Make flatc generate Rust files not requiring std (#7273)

* Set an explicit 2018 edition for Rust tests

* Replace all `std` usage with `core` and `alloc` in Rust code generator

* Update the generated files

* Make Rust tests actually use no_std when the corresponding feature is enabled

samples/rust_generated/my_game/sample/color_generated.rs

@@ -1,7 +1,11 @@
 // automatically generated by the FlatBuffers compiler, do not modify
+extern crate alloc;
 extern crate flatbuffers;
-use std::mem;
-use std::cmp::Ordering;
+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.")]
@@ -42,8 +46,8 @@ impl Color {
     }
   }
 }
-impl std::fmt::Debug for Color {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for Color {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     if let Some(name) = self.variant_name() {
       f.write_str(name)
     } else {

samples/rust_generated/my_game/sample/equipment_generated.rs

@@ -1,7 +1,11 @@
 // automatically generated by the FlatBuffers compiler, do not modify
+extern crate alloc;
 extern crate flatbuffers;
-use std::mem;
-use std::cmp::Ordering;
+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.")]
@@ -38,8 +42,8 @@ impl Equipment {
     }
   }
 }
-impl std::fmt::Debug for Equipment {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for Equipment {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     if let Some(name) = self.variant_name() {
       f.write_str(name)
     } else {
@@ -121,7 +125,7 @@ impl EquipmentT {
   /// If the union variant matches, return the owned WeaponT, setting the union to NONE.
   pub fn take_weapon(&mut self) -> Option<Box<WeaponT>> {
     if let Self::Weapon(_) = self {
-      let v = std::mem::replace(self, Self::NONE);
+      let v = core::mem::replace(self, Self::NONE);
       if let Self::Weapon(w) = v {
         Some(w)
       } else {

samples/rust_generated/my_game/sample/monster_generated.rs

@@ -1,7 +1,11 @@
 // automatically generated by the FlatBuffers compiler, do not modify
+extern crate alloc;
 extern crate flatbuffers;
-use std::mem;
-use std::cmp::Ordering;
+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::*;
 pub enum MonsterOffset {}
@@ -266,8 +270,8 @@ impl<'a: 'b, 'b> MonsterBuilder<'a, 'b> {
   }
 }
 
-impl std::fmt::Debug for Monster<'_> {
-  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for Monster<'_> {
+  fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
     let mut ds = f.debug_struct("Monster");
       ds.field("pos", &self.pos());
       ds.field("mana", &self.mana());

samples/rust_generated/my_game/sample/vec_3_generated.rs

@@ -1,7 +1,11 @@
 // automatically generated by the FlatBuffers compiler, do not modify
+extern crate alloc;
 extern crate flatbuffers;
-use std::mem;
-use std::cmp::Ordering;
+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::*;
 // struct Vec3, aligned to 4
@@ -13,8 +17,8 @@ impl Default for Vec3 {
     Self([0; 12])
   }
 }
-impl std::fmt::Debug for Vec3 {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for Vec3 {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     f.debug_struct("Vec3")
       .field("x", &self.x())
       .field("y", &self.y())
@@ -44,7 +48,7 @@ impl<'b> flatbuffers::Push for Vec3 {
     #[inline]
     fn push(&self, dst: &mut [u8], _rest: &[u8]) {
         let src = unsafe {
-            ::std::slice::from_raw_parts(self as *const Vec3 as *const u8, Self::size())
+            ::core::slice::from_raw_parts(self as *const Vec3 as *const u8, Self::size())
         };
         dst.copy_from_slice(src);
     }
@@ -55,7 +59,7 @@ impl<'b> flatbuffers::Push for &'b Vec3 {
     #[inline]
     fn push(&self, dst: &mut [u8], _rest: &[u8]) {
         let src = unsafe {
-            ::std::slice::from_raw_parts(*self as *const Vec3 as *const u8, Self::size())
+            ::core::slice::from_raw_parts(*self as *const Vec3 as *const u8, Self::size())
         };
         dst.copy_from_slice(src);
     }

samples/rust_generated/my_game/sample/weapon_generated.rs

@@ -1,7 +1,11 @@
 // automatically generated by the FlatBuffers compiler, do not modify
+extern crate alloc;
 extern crate flatbuffers;
-use std::mem;
-use std::cmp::Ordering;
+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::*;
 pub enum WeaponOffset {}
@@ -118,8 +122,8 @@ impl<'a: 'b, 'b> WeaponBuilder<'a, 'b> {
   }
 }
 
-impl std::fmt::Debug for Weapon<'_> {
-  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for Weapon<'_> {
+  fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
     let mut ds = f.debug_struct("Weapon");
       ds.field("name", &self.name());
       ds.field("damage", &self.damage());