error[E0618]: expected function, found struct `G`
  --> $DIR/issue-20714.rs:4:13
   |
LL | struct G;
   | -------- struct `G` defined here
...
LL |     let g = G();
   |             ^--
   |             |
   |             call expression requires function
   |
help: `G` is a unit struct, and does not take parentheses to be constructed
   |
LL -     let g = G();
LL +     let g = G;
   |

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0618`.