check for repr(transparent) in build.rs

build.rs

@@ -0,0 +1,117 @@
+// Copyright © 2018 Trevor Spiteri
+
+// Copying and distribution of this file, with or without
+// modification, are permitted in any medium without royalty provided
+// the copyright notice and this notice are preserved. This file is
+// offered as-is, without any warranty.
+
+use std::env;
+use std::ffi::OsString;
+use std::fs::{self, File};
+use std::io::{Result as IoResult, Write};
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+struct Environment {
+    out_dir: PathBuf,
+    rustc: OsString,
+}
+
+fn main() {
+    let env = Environment {
+        out_dir: PathBuf::from(cargo_env("OUT_DIR")),
+        rustc: cargo_env("RUSTC"),
+    };
+    env.check_feature(
+        "repr_transparent",
+        TRY_REPR_TRANSPARENT,
+        Some("repr_transparent"),
+    );
+}
+
+impl Environment {
+    fn check_feature(&self, name: &str, contents: &str, nightly_features: Option<&str>) {
+        let try_dir = self.out_dir.join(format!("try_{}", name));
+        let filename = format!("try_{}.rs", name);
+        create_dir_or_panic(&try_dir);
+        println!("$ cd {:?}", try_dir);
+
+        enum Iteration {
+            Stable,
+            Unstable,
+        }
+        for i in &[Iteration::Stable, Iteration::Unstable] {
+            let s;
+            let file_contents = match *i {
+                Iteration::Stable => contents,
+                Iteration::Unstable => match nightly_features {
+                    Some(features) => {
+                        s = format!("#![feature({})]\n{}", features, contents);
+                        &s
+                    }
+                    None => continue,
+                },
+            };
+            create_file_or_panic(&try_dir.join(&filename), file_contents);
+            let mut cmd = Command::new(&self.rustc);
+            cmd.current_dir(&try_dir)
+                .args(&[&filename, "--emit=dep-info,metadata"]);
+            println!("$ {:?}", cmd);
+            let status = cmd
+                .status()
+                .unwrap_or_else(|_| panic!("Unable to execute: {:?}", cmd));
+            if status.success() {
+                println!("cargo:rustc-cfg={}", name);
+                if let Iteration::Unstable = *i {
+                    println!("cargo:rustc-cfg=nightly_{}", name);
+                }
+                break;
+            }
+        }
+
+        remove_dir_or_panic(&try_dir);
+    }
+}
+
+fn cargo_env(name: &str) -> OsString {
+    env::var_os(name)
+        .unwrap_or_else(|| panic!("environment variable not found: {}, please use cargo", name))
+}
+
+fn remove_dir(dir: &Path) -> IoResult<()> {
+    if !dir.exists() {
+        return Ok(());
+    }
+    assert!(dir.is_dir(), "Not a directory: {:?}", dir);
+    println!("$ rm -r {:?}", dir);
+    fs::remove_dir_all(dir)
+}
+
+fn remove_dir_or_panic(dir: &Path) {
+    remove_dir(dir).unwrap_or_else(|_| panic!("Unable to remove directory: {:?}", dir));
+}
+
+fn create_dir(dir: &Path) -> IoResult<()> {
+    println!("$ mkdir -p {:?}", dir);
+    fs::create_dir_all(dir)
+}
+
+fn create_dir_or_panic(dir: &Path) {
+    create_dir(dir).unwrap_or_else(|_| panic!("Unable to create directory: {:?}", dir));
+}
+
+fn create_file_or_panic(filename: &Path, contents: &str) {
+    println!("$ printf '%s' {:?}... > {:?}", &contents[0..20], filename);
+    let mut file =
+        File::create(filename).unwrap_or_else(|_| panic!("Unable to create file: {:?}", filename));
+    file.write_all(contents.as_bytes())
+        .unwrap_or_else(|_| panic!("Unable to write to file: {:?}", filename));
+}
+
+const TRY_REPR_TRANSPARENT: &str = r#"// try_repr_transparent.rs
+#[repr(transparent)]
+struct Foo(i32);
+fn main() {
+    let _ = Foo(12);
+}
+"#;

src/lib.rs

@@ -81,6 +81,7 @@ additional terms or conditions.
 #![warn(missing_docs)]
 #![doc(html_root_url = "https://docs.rs/fixed/0.0.1")]
 #![doc(test(attr(deny(warnings))))]
+#![cfg_attr(nightly_repr_transparent, feature(repr_transparent))]
 
 extern crate typenum;
 
@@ -415,7 +416,7 @@ macro_rules! fixed {
                 "[const generics]: https://github.com/rust-lang/rust/issues/44580\n",
                 "[typenum crate]: https://crates.io/crates/typenum\n"
             ),
-            #[repr(transparent)]
+            #[cfg_attr(repr_transparent, repr(transparent))]
             pub struct $Fixed<Frac: Unsigned>(($Inner, PhantomData<Frac>));
         }