Support unnamed arguments in function pointers

Fixes #3089

Author: hanmertens

crates/ra_parser/src/grammar/params.rs

@@ -115,7 +115,7 @@ fn value_parameter(p: &mut Parser, flavor: Flavor) {
         // type Foo = fn(Bar::Baz);
         // type Qux = fn(baz: Bar::Baz);
         Flavor::FnPointer => {
-            if p.at(IDENT) && p.nth(1) == T![:] && !p.nth_at(1, T![::]) {
+            if (p.at(IDENT) || p.at(UNDERSCORE)) && p.nth(1) == T![:] && !p.nth_at(1, T![::]) {
                 patterns::pattern_single(p);
                 types::ascription(p);
             } else {