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由 Alex Crichton 提交于
This is an implementation of the `#[target_feature]` syntax-related changes of [RFC 2045][rfc]. Notably two changes have been implemented: * The new syntax is `#[target_feature(enable = "..")]` instead of `#[target_feature = "+.."]`. The `enable` key is necessary instead of the `+` to indicate that a feature is being enabled, and a sub-list is used for possible expansion in the future. Additionally within this syntax the feature names being enabled are now whitelisted against a known set of target feature names that we know about. * The `#[target_feature]` attribute can only be applied to unsafe functions. It was decided in the RFC that invoking an instruction possibly not defined for the current processor is undefined behavior, so to enable this feature for now it requires an `unsafe` intervention. [rfc]: https://github.com/rust-lang/rfcs/blob/master/text/2045-target-feature.md
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