Commits on Sep 18, 2019

1. Simplify some Iterator methods. …

   PR rust-lang#64545 got a big speed-up by replacing a hot call to `all()` with
   explicit iteration. This is because the implementation of `all()` is
   excessively complex: it wraps the given predicate in a closure that
   returns a `LoopState`, passes that closure to `try_for_each()`, which
   wraps the first closure in a second closure, passes that second closure
   to `try_fold()`, which does the actual iteration using the second
   closure.
   
   A sufficient smart compiler could optimize all this away; rustc is
   currently not sufficiently smart.
   
   This commit does the following.
   
   - Changes the implementations of `all()`, `any()`, `find()` and
     `find_map()` to use the simplest possible code, rather than using
     `try_for_each()`. (I am reminded of "The Evolution of a Haskell
     Programmer".) These are both shorter and faster than the current
     implementations, and will permit the undoing of the `all()` removal in
     rust-lang#64545.
   
   - Changes `ResultShunt::next()` so it doesn't call `self.find()`,
     because that was causing infinite recursion with the new
     implementation of `find()`, which itself calls `self.next()`. (I
     honestly don't know how the old implementation of
     `ResultShunt::next()` didn't cause an infinite loop, given that it
     also called `self.next()`, albeit via `try_for_each()` and
     `try_fold()`.)
   
   - Changes `nth()` to use `self.next()` in a while loop rather than `for
     x in self`, because using self-iteration within an iterator method
     seems dubious, and `self.next()` is used in all the other iterator
     methods.

   

   nnethercote committed Sep 18, 2019
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