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.

Author: Aaron1011

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
     // -------------------------------------------------------------------------

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);
 }

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.

src/libsyntax/parse/parser.rs

@@ -1486,6 +1486,15 @@ 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>> {

src/libsyntax/parse/parser/item.rs

@@ -149,17 +149,23 @@ impl<'a> Parser<'a> {
         }
         if self.eat_keyword(kw::Const) {
             let const_span = self.prev_span;
-            if self.check_keyword(kw::Fn)
-                || (self.check_keyword(kw::Unsafe)
-                    && self.is_keyword_ahead(1, &[kw::Fn])) {
+            if [kw::Fn, kw::Unsafe, kw::Extern].iter().any(|k| self.check_keyword(*k)) {
                 // CONST FUNCTION ITEM
+
                 let unsafety = self.parse_unsafety();
-                self.bump();
+
+                if self.check_keyword(kw::Extern) {
+                    self.sess.gated_spans.const_extern_fn.borrow_mut().push(
+                        lo.to(self.token.span)
+                    );
+                }
+                let abi = self.parse_extern_abi()?;
+
                 let header = FnHeader {
                     unsafety,
                     asyncness: respan(const_span, IsAsync::NotAsync),
                     constness: respan(const_span, Constness::Const),
-                    abi: Abi::Rust,
+                    abi,
                 };
                 return self.parse_item_fn(lo, visibility, attrs, header);
             }
@@ -257,11 +263,7 @@ impl<'a> Parser<'a> {
             self.bump(); // `unsafe`
             // `{` is also expected after `unsafe`; in case of error, include it in the diagnostic.
             self.check(&token::OpenDelim(token::Brace));
-            let abi = if self.eat_keyword(kw::Extern) {
-                self.parse_opt_abi()?.unwrap_or(Abi::C)
-            } else {
-                Abi::Rust
-            };
+            let abi = self.parse_extern_abi()?;
             self.expect_keyword(kw::Fn)?;
             let fn_span = self.prev_span;
             let header = FnHeader {
@@ -834,11 +836,7 @@ impl<'a> Parser<'a> {
         let (constness, unsafety, abi) = if is_const_fn {
             (respan(const_span, Constness::Const), unsafety, Abi::Rust)
         } else {
-            let abi = if self.eat_keyword(kw::Extern) {
-                self.parse_opt_abi()?.unwrap_or(Abi::C)
-            } else {
-                Abi::Rust
-            };
+            let abi = self.parse_extern_abi()?;
             (respan(self.prev_span, Constness::NotConst), unsafety, abi)
         };
         if !self.eat_keyword(kw::Fn) {
@@ -1278,14 +1276,30 @@ impl<'a> Parser<'a> {
         // Treat `const` as `static` for error recovery, but don't add it to expected tokens.
         if self.check_keyword(kw::Static) || self.token.is_keyword(kw::Const) {
             if self.token.is_keyword(kw::Const) {
-                self.diagnostic()
-                    .struct_span_err(self.token.span, "extern items cannot be `const`")
-                    .span_suggestion(
+                let mut err = self
+                    .struct_span_err(self.token.span, "extern items cannot be `const`");
+
+
+                // The user wrote 'const fn'
+                if self.is_keyword_ahead(1, &[kw::Fn, kw::Unsafe]) {
+                    err.emit();
+                    // Consume `const`
+                    self.bump();
+                    // Consume `unsafe` if present, since `extern` blocks
+                    // don't allow it. This will leave behind a plain 'fn'
+                    self.eat_keyword(kw::Unsafe);
+                    // Treat 'const fn` as a plain `fn` for error recovery purposes.
+                    // We've already emitted an error, so compilation is guaranteed
+                    // to fail
+                    return Ok(self.parse_item_foreign_fn(visibility, lo, attrs, extern_sp)?);
+                }
+                err.span_suggestion(
                         self.token.span,
                         "try using a static value",
                         "static".to_owned(),
                         Applicability::MachineApplicable
-                    ).emit();
+                );
+                err.emit();
             }
             self.bump(); // `static` or `const`
             return Ok(self.parse_item_foreign_static(visibility, lo, attrs)?);

src/libsyntax_pos/symbol.rs

@@ -196,6 +196,7 @@ symbols! {
         console,
         const_compare_raw_pointers,
         const_constructor,
+        const_extern_fn,
         const_fn,
         const_fn_union,
         const_generics,

src/test/ui/consts/const-extern-fn/const-extern-fn-call-extern-fn.rs

@@ -0,0 +1,23 @@
+#![feature(const_extern_fn)]
+
+extern "C" {
+    fn regular_in_block();
+}
+
+const extern fn bar() {
+    unsafe {
+        regular_in_block();
+        //~^ ERROR: cannot call functions with `"C"` abi in `min_const_fn`
+    }
+}
+
+extern fn regular() {}
+
+const extern fn foo() {
+    unsafe {
+        regular();
+        //~^ ERROR: cannot call functions with `"C"` abi in `min_const_fn`
+    }
+}
+
+fn main() {}

src/test/ui/consts/const-extern-fn/const-extern-fn-call-extern-fn.stderr

@@ -0,0 +1,21 @@
+error[E0723]: cannot call functions with `"C"` abi in `min_const_fn`
+  --> $DIR/const-extern-fn-call-extern-fn.rs:9:9
+   |
+LL |         regular_in_block();
+   |         ^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
+   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+
+error[E0723]: cannot call functions with `"C"` abi in `min_const_fn`
+  --> $DIR/const-extern-fn-call-extern-fn.rs:18:9
+   |
+LL |         regular();
+   |         ^^^^^^^^^
+   |
+   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
+   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0723`.

src/test/ui/consts/const-extern-fn/const-extern-fn-min-const-fn.rs

@@ -0,0 +1,13 @@
+#![feature(const_extern_fn)]
+
+const extern fn unsize(x: &[u8; 3]) -> &[u8] { x }
+//~^ ERROR unsizing casts are not allowed in const fn
+const unsafe extern "C" fn closure() -> fn() { || {} }
+//~^ ERROR function pointers in const fn are unstable
+const unsafe extern fn use_float() { 1.0 + 1.0; }
+//~^ ERROR only int, `bool` and `char` operations are stable in const fn
+const extern "C" fn ptr_cast(val: *const u8) { val as usize; }
+//~^ ERROR casting pointers to ints is unstable in const fn
+
+
+fn main() {}

src/test/ui/consts/const-extern-fn/const-extern-fn-min-const-fn.stderr

@@ -0,0 +1,39 @@
+error[E0723]: unsizing casts are not allowed in const fn
+  --> $DIR/const-extern-fn-min-const-fn.rs:3:48
+   |
+LL | const extern fn unsize(x: &[u8; 3]) -> &[u8] { x }
+   |                                                ^
+   |
+   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
+   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+
+error[E0723]: function pointers in const fn are unstable
+  --> $DIR/const-extern-fn-min-const-fn.rs:5:41
+   |
+LL | const unsafe extern "C" fn closure() -> fn() { || {} }
+   |                                         ^^^^
+   |
+   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
+   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+
+error[E0723]: only int, `bool` and `char` operations are stable in const fn
+  --> $DIR/const-extern-fn-min-const-fn.rs:7:38
+   |
+LL | const unsafe extern fn use_float() { 1.0 + 1.0; }
+   |                                      ^^^^^^^^^
+   |
+   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
+   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+
+error[E0723]: casting pointers to ints is unstable in const fn
+  --> $DIR/const-extern-fn-min-const-fn.rs:9:48
+   |
+LL | const extern "C" fn ptr_cast(val: *const u8) { val as usize; }
+   |                                                ^^^^^^^^^^^^
+   |
+   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
+   = help: add `#![feature(const_fn)]` to the crate attributes to enable
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0723`.