Feature gate extern fn methods

text/0000-extern-fn-methods.md

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+- Feature Name: gate-extern-fn-methods
+- Start Date: 2015-12-27
+- RFC PR: (leave this empty)
+- Rust Issue: #30235
+
+# Summary
+[summary]: #summary
+
+Restrict the use of non-Rust ABI methods to a feature gate.
+
+# Motivation
+[motivation]: #motivation
+
+Currently we allow this code:
+
+```rust
+trait Foo {
+    extern fn foo(&self);
+}
+```
+
+In the absense of an ABI string, `extern fn` defaults to using "C". This functionality does not
+work well and appears to frequently be a mistake on the part of the user. Since the decision to
+support this feature either way should be subject to discussion, this RFC is only proposing the
+addition of a feature gate that controls access to this feature.
+
+# Detailed design
+[design]: #detailed-design
+
+Detect use of `extern fn` in trait definitions and `impl` blocks that don't have a Rust ABI (either
+"Rust" or "rust-call") and disallow it unless a feature gate is enabled.
+
+The name of the feature gate is not important, but `extern-fn-methods` seems like a reasonable name.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+It may break some existing code, however evidence suggests that code attempting to use this feature
+runs into one of the bugs surrounding the feature and therefore already doesn't work.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Don't feature gate and instead skip straight to discussion about the feature's support. This isn't
+appealing as the feature will continue to be a source of confusion until the discussion is
+concluded.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None