Add support for 'extern const fn'

This works just as you might expect - an 'extern const fn' is a 'const
fn' that is callable from foreign code.

Currently, panicking is not allowed in consts. When RFC 2345 is
stabilized, then panicking in an 'extern const fn' will produce a
compile-time error when invoked at compile time, and an abort when
invoked at runtime.

Since this is extending the language (we're allowing the `const` keyword
in a new context), I believe that this will need an FCP. However, it's a
very minor change, so I didn't think that filing an RFC was necessary.

This will allow libc (and other FFI crates) to make many functions
`const`, without having to give up on making them `extern` as well.

2 files changed, 4 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate/active.rs b/src/libsyntax/feature_gate/active.rs
index 47ee41f0adc..20a77fa37cf 100644
--- a/src/libsyntax/feature_gate/active.rs
+++ b/src/libsyntax/feature_gate/active.rs
@@ -519,6 +519,9 @@ declare_features! (
     /// Allows the use of or-patterns (e.g., `0 | 1`).
     (active, or_patterns, "1.38.0", Some(54883), None),
 
+    /// Allows the definition of `const extern fn` and `const unsafe extern fn`.
+    (active, const_extern_fn, "1.40.0", Some(64926), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
diff --git a/src/libsyntax/feature_gate/check.rs b/src/libsyntax/feature_gate/check.rs
index d7fc74955bb..9e40b1a26ac 100644
--- a/src/libsyntax/feature_gate/check.rs
+++ b/src/libsyntax/feature_gate/check.rs
@@ -821,6 +821,7 @@ pub fn check_crate(krate: &ast::Crate,
     gate_all!(async_closure, "async closures are unstable");
     gate_all!(yields, generators, "yield syntax is experimental");
     gate_all!(or_patterns, "or-patterns syntax is experimental");
+    gate_all!(const_extern_fn, "`const extern fn` definitions are unstable");
 
     visit::walk_crate(&mut visitor, krate);
 }