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.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 2a745e979a7..1b2c2869cb8 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -316,6 +316,9 @@ declare_features! (
 
     // Allows `break {expr}` with a value inside `loop`s.
     (active, loop_break_value, "1.14.0", Some(37339)),
+
+    // Allows #[target_feature(...)]
+    (active, target_feature, "1.15.0", None),
 );
 
 declare_features! (
@@ -664,6 +667,10 @@ pub const BUILTIN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeG
                                  "the `#[naked]` attribute \
                                   is an experimental feature",
                                  cfg_fn!(naked_functions))),
+    ("target_feature", Whitelisted, Gated(
+        Stability::Unstable, "target_feature",
+        "the `#[target_feature]` attribute is an experimental feature",
+        cfg_fn!(target_feature))),
     ("export_name", Whitelisted, Ungated),
     ("inline", Whitelisted, Ungated),
     ("link", Whitelisted, Ungated),