From b4537fd9ac17573ded0fc6f548d0ca74bb92b844 Mon Sep 17 00:00:00 2001
From: Kevin Reid <kpreid@google.com>
Date: Fri, 17 Jun 2022 17:27:20 -0700
Subject: [PATCH] Make ouroboros `no_std` compatible.

* Mark the crate `#![no_std]`.
* Replace all references to `std` with `core` or `alloc`.
* Don't mention `Arc` if the target doesn't support it.
* Reexport `alloc` from `ouroboros::macro_help` so that the crate
  calling the macro doesn't need any `extern crate alloc`.
---
 ouroboros/src/lib.rs                    | 12 ++++++++----
 ouroboros_macro/src/generate/derives.rs |  4 ++--
 ouroboros_macro/src/info_structures.rs  | 20 ++++++++++++++++----
 3 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/ouroboros/src/lib.rs b/ouroboros/src/lib.rs
index 45c76de..87e50ac 100644
--- a/ouroboros/src/lib.rs
+++ b/ouroboros/src/lib.rs
@@ -3,6 +3,7 @@
 //! See the documentation of [`ouroboros_examples`](https://docs.rs/ouroboros_examples) for
 //! sample documentation of structs which have had the macro applied to them.
 
+#![no_std]
 #![allow(clippy::needless_doctest_main)]
 
 /// This macro is used to turn a regular struct into a self-referencing one. An example:
@@ -348,10 +349,12 @@ pub use ouroboros_macro::self_referencing;
 
 #[doc(hidden)]
 pub mod macro_help {
+    pub extern crate alloc;
+
     pub use aliasable::boxed::AliasableBox;
     use aliasable::boxed::UniqueBox;
 
-    pub struct CheckIfTypeIsStd<T>(std::marker::PhantomData<T>);
+    pub struct CheckIfTypeIsStd<T>(core::marker::PhantomData<T>);
 
     macro_rules! std_type_check {
         ($fn_name:ident $T:ident $check_for:ty) => {
@@ -361,9 +364,10 @@ pub mod macro_help {
         };
     }
 
-    std_type_check!(is_std_box_type T std::boxed::Box<T>);
-    std_type_check!(is_std_arc_type T std::sync::Arc<T>);
-    std_type_check!(is_std_rc_type T std::rc::Rc<T>);
+    std_type_check!(is_std_box_type T alloc::boxed::Box<T>);
+    #[cfg(target_has_atomic = "ptr")] // alloc::sync is missing if this is false
+    std_type_check!(is_std_arc_type T alloc::sync::Arc<T>);
+    std_type_check!(is_std_rc_type T alloc::rc::Rc<T>);
 
     pub fn aliasable_boxed<T>(data: T) -> AliasableBox<T> {
         AliasableBox::from_unique(UniqueBox::new(data))
diff --git a/ouroboros_macro/src/generate/derives.rs b/ouroboros_macro/src/generate/derives.rs
index 5a40841..0233a3f 100644
--- a/ouroboros_macro/src/generate/derives.rs
+++ b/ouroboros_macro/src/generate/derives.rs
@@ -40,10 +40,10 @@ fn impl_debug(info: &StructInfo) -> Result<TokenStream, Error> {
             }
         })
         .collect::<Vec<_>>();
-    let trait_name = syn::parse_quote! { ::std::fmt::Debug };
+    let trait_name = syn::parse_quote! { ::core::fmt::Debug };
     let struct_name = &info.ident;
     let body = quote! {
-        fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+        fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
             self.with(|safe_self| {
                 f.debug_struct(stringify!(#struct_name))
                 #(.#fields)*
diff --git a/ouroboros_macro/src/info_structures.rs b/ouroboros_macro/src/info_structures.rs
index a289f37..05dc734 100644
--- a/ouroboros_macro/src/info_structures.rs
+++ b/ouroboros_macro/src/info_structures.rs
@@ -261,10 +261,14 @@ impl StructFieldInfo {
         let field_type = &self.typ;
         let return_ty_constructor = || match builder_type {
             BuilderType::AsyncSend => {
-                quote! { ::std::pin::Pin<::std::boxed::Box<dyn ::core::future::Future<Output=#field_type> + ::core::marker::Send + 'this>> }
+                quote! {
+                    ::core::pin::Pin<::ouroboros::macro_help::alloc::boxed::Box<
+                        dyn ::core::future::Future<Output=#field_type> + ::core::marker::Send + 'this>>
+                }
             }
             BuilderType::Async => {
-                quote! { ::std::pin::Pin<::std::boxed::Box<dyn ::core::future::Future<Output=#field_type> + 'this>> }
+                quote! { ::core::pin::Pin<::ouroboros::macro_help::alloc::boxed::Box<
+                dyn ::core::future::Future<Output=#field_type> + 'this>> }
             }
             BuilderType::Sync => quote! { #field_type },
         };
@@ -280,10 +284,18 @@ impl StructFieldInfo {
         let field_type = &self.typ;
         let return_ty_constructor = || match builder_type {
             BuilderType::AsyncSend => {
-                quote! { ::std::pin::Pin<::std::boxed::Box<dyn ::core::future::Future<Output=::core::result::Result<#field_type, Error_>> + ::core::marker::Send + 'this>> }
+                quote! {
+                    ::core::pin::Pin<::ouroboros::macro_help::alloc::boxed::Box<
+                        dyn ::core::future::Future<Output=::core::result::Result<#field_type, Error_>>
+                            + ::core::marker::Send + 'this>>
+                }
             }
             BuilderType::Async => {
-                quote! { ::std::pin::Pin<::std::boxed::Box<dyn ::core::future::Future<Output=::core::result::Result<#field_type, Error_>> + 'this>> }
+                quote! {
+                    ::core::pin::Pin<::ouroboros::macro_help::alloc::boxed::Box<
+                        dyn ::core::future::Future<Output=::core::result::Result<#field_type, Error_>>
+                            + 'this>>
+                }
             }
             BuilderType::Sync => quote! { ::core::result::Result<#field_type, Error_> },
         };