Stable compiler features

text/0000-stable-compiler-features.md

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+- Feature Name: stable-compiler-features
+- Start Date: 2016-01-22
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Stabilize some low level feature flags.
+
+# Motivation
+[motivation]: #motivation
+
+Currently there is no way to use LLVM intrinsics or `asm!` block in stable Rust
+as they are considered incompatible with hypothetical alternative
+implementations. But some of these features are highly desired in some
+environments where currently it is required either to:
+
+- use nightly
+- compile `rustc` from source
+- use external files (assembly)
+
+# Detailed design
+[design]: #detailed-design
+
+Some low-level features should be hidden behind stable feature gates. This would
+allow compiler to signalize user which features aren't supported on current
+platform/implementation.
+
+Ex. `#[feature(asm)]` would be stable in mainline Rust but not on GCC Rust, so
+GCC Rust can at compile time inform user that `asm` feature is not available and
+provide alternative (like `asm_gcc`) flag.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+There will be some crates that will require given compiler to work, but I think
+that it can be mitigated by Cargo's feature flags.
+
+Also there is non-0 possibility that these flags can be overused but I think
+that most Rust users would not be interested in them if we keep only some
+low-level stuff behind stable gates.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Keep current status quo.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- Which flags should be stabilized? I think that for now only:
+  + `asm`
+  * `llvm_intrinsics`