Since is_x86_feature_detected requires operating-system support it cannot be provided in libcore.

@gnzlbg I'm a bit confused by this. For x86, all features are detected via CPUID or via XCR0 (which requires the OS to enable osxsave), both of which are available in usersapce. This means that while OS support is needed to enable these features, no OS support is needed to detect these features. The detect_features function doesn't use anything that's not already in libcore. std_detect even notes that "On x86/x86_64 no OS specific functionality is required":

stdarch/crates/std_detect/src/detect/mod.rs

Line 97 in f1934fd

// On x86/x86_64 no OS specific functionality is required.

Both ring (see the linked issue) and getrandom (for the RDRAND implementation) have had to work around this by duplicating parts of std_detect's code in their own crates. This seems unnecessary.

I know the story for other arches is different, but it seems like nothing is preventing providing is_x86_feature_detected!() as part of libcore.

Expose is_x86_feature_detected to libcore #464

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g2popenedon May 28, 2018

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g2p
openedon May 28, 2018