Rollup merge of #77514 - scottmcm:less-once-chain-once, r=estebank

Replace some once(x).chain(once(y)) with [x, y] IntoIter

Now that we have by-value array iterators that are [already used](https://github.com/rust-lang/rust/blob/25c8c53dd994acb3f4f7c02fe6bb46076393f8b0/compiler/rustc_hir/src/def.rs#L305-L307)...

For example,
```diff
-        once(self.type_ns).chain(once(self.value_ns)).chain(once(self.macro_ns)).filter_map(|it| it)
+        IntoIter::new([self.type_ns, self.value_ns, self.macro_ns]).filter_map(|it| it)
```

compiler/rustc_hir/src/def.rs

@@ -341,9 +341,7 @@ impl<T> PerNS<Option<T>> {
 
     /// Returns an iterator over the items which are `Some`.
     pub fn present_items(self) -> impl Iterator<Item = T> {
-        use std::iter::once;
-
-        once(self.type_ns).chain(once(self.value_ns)).chain(once(self.macro_ns)).filter_map(|it| it)
+        IntoIter::new([self.type_ns, self.value_ns, self.macro_ns]).filter_map(|it| it)
     }
 }
 

compiler/rustc_trait_selection/src/lib.rs

@@ -11,6 +11,7 @@
 //! This API is completely unstable and subject to change.
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(array_value_iter)]
 #![feature(bool_to_option)]
 #![feature(box_patterns)]
 #![feature(drain_filter)]

compiler/rustc_trait_selection/src/traits/object_safety.rs

@@ -24,6 +24,7 @@ use rustc_span::symbol::Symbol;
 use rustc_span::Span;
 use smallvec::SmallVec;
 
+use std::array;
 use std::iter;
 
 pub use crate::traits::{MethodViolationCode, ObjectSafetyViolation};
@@ -652,8 +653,7 @@ fn receiver_is_dispatchable<'tcx>(
         let caller_bounds: Vec<Predicate<'tcx>> = param_env
             .caller_bounds()
             .iter()
-            .chain(iter::once(unsize_predicate))
-            .chain(iter::once(trait_predicate))
+            .chain(array::IntoIter::new([unsize_predicate, trait_predicate]))
             .collect();
 
         ty::ParamEnv::new(tcx.intern_predicates(&caller_bounds), param_env.reveal())

compiler/rustc_typeck/src/astconv/mod.rs

@@ -35,8 +35,8 @@ use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
 use rustc_trait_selection::traits::wf::object_region_bounds;
 
 use smallvec::SmallVec;
+use std::array;
 use std::collections::BTreeSet;
-use std::iter;
 use std::slice;
 
 #[derive(Debug)]
@@ -1346,7 +1346,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             debug!("one_bound_for_assoc_type: bound2 = {:?}", bound2);
 
             let is_equality = is_equality();
-            let bounds = iter::once(bound).chain(iter::once(bound2)).chain(matching_candidates);
+            let bounds = array::IntoIter::new([bound, bound2]).chain(matching_candidates);
             let mut err = if is_equality.is_some() {
                 // More specific Error Index entry.
                 struct_span_err!(

compiler/rustc_typeck/src/lib.rs

@@ -56,6 +56,7 @@ This API is completely unstable and subject to change.
 */
 
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(array_value_iter)]
 #![feature(bool_to_option)]
 #![feature(box_syntax)]
 #![feature(crate_visibility_modifier)]

library/alloc/src/collections/vec_deque.rs

@@ -9,10 +9,11 @@
 
 // ignore-tidy-filelength
 
+use core::array;
 use core::cmp::{self, Ordering};
 use core::fmt;
 use core::hash::{Hash, Hasher};
-use core::iter::{once, repeat_with, FromIterator, FusedIterator};
+use core::iter::{repeat_with, FromIterator, FusedIterator};
 use core::mem::{self, replace, ManuallyDrop};
 use core::ops::{Index, IndexMut, Range, RangeBounds, Try};
 use core::ptr::{self, NonNull};
@@ -99,7 +100,7 @@ impl<'a, 'b, T> PairSlices<'a, 'b, T> {
     }
 
     fn remainder(self) -> impl Iterator<Item = &'b [T]> {
-        once(self.b0).chain(once(self.b1))
+        array::IntoIter::new([self.b0, self.b1])
     }
 }
 

library/alloc/src/lib.rs

@@ -77,6 +77,7 @@
 #![cfg_attr(test, feature(new_uninit))]
 #![feature(allocator_api)]
 #![feature(array_chunks)]
+#![feature(array_value_iter)]
 #![feature(array_windows)]
 #![feature(allow_internal_unstable)]
 #![feature(arbitrary_self_types)]