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

tests/namespace_test/namespace_a/namespace_b/struct_in_nested_ns_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 StructInNestedNS, aligned to 4
@@ -13,8 +17,8 @@ impl Default for StructInNestedNS {
     Self([0; 8])
   }
 }
-impl std::fmt::Debug for StructInNestedNS {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for StructInNestedNS {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     f.debug_struct("StructInNestedNS")
       .field("a", &self.a())
       .field("b", &self.b())
@@ -43,7 +47,7 @@ impl<'b> flatbuffers::Push for StructInNestedNS {
     #[inline]
     fn push(&self, dst: &mut [u8], _rest: &[u8]) {
         let src = unsafe {
-            ::std::slice::from_raw_parts(self as *const StructInNestedNS as *const u8, Self::size())
+            ::core::slice::from_raw_parts(self as *const StructInNestedNS as *const u8, Self::size())
         };
         dst.copy_from_slice(src);
     }
@@ -54,7 +58,7 @@ impl<'b> flatbuffers::Push for &'b StructInNestedNS {
     #[inline]
     fn push(&self, dst: &mut [u8], _rest: &[u8]) {
         let src = unsafe {
-            ::std::slice::from_raw_parts(*self as *const StructInNestedNS as *const u8, Self::size())
+            ::core::slice::from_raw_parts(*self as *const StructInNestedNS as *const u8, Self::size())
         };
         dst.copy_from_slice(src);
     }

tests/namespace_test/namespace_a/namespace_b/table_in_nested_ns_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 TableInNestedNSOffset {}
@@ -101,8 +105,8 @@ impl<'a: 'b, 'b> TableInNestedNSBuilder<'a, 'b> {
   }
 }
 
-impl std::fmt::Debug for TableInNestedNS<'_> {
-  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for TableInNestedNS<'_> {
+  fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
     let mut ds = f.debug_struct("TableInNestedNS");
       ds.field("foo", &self.foo());
       ds.finish()

tests/namespace_test/namespace_a/namespace_b/union_in_nested_ns_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 UnionInNestedNS {
     }
   }
 }
-impl std::fmt::Debug for UnionInNestedNS {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for UnionInNestedNS {
+  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 UnionInNestedNST {
   /// If the union variant matches, return the owned TableInNestedNST, setting the union to NONE.
   pub fn take_table_in_nested_ns(&mut self) -> Option<Box<TableInNestedNST>> {
     if let Self::TableInNestedNS(_) = self {
-      let v = std::mem::replace(self, Self::NONE);
+      let v = core::mem::replace(self, Self::NONE);
       if let Self::TableInNestedNS(w) = v {
         Some(w)
       } else {

tests/namespace_test/namespace_a/second_table_in_a_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 SecondTableInAOffset {}
@@ -103,8 +107,8 @@ impl<'a: 'b, 'b> SecondTableInABuilder<'a, 'b> {
   }
 }
 
-impl std::fmt::Debug for SecondTableInA<'_> {
-  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for SecondTableInA<'_> {
+  fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
     let mut ds = f.debug_struct("SecondTableInA");
       ds.field("refer_to_c", &self.refer_to_c());
       ds.finish()

tests/namespace_test/namespace_a/table_in_first_ns_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 TableInFirstNSOffset {}
@@ -185,8 +189,8 @@ impl<'a: 'b, 'b> TableInFirstNSBuilder<'a, 'b> {
   }
 }
 
-impl std::fmt::Debug for TableInFirstNS<'_> {
-  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for TableInFirstNS<'_> {
+  fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
     let mut ds = f.debug_struct("TableInFirstNS");
       ds.field("foo_table", &self.foo_table());
       ds.field("foo_enum", &self.foo_enum());

tests/namespace_test/namespace_c/table_in_c_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 TableInCOffset {}
@@ -120,8 +124,8 @@ impl<'a: 'b, 'b> TableInCBuilder<'a, 'b> {
   }
 }
 
-impl std::fmt::Debug for TableInC<'_> {
-  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for TableInC<'_> {
+  fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
     let mut ds = f.debug_struct("TableInC");
       ds.field("refer_to_a1", &self.refer_to_a1());
       ds.field("refer_to_a2", &self.refer_to_a2());