better error messages for target-feature detection macros (rust-lang#352)

Better error messages for target-feature detection macros

Author: gnzlbg

stdsimd/arch/detect/mod.rs

@@ -36,3 +36,72 @@ pub use self::arch::Feature;
 pub fn check_for(x: Feature) -> bool {
     cache::test(x as u32, arch::detect_features)
 }
+
+#[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
+#[macro_export]
+#[unstable(feature = "stdsimd", issue = "0")]
+macro_rules! is_x86_feature_detected {
+    ($t:tt) => {
+        compile_error!(r#"
+is_x86_feature_detected can only be used on x86 and x86_64 targets.
+You can prevent it from being used in other architectures by
+guarding it behind a cfg(target_arch) as follows:
+
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] {
+        if is_x86_feature_detected(...) { ... }
+    }
+"#)
+    };
+}
+
+#[cfg(not(target_arch = "arm"))]
+#[macro_export]
+#[unstable(feature = "stdsimd", issue = "0")]
+macro_rules! is_arm_feature_detected {
+    ($t:tt) => {
+        compile_error!(r#"
+is_arm_feature_detected can only be used on ARM targets.
+You can prevent it from being used in other architectures by
+guarding it behind a cfg(target_arch) as follows:
+
+    #[cfg(target_arch = "arm")] {
+        if is_arm_feature_detected(...) { ... }
+    }
+"#)
+    };
+}
+
+#[cfg(not(target_arch = "aarch64"))]
+#[macro_export]
+#[unstable(feature = "stdsimd", issue = "0")]
+macro_rules! is_aarch64_feature_detected {
+    ($t:tt) => {
+        compile_error!(r#"
+is_aarch64_feature_detected can only be used on AArch64 targets.
+You can prevent it from being used in other architectures by
+guarding it behind a cfg(target_arch) as follows:
+
+    #[cfg(target_arch = "aarch64")] {
+        if is_aarch64_feature_detected(...) { ... }
+    }
+"#)
+    };
+}
+
+#[cfg(not(target_arch = "powerpc64"))]
+#[macro_export]
+#[unstable(feature = "stdsimd", issue = "0")]
+macro_rules! is_powerpc64_feature_detected {
+    ($t:tt) => {
+        compile_error!(r#"
+is_powerpc64_feature_detected can only be used on PowerPC64 targets.
+You can prevent it from being used in other architectures by
+guarding it behind a cfg(target_arch) as follows:
+
+    #[cfg(target_arch = "powerpc64")] {
+        if is_powerpc64_feature_detected(...) { ... }
+    }
+"#)
+    };
+}
+