error[E0283]: type annotations needed
  --> $DIR/question-mark-type-infer.rs:10:21
   |
LL |     l.iter().map(f).collect()?
   |                     ^^^^^^^ cannot infer type of the type parameter `B` declared on the method `collect`
   |
   = note: cannot satisfy `_: FromIterator<Result<i32, ()>>`
note: required by a bound in `collect`
  --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
help: consider specifying the generic argument
   |
LL |     l.iter().map(f).collect::<Vec<_>>()?
   |                            ++++++++++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0283`.