error[E0282]: type annotations needed
  --> $DIR/type-annotations-needed-expr.rs:2:39
   |
LL |     let _ = (vec![1,2,3]).into_iter().sum() as f64;
   |                                       ^^^ cannot infer type of the type parameter `S` declared on the associated function `sum`
   |
help: consider specifying the generic argument
   |
LL |     let _ = (vec![1,2,3]).into_iter().sum::<S>() as f64;
   |                                          +++++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0282`.