os: add support for xous

[xobs]

Add support for riscv32imac-unknown-xous-elf.

The entire system is based on UTF-8. The OsStrExt and OsStringExt modules provide identical implementations to unix and wasi.

[dylni]

Do you have a link to where std::os::xous is defined? I don't see it in this file:
https://github.com/rust-lang/rust/blob/1898c34e923bad763e723c68dd9f23a09f9eb0fc/library/std/src/os/mod.rs

However, if OsStrExt and OsStringExt will not be defined for this target and only UTF-8 will be supported, it could be treated similarly to WebAssembly.

[xobs]

Thank you for taking a look at this.

I'm still upstreaming parts, and am working on 3rd party libraries before I get libstd upstreamed. However, since we're only using UTF-8, and OS strings should only ever be UTF-8, perhaps you're right and I just misunderstood how to use this.

If we do duplicate wasm, which approach would you prefer?

1. Just use wasm on xous
2. Rename wasm to something like utf8native and use that

For example, this does appear to work and build. I can take this approach if you prefer:

diff --git a/src/lib.rs b/src/lib.rs
index ff24d5b..70986ea 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -236,12 +236,19 @@ macro_rules! if_raw_str {
 }

 #[cfg_attr(
-    all(target_family = "wasm", target_os = "unknown"),
+    any(
+        all(target_family = "wasm", target_os = "unknown"),
+        target_os = "xous"
+    ),
     path = "wasm/mod.rs"
 )]
 #[cfg_attr(windows, path = "windows/mod.rs")]
 #[cfg_attr(
-    not(any(all(target_family = "wasm", target_os = "unknown"), windows)),
+    not(any(
+        all(target_family = "wasm", target_os = "unknown"),
+        windows,
+        target_os = "xous"
+    )),
     path = "common/mod.rs"
 )]
 mod imp;
@@ -252,6 +259,7 @@ mod imp;
         target_family = "wasm",
         target_os = "unknown",
     ),
+    target_os = "xous",
     windows,
 ))]
 mod util;

[dylni]

Thanks for the detailed information and sorry for the late replies! As long as the traits are planned to be added to libstd and only return UTF-8 bytes, the current approach would be better. The wasm module is a hack I'd rather avoid for new targets unless needed.

However, support for this platform would need to be soft-unstable until the traits are stabilized by libstd. If that works for you, I can merge this and release a new version with the change.

[xobs]

That'd be perfect, thanks!

We keep a parallel fork of libstd that everyone uses, and we're working on getting things upstream. As long as they're okay with the same implementation of OsStrExt that Unix has, we'll keep that approach.

[dylni]

Thanks for the PR! I'll release this soon.