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/keyword_test/keyword_test/abc_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 ABC {
     }
   }
 }
-impl std::fmt::Debug for ABC {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for ABC {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     if let Some(name) = self.variant_name() {
       f.write_str(name)
     } else {

tests/keyword_test/keyword_test/keywords_in_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 KeywordsInTableOffset {}
@@ -146,8 +150,8 @@ impl<'a: 'b, 'b> KeywordsInTableBuilder<'a, 'b> {
   }
 }
 
-impl std::fmt::Debug for KeywordsInTable<'_> {
-  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for KeywordsInTable<'_> {
+  fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
     let mut ds = f.debug_struct("KeywordsInTable");
       ds.field("is", &self.is());
       ds.field("private", &self.private());

tests/keyword_test/keyword_test/keywords_in_union_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 KeywordsInUnion {
     }
   }
 }
-impl std::fmt::Debug for KeywordsInUnion {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for KeywordsInUnion {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     if let Some(name) = self.variant_name() {
       f.write_str(name)
     } else {
@@ -128,7 +132,7 @@ impl KeywordsInUnionT {
   /// If the union variant matches, return the owned KeywordsInTableT, setting the union to NONE.
   pub fn take_static_(&mut self) -> Option<Box<KeywordsInTableT>> {
     if let Self::Static_(_) = self {
-      let v = std::mem::replace(self, Self::NONE);
+      let v = core::mem::replace(self, Self::NONE);
       if let Self::Static_(w) = v {
         Some(w)
       } else {
@@ -149,7 +153,7 @@ impl KeywordsInUnionT {
   /// If the union variant matches, return the owned KeywordsInTableT, setting the union to NONE.
   pub fn take_internal(&mut self) -> Option<Box<KeywordsInTableT>> {
     if let Self::Internal(_) = self {
-      let v = std::mem::replace(self, Self::NONE);
+      let v = core::mem::replace(self, Self::NONE);
       if let Self::Internal(w) = v {
         Some(w)
       } else {

tests/keyword_test/keyword_test/public_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.")]
@@ -34,8 +38,8 @@ impl public {
     }
   }
 }
-impl std::fmt::Debug for public {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for public {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     if let Some(name) = self.variant_name() {
       f.write_str(name)
     } else {