Auto merge of #38079 - BurntSushi:attrtarget, r=alexcrichton

Add new #[target_feature = "..."] attribute.

This commit adds a new attribute that instructs the compiler to emit
target specific code for a single function. For example, the following
function is permitted to use instructions that are part of SSE 4.2:

    #[target_feature = "+sse4.2"]
    fn foo() { ... }

In particular, use of this attribute does not require setting the
-C target-feature or -C target-cpu options on rustc.

This attribute does not have any protections built into it. For example,
nothing stops one from calling the above `foo` function on hosts without
SSE 4.2 support. Doing so may result in a SIGILL.

I've also expanded the x86 target feature whitelist.

0 files changed, 0 insertions, 0 deletions