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.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index fa4c1043122..1518da23b09 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -57,6 +57,8 @@ pub struct GatedSpans {
     pub yields: Lock<Vec<Span>>,
     /// Spans collected for gating `or_patterns`, e.g. `Some(Foo | Bar)`.
     pub or_patterns: Lock<Vec<Span>>,
+    /// Spans collected for gating `const_extern_fn`, e.g. `const extern fn foo`.
+    pub const_extern_fn: Lock<Vec<Span>>,
 }
 
 /// Info about a parsing session.