Rework enums in rust. (#6098)

* Rework enums in rust.

They're now a unit struct, rather than an enum. This is a
backwards incompatible change but the previous version had UB
and was also backwards incompatible so...

* Update and test sample rust flatbuffers

* Use bitflags crate to properly support rust enums.

Previously, the bitflags attribute was just ignored. This is a breaking change
as the bitflgs API is not like a normal rust enum (duh).

* variant_name() -> Option<_>

* repr transparent

* Reexport bitflags from flatbuffers

* Make bitflags constants CamelCase, matching normal enums

* Deprecate c-style associated enum constants

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

tests/optional_scalars2_generated.rs

@@ -1,6 +1,7 @@
 // automatically generated by the FlatBuffers compiler, do not modify
 
 
+#![allow(unused_imports, dead_code)]
 
 use std::mem;
 use std::cmp::Ordering;
@@ -17,39 +18,58 @@ pub mod optional_scalars {
   extern crate flatbuffers;
   use self::flatbuffers::EndianScalar;
 
-#[allow(non_camel_case_types)]
-#[repr(i8)]
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
-pub enum OptionalByte {
-  None = 0,
-  One = 1,
-  Two = 2,
-
-}
-
+#[deprecated(since = "1.13", note = "Use associated constants instead. This will no longer be generated in 2021.")]
 pub const ENUM_MIN_OPTIONAL_BYTE: i8 = 0;
+#[deprecated(since = "1.13", note = "Use associated constants instead. This will no longer be generated in 2021.")]
 pub const ENUM_MAX_OPTIONAL_BYTE: i8 = 2;
+#[deprecated(since = "1.13", note = "Use associated constants instead. This will no longer be generated in 2021.")]
+#[allow(non_camel_case_types)]
+pub const ENUM_VALUES_OPTIONAL_BYTE: [OptionalByte; 3] = [
+  OptionalByte::None,
+  OptionalByte::One,
+  OptionalByte::Two,
+];
 
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[repr(transparent)]
+pub struct OptionalByte(pub i8);
+#[allow(non_upper_case_globals)]
+impl OptionalByte {
+  pub const None: Self = Self(0);
+  pub const One: Self = Self(1);
+  pub const Two: Self = Self(2);
+
+  pub const ENUM_MIN: i8 = 0;
+  pub const ENUM_MAX: i8 = 2;
+  pub const ENUM_VALUES: &'static [Self] = &[
+    Self::None,
+    Self::One,
+    Self::Two,
+  ];
+  /// Returns the variant's name or "" if unknown.
+  pub fn variant_name(self) -> Option<&'static str> {
+    match self {
+      Self::None => Some("None"),
+      Self::One => Some("One"),
+      Self::Two => Some("Two"),
+      _ => None,
+    }
+  }
+}
+impl std::fmt::Debug for OptionalByte {
+  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::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 OptionalByte {
   type Inner = Self;
   #[inline]
   fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {
-    flatbuffers::read_scalar_at::<Self>(buf, loc)
-  }
-}
-
-impl flatbuffers::EndianScalar for OptionalByte {
-  #[inline]
-  fn to_little_endian(self) -> Self {
-    let n = i8::to_le(self as i8);
-    let p = &n as *const i8 as *const OptionalByte;
-    unsafe { *p }
-  }
-  #[inline]
-  fn from_little_endian(self) -> Self {
-    let n = i8::from_le(self as i8);
-    let p = &n as *const i8 as *const OptionalByte;
-    unsafe { *p }
+    Self(flatbuffers::read_scalar_at::<i8>(buf, loc))
   }
 }
 
@@ -57,27 +77,19 @@ impl flatbuffers::Push for OptionalByte {
     type Output = OptionalByte;
     #[inline]
     fn push(&self, dst: &mut [u8], _rest: &[u8]) {
-        flatbuffers::emplace_scalar::<OptionalByte>(dst, *self);
+        flatbuffers::emplace_scalar::<i8>(dst, self.0);
     }
 }
 
-#[allow(non_camel_case_types)]
-pub const ENUM_VALUES_OPTIONAL_BYTE: [OptionalByte; 3] = [
-  OptionalByte::None,
-  OptionalByte::One,
-  OptionalByte::Two
-];
-
-#[allow(non_camel_case_types)]
-pub const ENUM_NAMES_OPTIONAL_BYTE: [&str; 3] = [
-    "None",
-    "One",
-    "Two"
-];
-
-pub fn enum_name_optional_byte(e: OptionalByte) -> &'static str {
-  let index = e as i8;
-  ENUM_NAMES_OPTIONAL_BYTE[index as usize]
+impl flatbuffers::EndianScalar for OptionalByte {
+  #[inline]
+  fn to_little_endian(self) -> Self {
+    Self(i8::to_le(self.0))
+  }
+  #[inline]
+  fn from_little_endian(self) -> Self {
+    Self(i8::from_le(self.0))
+  }
 }
 
 pub enum ScalarStuffOffset {}