chore: get rid of now-stabilized error_in_core feature

Author: TheButlah

optee-utee/Cargo.toml

@@ -42,6 +42,9 @@ serde_json = { version = "1.0.133" }
 optee-utee-sys = { version = "0.7.0", path = "optee-utee-sys", features = ["no_link"] }
 optee-utee-mock = { version = "0.7.0", path = "optee-utee-mock" }
 
+[build-dependencies]
+version_check = "0.9.5"
+
 [features]
 default = []
 std = ["optee-utee-sys/std", "optee-utee-macros/std"]

optee-utee/build.rs

@@ -0,0 +1,27 @@
+use std::fs;
+
+/// Outputs unstable #[feature = "foo"] iff the rustc version is older than $version
+macro_rules! maybe_feat {
+    ($out:expr, $feat:literal, $version:literal) => {
+        {
+            let filename = $out.join(concat!($feat, ".rs"));
+            
+            let s = if version_check::is_min_version($version).unwrap_or(false) {
+                String::new()
+            } else {
+                format!("#![feature = \"{}\"]\n", $feat)
+            };
+            fs::write(filename, s).expect("failed to write to {filename:?}");
+        }
+    }
+}
+
+fn main() {
+    let out = std::path::PathBuf::from(std::env::var("OUT_DIR").expect("infallible"));
+
+    // The custom patched std version is currently on 1.80. When we upgrade, we should
+    // bump the MSRV accordingly and remove any of these features that are stablized.
+    maybe_feat!(out, "error_in_core", "1.81.0");
+}
+
+

optee-utee/src/lib.rs

@@ -16,7 +16,20 @@
 // under the License.
 
 #![cfg_attr(not(feature = "std"), no_std)]
-#![cfg_attr(not(feature = "std"), feature(error_in_core))]
+
+#[allow(unused_macros)]
+/// Cargo will complain if your source file contains `#![features = ..]` even
+/// whe the current cfg wouldn't enable the attribute. To get around this, we
+/// `include!()` a source file with the offending attr *only* present when the compiler
+/// is too old.
+macro_rules! enable_feat {
+    ($feature:literal) => {
+        include!(concat!(env!("OUT_DIR"), "/", $feature, ".rs"));
+    }
+}
+
+#[cfg(not(feature = "std"))]
+enable_feat!("error_in_core");
 
 // Requires `alloc`.
 #[macro_use]