impl IoSafe for std::io::PipeReader & std::io::PipeWriter

It would be handy to have async wrappers for the pipe types (from
std::io::pipe()), so impl IoSafe for both.

The pipe types were introduced in 1.87, but the MSRV for async-io
appears to be 1.63 currently. So, protect the impls with cfg()s
and use autocfg to detect the type availbility, similar to what was done
in a20076f ("Implement I/O-safe traits (smol-rs#84)")

Signed-off-by: Jeremy Kerr <[email protected]>

Author: jk-ozlabs

Cargo.toml

@@ -40,6 +40,9 @@ tracing = { version = "0.1.37", default-features = false, optional = true }
 [target.'cfg(windows)'.dependencies]
 windows-sys = { version = "0.60.0", features = ["Win32_Foundation"] }
 
+[build-dependencies]
+autocfg = "1"
+
 [dev-dependencies]
 async-channel = "2.0.0"
 async-net = "2.0.0"

build.rs

@@ -0,0 +1,20 @@
+fn main() {
+    let cfg = match autocfg::AutoCfg::new() {
+        Ok(cfg) => cfg,
+        Err(e) => {
+            println!("cargo:warning=async-io: failed to detect compiler features: {e}");
+            return;
+        }
+    };
+
+    // emit expected config settings; may be replaced with
+    // autocfg::emit_possibility if we update to autocfg>=1.5
+    println!("cargo:rustc-check-cfg=cfg(async_io_no_pipe)");
+
+    // We use "no_*" instead of "has_*" here. For (non-Cargo) build processes
+    // that don't run build.rs, the negated version gives us a recent
+    // feature-set by default.
+    if !cfg.probe_rustc_version(1, 87) {
+        autocfg::emit("async_io_no_pipe");
+    }
+}

src/lib.rs

@@ -1252,6 +1252,13 @@ unsafe impl IoSafe for std::process::ChildStderr {}
 #[cfg(unix)]
 unsafe impl IoSafe for std::os::unix::net::UnixStream {}
 
+// PipeReader & PipeWriter require std >= 1.87, our MSRV is 1.63, hence
+// conditional on cfg()s, generated from build.rs
+#[cfg(not(async_io_no_pipe))]
+unsafe impl IoSafe for std::io::PipeReader {}
+#[cfg(not(async_io_no_pipe))]
+unsafe impl IoSafe for std::io::PipeWriter {}
+
 unsafe impl<T: IoSafe + Read> IoSafe for std::io::BufReader<T> {}
 unsafe impl<T: IoSafe + Write> IoSafe for std::io::BufWriter<T> {}
 unsafe impl<T: IoSafe + Write> IoSafe for std::io::LineWriter<T> {}