Started implementation for private flags in rust (#7269)

Adds flag to the cpp to fix bad annotations in all the languages

Addresses comments to the PR, and fixes logic for leaking annotations

tests/private_annotation_test/ab_generated.rs

@@ -0,0 +1,97 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+extern crate alloc;
+extern crate flatbuffers;
+use alloc::boxed::Box;
+use alloc::string::{String, ToString};
+use alloc::vec::Vec;
+use core::mem;
+use core::cmp::Ordering;
+use self::flatbuffers::{EndianScalar, Follow};
+use super::*;
+#[deprecated(since = "2.0.0", note = "Use associated constants instead. This will no longer be generated in 2021.")]
+pub const ENUM_MIN_AB: i8 = 0;
+#[deprecated(since = "2.0.0", note = "Use associated constants instead. This will no longer be generated in 2021.")]
+pub const ENUM_MAX_AB: i8 = 1;
+#[deprecated(since = "2.0.0", note = "Use associated constants instead. This will no longer be generated in 2021.")]
+#[allow(non_camel_case_types)]
+pub const ENUM_VALUES_AB: [AB; 2] = [
+  AB::A,
+  AB::B,
+];
+
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Default)]
+#[repr(transparent)]
+pub(crate) struct AB(pub i8);
+#[allow(non_upper_case_globals)]
+impl AB {
+  pub const A: Self = Self(0);
+  pub const B: Self = Self(1);
+
+  pub const ENUM_MIN: i8 = 0;
+  pub const ENUM_MAX: i8 = 1;
+  pub const ENUM_VALUES: &'static [Self] = &[
+    Self::A,
+    Self::B,
+  ];
+  /// Returns the variant's name or "" if unknown.
+  pub fn variant_name(self) -> Option<&'static str> {
+    match self {
+      Self::A => Some("A"),
+      Self::B => Some("B"),
+      _ => None,
+    }
+  }
+}
+impl core::fmt::Debug for AB {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
+    if let Some(name) = self.variant_name() {
+      f.write_str(name)
+    } else {
+      f.write_fmt(format_args!("<UNKNOWN {:?}>", self.0))
+    }
+  }
+}
+impl<'a> flatbuffers::Follow<'a> for AB {
+  type Inner = Self;
+  #[inline]
+  fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {
+    let b = unsafe {
+      flatbuffers::read_scalar_at::<i8>(buf, loc)
+    };
+    Self(b)
+  }
+}
+
+impl flatbuffers::Push for AB {
+    type Output = AB;
+    #[inline]
+    fn push(&self, dst: &mut [u8], _rest: &[u8]) {
+        unsafe { flatbuffers::emplace_scalar::<i8>(dst, self.0); }
+    }
+}
+
+impl flatbuffers::EndianScalar for AB {
+  #[inline]
+  fn to_little_endian(self) -> Self {
+    let b = i8::to_le(self.0);
+    Self(b)
+  }
+  #[inline]
+  #[allow(clippy::wrong_self_convention)]
+  fn from_little_endian(self) -> Self {
+    let b = i8::from_le(self.0);
+    Self(b)
+  }
+}
+
+impl<'a> flatbuffers::Verifiable for AB {
+  #[inline]
+  fn run_verifier(
+    v: &mut flatbuffers::Verifier, pos: usize
+  ) -> Result<(), flatbuffers::InvalidFlatbuffer> {
+    use self::flatbuffers::Verifiable;
+    i8::run_verifier(v, pos)
+  }
+}
+
+impl flatbuffers::SimpleToVerifyInSlice for AB {}