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/namespace_test/namespace_test1_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;
@@ -25,39 +26,58 @@ pub mod namespace_b {
   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 EnumInNestedNS {
-  A = 0,
-  B = 1,
-  C = 2,
-
-}
-
+#[deprecated(since = "1.13", note = "Use associated constants instead. This will no longer be generated in 2021.")]
 pub const ENUM_MIN_ENUM_IN_NESTED_NS: i8 = 0;
+#[deprecated(since = "1.13", note = "Use associated constants instead. This will no longer be generated in 2021.")]
 pub const ENUM_MAX_ENUM_IN_NESTED_NS: 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_ENUM_IN_NESTED_NS: [EnumInNestedNS; 3] = [
+  EnumInNestedNS::A,
+  EnumInNestedNS::B,
+  EnumInNestedNS::C,
+];
 
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[repr(transparent)]
+pub struct EnumInNestedNS(pub i8);
+#[allow(non_upper_case_globals)]
+impl EnumInNestedNS {
+  pub const A: Self = Self(0);
+  pub const B: Self = Self(1);
+  pub const C: Self = Self(2);
+
+  pub const ENUM_MIN: i8 = 0;
+  pub const ENUM_MAX: i8 = 2;
+  pub const ENUM_VALUES: &'static [Self] = &[
+    Self::A,
+    Self::B,
+    Self::C,
+  ];
+  /// 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"),
+      Self::C => Some("C"),
+      _ => None,
+    }
+  }
+}
+impl std::fmt::Debug for EnumInNestedNS {
+  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 EnumInNestedNS {
   type Inner = Self;
   #[inline]
   fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {
-    flatbuffers::read_scalar_at::<Self>(buf, loc)
-  }
-}
-
-impl flatbuffers::EndianScalar for EnumInNestedNS {
-  #[inline]
-  fn to_little_endian(self) -> Self {
-    let n = i8::to_le(self as i8);
-    let p = &n as *const i8 as *const EnumInNestedNS;
-    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 EnumInNestedNS;
-    unsafe { *p }
+    Self(flatbuffers::read_scalar_at::<i8>(buf, loc))
   }
 }
 
@@ -65,27 +85,19 @@ impl flatbuffers::Push for EnumInNestedNS {
     type Output = EnumInNestedNS;
     #[inline]
     fn push(&self, dst: &mut [u8], _rest: &[u8]) {
-        flatbuffers::emplace_scalar::<EnumInNestedNS>(dst, *self);
+        flatbuffers::emplace_scalar::<i8>(dst, self.0);
     }
 }
 
-#[allow(non_camel_case_types)]
-pub const ENUM_VALUES_ENUM_IN_NESTED_NS: [EnumInNestedNS; 3] = [
-  EnumInNestedNS::A,
-  EnumInNestedNS::B,
-  EnumInNestedNS::C
-];
-
-#[allow(non_camel_case_types)]
-pub const ENUM_NAMES_ENUM_IN_NESTED_NS: [&str; 3] = [
-    "A",
-    "B",
-    "C"
-];
-
-pub fn enum_name_enum_in_nested_ns(e: EnumInNestedNS) -> &'static str {
-  let index = e as i8;
-  ENUM_NAMES_ENUM_IN_NESTED_NS[index as usize]
+impl flatbuffers::EndianScalar for EnumInNestedNS {
+  #[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))
+  }
 }
 
 // struct StructInNestedNS, aligned to 4

tests/namespace_test/namespace_test2_generated.rs

@@ -1,6 +1,7 @@
 // automatically generated by the FlatBuffers compiler, do not modify
 
 
+#![allow(unused_imports, dead_code)]
 
 use crate::namespace_test1_generated::*;
 use std::mem;