(I created this issue to avoid duplication of discussion between them.)
In most cases, the APIs where people are unhappy seem to be filter-related. (Note that this does not necessarily mean that "many people are unhappy with the current filter-related API" since those who are not unhappy with the current API will not leave comments.)
This is a tracking issue for consistency in APIs that use closures in futures 0.4.
For individual cases, see below. (Especially #1889, #2470, and #2518 have some discussion on this)
(I created this issue to avoid duplication of discussion between them.)
In most cases, the APIs where people are unhappy seem to be filter-related. (Note that this does not necessarily mean that "many people are unhappy with the current filter-related API" since those who are not unhappy with the current API will not leave comments.)