expect fn after const unsafe / const extern

(cherry picked from commit 915db7a)

Author: Centril

src/librustc_parse/parser/item.rs

@@ -144,7 +144,7 @@ impl<'a> Parser<'a> {
                     self.sess.gated_spans.gate(sym::const_extern_fn, lo.to(self.token.span));
                 }
                 let ext = self.parse_extern()?;
-                self.bump(); // `fn`
+                self.expect_keyword(kw::Fn)?;
 
                 let header = FnHeader {
                     unsafety,

src/test/ui/consts/const-extern-fn/issue-68062-const-extern-fns-dont-need-fn-specifier-2.rs

@@ -0,0 +1,7 @@
+fn main() {}
+
+#[cfg(FALSE)]
+fn container() {
+    const unsafe WhereIsFerris Now() {}
+    //~^ ERROR expected one of `extern` or `fn`
+}

src/test/ui/consts/const-extern-fn/issue-68062-const-extern-fns-dont-need-fn-specifier-2.stderr

@@ -0,0 +1,8 @@
+error: expected one of `extern` or `fn`, found `WhereIsFerris`
+  --> $DIR/issue-68062-const-extern-fns-dont-need-fn-specifier-2.rs:5:18
+   |
+LL |     const unsafe WhereIsFerris Now() {}
+   |                  ^^^^^^^^^^^^^ expected one of `extern` or `fn`
+
+error: aborting due to previous error
+

src/test/ui/consts/const-extern-fn/issue-68062-const-extern-fns-dont-need-fn-specifier.rs

@@ -0,0 +1,8 @@
+fn main() {}
+
+#[cfg(FALSE)]
+fn container() {
+    const extern "Rust" PUT_ANYTHING_YOU_WANT_HERE bug() -> usize { 1 }
+    //~^ ERROR expected `fn`
+    //~| ERROR `const extern fn` definitions are unstable
+}

src/test/ui/consts/const-extern-fn/issue-68062-const-extern-fns-dont-need-fn-specifier.stderr

@@ -0,0 +1,18 @@
+error: expected `fn`, found `PUT_ANYTHING_YOU_WANT_HERE`
+  --> $DIR/issue-68062-const-extern-fns-dont-need-fn-specifier.rs:5:25
+   |
+LL |     const extern "Rust" PUT_ANYTHING_YOU_WANT_HERE bug() -> usize { 1 }
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `fn`
+
+error[E0658]: `const extern fn` definitions are unstable
+  --> $DIR/issue-68062-const-extern-fns-dont-need-fn-specifier.rs:5:5
+   |
+LL |     const extern "Rust" PUT_ANYTHING_YOU_WANT_HERE bug() -> usize { 1 }
+   |     ^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/64926
+   = help: add `#![feature(const_extern_fn)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.