Once a target feature is enabled for a function that means that it in general
can't be inlined into other functions which don't have that target feature
enabled. This can cause both safety and LLVM issues if we were to actually
inline it, so `#[inline(always)]` both can't be respected and would be an error
if we did so!
Today LLVM doesn't inline functions with different `#[target_feature]`
annotations, but it turns out that if one is tagged with `#[inline(always)]`
it'll override this and cause scary LLVM error to arise!
This commit fixes this issue by forbidding these two attributes to be used in
conjunction with one another.
cc rust-lang-nursery/stdsimd#404
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
