Target Feature: target_feature 1.1

[Centril]

• RFC: target_feature 1.1 rust-lang/rfcs#2396
• Old RFC: Minimal target feature unsafe rust-lang/rfcs#2212
• RFC 2045: RFC: target_feature rust-lang/rfcs#2045

[gnzlbg]

One major problem with #[target_feature] as proposed is that it cannot propagate effects properly. This makes it impossible for libraries to work around code-generation issues: rust-lang/stdarch#362 (comment)

An effect systems for target features would need to allow a caller to enable target features for their callees. This might mean that all Rust functions my need to be always polymorphic with respect to target-feature effects.