suppress internal error

Author: rchaser53

rustfmt-core/rustfmt-lib/src/visitor.rs

@@ -364,7 +364,7 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
                     mk_sp(s.lo(), b.span.lo()),
                 )
             }
-            _ => unreachable!(),
+            _ => return,
         };
 
         if let Some((fn_str, fn_brace_style)) = rewrite {

rustfmt-core/rustfmt-lib/tests/target/issue-4068.rs

@@ -0,0 +1,4 @@
+// following code is invalid but should not cause an internal error
+fn main() {
+extern "C" fn packet_records_options_impl_layout_length_encoding_option_len_multiplier();
+}