Auto merge of #64906 - Aaron1011:feature/extern-const-fn, r=Centril

Add support for `const unsafe? extern fn`

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

Currently, panicking is not allowed in `const`s. When https://github.com/rust-lang/rfcs/pull/2345 (https://github.com/rust-lang/rust/issues/51999) is stabilized, then panicking in an `const extern 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.

Tracking issue: https://github.com/rust-lang/rust/issues/64926.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index d4a6e9f6c6b..93165b09649 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -1487,6 +1487,15 @@ impl<'a> Parser<'a> {
         Ok(())
     }
 
+    /// Parses `extern` followed by an optional ABI string, or nothing.
+    fn parse_extern_abi(&mut self) -> PResult<'a, Abi> {
+        if self.eat_keyword(kw::Extern) {
+            Ok(self.parse_opt_abi()?.unwrap_or(Abi::C))
+        } else {
+            Ok(Abi::Rust)
+        }
+    }
+
     /// Parses a string as an ABI spec on an extern type or module. Consumes
     /// the `extern` keyword, if one is found.
     fn parse_opt_abi(&mut self) -> PResult<'a, Option<Abi>> {