Use feature() on nightly toolchains only.

`feature()` is only allowed on Nightly, it's completely disallowed on stable and beta
even for already-stabilized features. So, we autodetect whether the user is using nightly
and conditionally use `feature()`. This allows the crates to Just Work on current 1.75 beta
and will also Just Work when 1.75 stable is out.

Keeping `feature()` is desirable to keep support for:

- Espressif's xtensa rustc fork. (they build from the stable branch but enable use of `feature()`, so latest xtensa rustc still requires `feature()`)
- Users of older nightlies

Once xtensa rust 1.75 is out, we can remove this (upstream nightlies that require `feature()` will be quite old by then, so dropping support for them should be OK).

I decided to not use already-made crates like `rustversion` to do this because they're quite big and do way more than what we need, so I felt badd adding another dep. The code is inspired from `rustversion`'s build.rs.

embedded-hal-async/build.rs

@@ -0,0 +1,18 @@
+use std::env;
+use std::ffi::OsString;
+use std::process::Command;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
+
+    let output = Command::new(rustc)
+        .arg("--version")
+        .output()
+        .expect("failed to run `rustc --version`");
+
+    if String::from_utf8_lossy(&output.stdout).contains("nightly") {
+        println!("cargo:rustc-cfg=nightly");
+    }
+}

embedded-hal-async/src/lib.rs

@@ -5,10 +5,9 @@
 // Needed to pass CI, because we deny warnings.
 // We don't immediately remove them to not immediately break older nightlies.
 // When all features are stable, we'll remove them.
-#![allow(stable_features)]
-#![allow(unknown_lints)]
+#![cfg_attr(nightly, allow(stable_features, unknown_lints))]
+#![cfg_attr(nightly, feature(async_fn_in_trait, impl_trait_projections))]
 #![allow(async_fn_in_trait)]
-#![feature(async_fn_in_trait, impl_trait_projections)]
 
 pub mod delay;
 pub mod digital;

embedded-hal-bus/build.rs

@@ -0,0 +1,18 @@
+use std::env;
+use std::ffi::OsString;
+use std::process::Command;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
+
+    let output = Command::new(rustc)
+        .arg("--version")
+        .output()
+        .expect("failed to run `rustc --version`");
+
+    if String::from_utf8_lossy(&output.stdout).contains("nightly") {
+        println!("cargo:rustc-cfg=nightly");
+    }
+}

embedded-hal-bus/src/lib.rs

@@ -6,8 +6,11 @@
 // Needed to pass CI, because we deny warnings.
 // We don't immediately remove them to not immediately break older nightlies.
 // When all features are stable, we'll remove them.
-#![cfg_attr(feature = "async", allow(stable_features))]
-#![cfg_attr(feature = "async", feature(async_fn_in_trait, impl_trait_projections))]
+#![cfg_attr(all(feature = "async", nightly), allow(stable_features))]
+#![cfg_attr(
+    all(feature = "async", nightly),
+    feature(async_fn_in_trait, impl_trait_projections)
+)]
 
 // needed to prevent defmt macros from breaking, since they emit code that does `defmt::blahblah`.
 #[cfg(feature = "defmt-03")]

embedded-io-adapters/build.rs

@@ -0,0 +1,18 @@
+use std::env;
+use std::ffi::OsString;
+use std::process::Command;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
+
+    let output = Command::new(rustc)
+        .arg("--version")
+        .output()
+        .expect("failed to run `rustc --version`");
+
+    if String::from_utf8_lossy(&output.stdout).contains("nightly") {
+        println!("cargo:rustc-cfg=nightly");
+    }
+}

embedded-io-adapters/src/lib.rs

@@ -7,11 +7,11 @@
 // We don't immediately remove them to not immediately break older nightlies.
 // When all features are stable, we'll remove them.
 #![cfg_attr(
-    any(feature = "tokio-1", feature = "futures-03"),
+    all(any(feature = "tokio-1", feature = "futures-03"), nightly),
     allow(stable_features)
 )]
 #![cfg_attr(
-    any(feature = "tokio-1", feature = "futures-03"),
+    all(any(feature = "tokio-1", feature = "futures-03"), nightly),
     feature(async_fn_in_trait, impl_trait_projections)
 )]
 

embedded-io-async/build.rs

@@ -0,0 +1,18 @@
+use std::env;
+use std::ffi::OsString;
+use std::process::Command;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
+
+    let output = Command::new(rustc)
+        .arg("--version")
+        .output()
+        .expect("failed to run `rustc --version`");
+
+    if String::from_utf8_lossy(&output.stdout).contains("nightly") {
+        println!("cargo:rustc-cfg=nightly");
+    }
+}

embedded-io-async/src/lib.rs

@@ -6,10 +6,9 @@
 // Needed to pass CI, because we deny warnings.
 // We don't immediately remove them to not immediately break older nightlies.
 // When all features are stable, we'll remove them.
-#![allow(stable_features)]
-#![allow(unknown_lints)]
+#![cfg_attr(nightly, allow(stable_features, unknown_lints))]
+#![cfg_attr(nightly, feature(async_fn_in_trait, impl_trait_projections))]
 #![allow(async_fn_in_trait)]
-#![feature(async_fn_in_trait, impl_trait_projections)]
 
 #[cfg(feature = "alloc")]
 extern crate alloc;