Support option on no_std

[d3d9]

Hi, would it be possible to allow usage of the option feature on no_std targets by using core imports like core::option::Option::{self, *} and core::convert::{From, Into}?

[d3d9]

I just remembered the core prelude gets imported automatically on no_std so is this just the only change that's required? Maybe the option feature code could also be moved out of the std module to avoid confusion.

-#![cfg_attr(not(any(test, feature = "box", feature = "option")), no_std)]
+#![cfg_attr(not(any(test, feature = "box")), no_std)]

I haven't had a look at the box feature but maybe something similar could be done with alloc instead of std to improve compatibility.

[yanganto]

@d3d9

The PR is ready, I hope you can help me review, because it seems you have a better knowledge on no_std.

Sorry about I do things in one commit, because it is late in my time. Here is the details I have done.

• The std mod is split into option mod and box mod,
• two bin in no-std-example for test are added.
• update ci

Thank you.

[d3d9]

Thank you, I've had a look at it and prepared a little patch with my ideas.
I tried to check how other crates that I use in my embedded projects tend to do it.
Usually, there is a feature alloc for things that depend on alloc, and std only in case there is something that really relies on std and is not part of the alloc library.

• lib/Cargo.toml:
  • Basically, the feature std in this crate would not be required anymore and only remain for compatibility to previous versions for existing users.
  • make box dependent on new feature alloc
• lib/src/box.rs
  • the alloc::boxed::Box is exactly the same as in std, so it can always be used, even in projects that are not no_std.
• lib/src/lib.rs
  • always defining #![no_std] because the library is not dependent on std, no matter which features enabled
  • defining extern crate alloc; when the feature alloc is enabled
• Tests
  • because Strings are used in the examples, the test modules also get extern crate alloc; because they might get run without the alloc feature enabled.
  • it might be cleaner to remove the dependency on alloc for most general tests by using another type in the test structs, and maybe add some tests with String in a separate test module that only gets enabled when alloc is enabled? Currently the tests wouldn't be able to run on a platform that doesn't provide alloc.

diff --git a/lib/Cargo.toml b/lib/Cargo.toml
index 6eab5ce..f5a99a4 100644
--- a/lib/Cargo.toml
+++ b/lib/Cargo.toml
@@ -34,8 +34,9 @@ merge = [
     "struct-patch-derive/merge"
 ]
 
+alloc = []
 std = ["box", "option"]
-box = []
+box = ["alloc"]
 option = []
 nesting = [
     "struct-patch-derive/nesting"
diff --git a/lib/src/box.rs b/lib/src/box.rs
index 9a0f624..ad16364 100644
--- a/lib/src/box.rs
+++ b/lib/src/box.rs
@@ -1,14 +1,9 @@
 #![cfg(feature = "box")]
 use crate::Patch;
 
-#[cfg(not(feature = "std"))]
 extern crate alloc;
-#[cfg(not(feature = "std"))]
 use alloc::boxed::Box;
 
-#[cfg(feature = "std")]
-use std::boxed::Box;
-
 impl<T, P> Patch<Box<P>> for T
 where
     T: Patch<P>,
@@ -33,6 +28,7 @@ where
 #[cfg(test)]
 mod tests {
     use super::*;
+    use alloc::string::String;
     use crate as struct_patch;
     use crate::Patch;
 
diff --git a/lib/src/lib.rs b/lib/src/lib.rs
index 65062dc..ef2b307 100644
--- a/lib/src/lib.rs
+++ b/lib/src/lib.rs
@@ -73,7 +73,10 @@
 //! item.apply(filler_2);
 //! assert_eq!(item.maybe_field_int, Some(7));
 //! ```
-#![cfg_attr(not(any(test, feature = "std")), no_std)]
+#![no_std]
+
+#[cfg(feature = "alloc")]
+extern crate alloc;
 
 #[doc(hidden)]
 pub use struct_patch_derive::Filler;
@@ -86,6 +89,8 @@ pub use traits::*;
 
 #[cfg(test)]
 mod tests {
+    extern crate alloc;
+    use alloc::string::String;
     use serde::Deserialize;
     #[cfg(feature = "merge")]
     use struct_patch::Merge;
diff --git a/lib/src/option.rs b/lib/src/option.rs
index e7c0974..4a6781a 100644
--- a/lib/src/option.rs
+++ b/lib/src/option.rs
@@ -140,6 +140,8 @@ where
 #[cfg(test)]
 mod tests {
     use super::*;
+    extern crate alloc;
+    use alloc::string::String;
     use crate as struct_patch;
     use crate::Patch;