Auto merge of #50941 - kennytm:never-ok, r=sfackler

Replace `core::iter::AlwaysOk<T>` by `Result<T, !>`

#43278 has been fixed, so we don't need this struct anymore.

(Actually we don't even need `.unwrap()` thanks to `#![feature(exhaustive_patterns)]`)

Author: bors

src/libcore/iter/iterator.rs

@@ -11,7 +11,7 @@
 use cmp::Ordering;
 use ops::Try;
 
-use super::{AlwaysOk, LoopState};
+use super::LoopState;
 use super::{Chain, Cycle, Cloned, Enumerate, Filter, FilterMap, Fuse};
 use super::{Flatten, FlatMap, flatten_compat};
 use super::{Inspect, Map, Peekable, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile, Rev};
@@ -1614,7 +1614,7 @@ pub trait Iterator {
     fn fold<B, F>(mut self, init: B, mut f: F) -> B where
         Self: Sized, F: FnMut(B, Self::Item) -> B,
     {
-        self.try_fold(init, move |acc, x| AlwaysOk(f(acc, x))).0
+        self.try_fold(init, move |acc, x| Ok::<B, !>(f(acc, x))).unwrap()
     }
 
     /// Tests if every element of the iterator matches a predicate.

src/libcore/iter/mod.rs

@@ -354,21 +354,6 @@ mod range;
 mod sources;
 mod traits;
 
-/// Transparent newtype used to implement foo methods in terms of try_foo.
-/// Important until #43278 is fixed; might be better as `Result<T, !>` later.
-struct AlwaysOk<T>(pub T);
-
-impl<T> Try for AlwaysOk<T> {
-    type Ok = T;
-    type Error = !;
-    #[inline]
-    fn into_result(self) -> Result<Self::Ok, Self::Error> { Ok(self.0) }
-    #[inline]
-    fn from_error(v: Self::Error) -> Self { v }
-    #[inline]
-    fn from_ok(v: Self::Ok) -> Self { AlwaysOk(v) }
-}
-
 /// Used to make try_fold closures more like normal loops
 #[derive(PartialEq)]
 enum LoopState<C, B> {

src/libcore/iter/traits.rs

@@ -10,7 +10,7 @@
 use ops::{Mul, Add, Try};
 use num::Wrapping;
 
-use super::{AlwaysOk, LoopState};
+use super::LoopState;
 
 /// Conversion from an `Iterator`.
 ///
@@ -524,7 +524,7 @@ pub trait DoubleEndedIterator: Iterator {
     fn rfold<B, F>(mut self, accum: B, mut f: F) -> B where
         Self: Sized, F: FnMut(B, Self::Item) -> B,
     {
-        self.try_rfold(accum, move |acc, x| AlwaysOk(f(acc, x))).0
+        self.try_rfold(accum, move |acc, x| Ok::<B, !>(f(acc, x))).unwrap()
     }
 
     /// Searches for an element of an iterator from the back that satisfies a predicate.