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

tests/arrays_test/my_game/example/array_struct_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 ArrayStruct, aligned to 8
@@ -13,8 +17,8 @@ impl Default for ArrayStruct {
     Self([0; 160])
   }
 }
-impl std::fmt::Debug for ArrayStruct {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for ArrayStruct {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     f.debug_struct("ArrayStruct")
       .field("a", &self.a())
       .field("b", &self.b())
@@ -47,7 +51,7 @@ impl<'b> flatbuffers::Push for ArrayStruct {
     #[inline]
     fn push(&self, dst: &mut [u8], _rest: &[u8]) {
         let src = unsafe {
-            ::std::slice::from_raw_parts(self as *const ArrayStruct as *const u8, Self::size())
+            ::core::slice::from_raw_parts(self as *const ArrayStruct as *const u8, Self::size())
         };
         dst.copy_from_slice(src);
     }
@@ -58,7 +62,7 @@ impl<'b> flatbuffers::Push for &'b ArrayStruct {
     #[inline]
     fn push(&self, dst: &mut [u8], _rest: &[u8]) {
         let src = unsafe {
-            ::std::slice::from_raw_parts(*self as *const ArrayStruct as *const u8, Self::size())
+            ::core::slice::from_raw_parts(*self as *const ArrayStruct as *const u8, Self::size())
         };
         dst.copy_from_slice(src);
     }
@@ -158,7 +162,7 @@ impl<'a> ArrayStruct {
 
   pub fn set_d(&mut self, x: &[NestedStruct; 2]) {
     unsafe {
-      std::ptr::copy(
+      core::ptr::copy(
         x.as_ptr() as *const u8,
         self.0.as_mut_ptr().add(72),
         64,

tests/arrays_test/my_game/example/array_table_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 ArrayTableOffset {}
@@ -103,8 +107,8 @@ impl<'a: 'b, 'b> ArrayTableBuilder<'a, 'b> {
   }
 }
 
-impl std::fmt::Debug for ArrayTable<'_> {
-  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for ArrayTable<'_> {
+  fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
     let mut ds = f.debug_struct("ArrayTable");
       ds.field("a", &self.a());
       ds.finish()

tests/arrays_test/my_game/example/nested_struct_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 NestedStruct, aligned to 8
@@ -13,8 +17,8 @@ impl Default for NestedStruct {
     Self([0; 32])
   }
 }
-impl std::fmt::Debug for NestedStruct {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for NestedStruct {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     f.debug_struct("NestedStruct")
       .field("a", &self.a())
       .field("b", &self.b())
@@ -45,7 +49,7 @@ impl<'b> flatbuffers::Push for NestedStruct {
     #[inline]
     fn push(&self, dst: &mut [u8], _rest: &[u8]) {
         let src = unsafe {
-            ::std::slice::from_raw_parts(self as *const NestedStruct as *const u8, Self::size())
+            ::core::slice::from_raw_parts(self as *const NestedStruct as *const u8, Self::size())
         };
         dst.copy_from_slice(src);
     }
@@ -56,7 +60,7 @@ impl<'b> flatbuffers::Push for &'b NestedStruct {
     #[inline]
     fn push(&self, dst: &mut [u8], _rest: &[u8]) {
         let src = unsafe {
-            ::std::slice::from_raw_parts(*self as *const NestedStruct as *const u8, Self::size())
+            ::core::slice::from_raw_parts(*self as *const NestedStruct as *const u8, Self::size())
         };
         dst.copy_from_slice(src);
     }
@@ -129,7 +133,7 @@ impl<'a> NestedStruct {
 
   pub fn set_c(&mut self, x: &[TestEnum; 2]) {
     unsafe {
-      std::ptr::copy(
+      core::ptr::copy(
         x.as_ptr() as *const u8,
         self.0.as_mut_ptr().add(9),
         2,

tests/arrays_test/my_game/example/test_enum_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 TestEnum {
     }
   }
 }
-impl std::fmt::Debug for TestEnum {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for TestEnum {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     if let Some(name) = self.variant_name() {
       f.write_str(name)
     } else {