  }
+}
+#[non_exhaustive]
+#[derive(Debug, Clone, PartialEq)]
+pub struct TableAT {
+  pub b: Option<Box<my_game::other_name_space::TableBT>>,
+}
+impl Default for TableAT {
+  fn default() -> Self {
+    Self {
+      b: None,
+    }
+  }
+}
+impl TableAT {
+  pub fn pack<'b>(
+    &self,
+    _fbb: &mut flatbuffers::FlatBufferBuilder<'b>
+  ) -> flatbuffers::WIPOffset<TableA<'b>> {
+    let b = self.b.as_ref().map(|x|{
+      x.pack(_fbb)
+    });
+    TableA::create(_fbb, &TableAArgs{
+      b,
+    })
+  }
+}