Automatically detect support for i128/u128

Author: cuviper

Cargo.toml

@@ -10,12 +10,13 @@ repository = "https://github.com/rust-num/num-integer"
 name = "num-integer"
 version = "0.1.37"
 readme = "README.md"
+build = "build.rs"
 
 [package.metadata.docs.rs]
-all-features = true
+features = ["std"]
 
 [dependencies.num-traits]
-version = "0.2.3"
+version = "0.2.4"
 default-features = false
 
 [features]

README.md

@@ -36,7 +36,10 @@ default-features = false
 There is no functional difference with and without `std` at this time, but
 there may be in the future.
 
-Implementations for `i128` and `u128` are only available when `i128` is enabled.
+Implementations for `i128` and `u128` are only available with Rust 1.26 and
+later.  The build script automatically detects this, but you can make it
+mandatory by enabling the `i128` crate feature.
+
 
 ## Releases
 

build.rs

@@ -0,0 +1,35 @@
+use std::env;
+use std::io::Write;
+use std::process::{Command, Stdio};
+
+fn main() {
+    if probe("fn main() { 0i128; }") {
+        println!("cargo:rustc-cfg=has_i128");
+    } else if env::var_os("CARGO_FEATURE_I128").is_some() {
+        panic!("i128 support was not detected!");
+    }
+}
+
+/// Test if a code snippet can be compiled
+fn probe(code: &str) -> bool {
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| "rustc".into());
+    let out_dir = env::var_os("OUT_DIR").expect("environment variable OUT_DIR");
+
+    let mut child = Command::new(rustc)
+        .arg("--out-dir")
+        .arg(out_dir)
+        .arg("--emit=obj")
+        .arg("-")
+        .stdin(Stdio::piped())
+        .spawn()
+        .expect("rustc probe");
+
+    child
+        .stdin
+        .as_mut()
+        .expect("rustc stdin")
+        .write_all(code.as_bytes())
+        .expect("write rustc stdin");
+
+    child.wait().expect("rustc probe").success()
+}

src/lib.rs

@@ -503,7 +503,7 @@ impl_integer_for_isize!(i16, test_integer_i16);
 impl_integer_for_isize!(i32, test_integer_i32);
 impl_integer_for_isize!(i64, test_integer_i64);
 impl_integer_for_isize!(isize, test_integer_isize);
-#[cfg(feature = "i128")]
+#[cfg(has_i128)]
 impl_integer_for_isize!(i128, test_integer_i128);
 
 macro_rules! impl_integer_for_usize {
@@ -677,7 +677,7 @@ impl_integer_for_usize!(u16, test_integer_u16);
 impl_integer_for_usize!(u32, test_integer_u32);
 impl_integer_for_usize!(u64, test_integer_u64);
 impl_integer_for_usize!(usize, test_integer_usize);
-#[cfg(feature = "i128")]
+#[cfg(has_i128)]
 impl_integer_for_usize!(u128, test_integer_u128);
 
 /// An iterator over binomial coefficients.