Rollup merge of rust-lang#149597 - jdonszelmann:revert-iterator-exactly-one, r=wafflelapkin

Revert "implement and test `Iterator::{exactly_one, collect_array}`"

This reverts rust-lang#149270

I was quite excited it merged, and immediately realized with `@WaffleLapkin` that this is a breaking change on nightly! Despite still being marked as unstable, the name conflicts with the name on itertools as was discussed on the PR itself: rust-lang#149270 (comment).

I'll reopen the PR though, and mark it as blocked on rust-lang#148605

Author: matthiaskrgr

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -86,8 +86,7 @@ impl MetadataLoader for DefaultMetadataLoader {
                     format!("failed to parse aix dylib '{}': {}", path.display(), e)
                 })?;
 
-                // FIXME: rewrite in terms of `#![feature(exact_length_collection)]`. See: #149266
-                match Itertools::exactly_one(archive.members()) {
+                match archive.members().exactly_one() {
                     Ok(lib) => {
                         let lib = lib.map_err(|e| {
                             format!("failed to parse aix dylib '{}': {}", path.display(), e)

library/core/src/iter/traits/iterator.rs

@@ -4034,62 +4034,6 @@ pub trait Iterator {
     {
         unreachable!("Always specialized");
     }
-
-    /// Checks if the iterator contains *exactly* one element.
-    /// If so, returns this one element.
-    ///
-    /// See also [`collect_array`](Iterator::collect_array) for lengths other than `1`.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(exact_length_collection)]
-    ///
-    /// assert_eq!([1].into_iter().exactly_one(), Some(1));
-    /// assert_eq!([].into_iter().exactly_one(), None::<()>);
-    ///
-    /// // There is exactly one even integer in the array:
-    /// assert_eq!([1, 2, 3].into_iter().filter(|x| x % 2 == 0).exactly_one(), Some(2));
-    /// // But there are two odds, which is too many:
-    /// assert_eq!([1, 2, 3].into_iter().filter(|x| x % 2 == 1).exactly_one(), None);
-    /// ```
-    #[inline]
-    #[unstable(feature = "exact_length_collection", issue = "149266")]
-    fn exactly_one(self) -> Option<Self::Item>
-    where
-        Self: Sized,
-    {
-        self.collect_array::<1>().map(|[i]| i)
-    }
-
-    /// Checks if an iterator has *exactly* `N` elements.
-    /// If so, returns those `N` elements in an array.
-    ///
-    /// See also [`exactly_one`](Iterator::exactly_one) when expecting a single element.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// #![feature(exact_length_collection)]
-    ///
-    /// assert_eq!([1, 2, 3, 4].into_iter().collect_array(), Some([1, 2, 3, 4]));
-    /// assert_eq!([1, 2].into_iter().chain([3, 4]).collect_array(), Some([1, 2, 3, 4]));
-    ///
-    /// // Iterator contains too few elements:
-    /// assert_eq!([1, 2].into_iter().collect_array::<4>(), None);
-    /// // Iterator contains too many elements:
-    /// assert_eq!([1, 2, 3, 4, 5].into_iter().collect_array::<4>(), None);
-    /// // Taking 4 makes it work again:
-    /// assert_eq!([1, 2, 3, 4, 5].into_iter().take(4).collect_array(), Some([1, 2, 3, 4]));
-    /// ```
-    #[inline]
-    #[unstable(feature = "exact_length_collection", issue = "149266")]
-    fn collect_array<const N: usize>(mut self) -> Option<[Self::Item; N]>
-    where
-        Self: Sized,
-    {
-        self.next_chunk().ok().filter(|_| self.next().is_none())
-    }
 }
 
 trait SpecIterEq<B: Iterator>: Iterator {

src/librustdoc/html/format.rs

@@ -1126,8 +1126,7 @@ pub(crate) fn print_impl(
             }
             if impl_.kind.is_fake_variadic()
                 && let Some(generics) = ty.generics()
-                // FIXME: rewrite in terms of `#![feature(exact_length_collection)]`. See: #149266
-                && let Ok(inner_type) = Itertools::exactly_one(generics)
+                && let Ok(inner_type) = generics.exactly_one()
             {
                 let last = ty.last();
                 if f.alternate() {
@@ -1207,8 +1206,7 @@ impl clean::Impl {
             }
         } else if let clean::Type::Path { path } = type_
             && let Some(generics) = path.generics()
-            // FIXME: rewrite in terms of `#![feature(exact_length_collection)]`. See: #149266
-            && let Ok(ty) = Itertools::exactly_one(generics)
+            && let Ok(ty) = generics.exactly_one()
             && self.kind.is_fake_variadic()
         {
             print_anchor(path.def_id(), path.last(), cx).fmt(f)?;

src/tools/clippy/clippy_lints/src/manual_non_exhaustive.rs

@@ -92,8 +92,7 @@ impl<'tcx> LateLintPass<'tcx> for ManualNonExhaustive {
                     (matches!(v.data, VariantData::Unit(_, _)) && is_doc_hidden(cx.tcx.hir_attrs(v.hir_id)))
                         .then_some((v.def_id, v.span))
                 });
-                // FIXME: rewrite in terms of `#![feature(exact_length_collection)]`. See: #149266
-                if let Ok((id, span)) = Itertools::exactly_one(iter)
+                if let Ok((id, span)) = iter.exactly_one()
                     && !find_attr!(cx.tcx.hir_attrs(item.hir_id()), AttributeKind::NonExhaustive(..))
                 {
                     self.potential_enums.push((item.owner_id.def_id, id, item.span, span));
@@ -105,8 +104,7 @@ impl<'tcx> LateLintPass<'tcx> for ManualNonExhaustive {
                     .iter()
                     .filter(|field| !cx.effective_visibilities.is_exported(field.def_id));
                 if fields.len() > 1
-                    // FIXME: rewrite in terms of `#![feature(exact_length_collection)]`. See: #149266
-                    && let Ok(field) = Itertools::exactly_one(private_fields)
+                    && let Ok(field) = private_fields.exactly_one()
                     && let TyKind::Tup([]) = field.ty.kind
                 {
                     span_lint_and_then(

tests/ui/const-generics/type-dependent/issue-71805.rs

@@ -4,7 +4,7 @@ use std::mem::MaybeUninit;
 trait CollectSlice<'a>: Iterator {
     fn inner_array<const N: usize>(&mut self) -> [Self::Item; N];
 
-    fn custom_collect_array<const N: usize>(&mut self) -> [Self::Item; N] {
+    fn collect_array<const N: usize>(&mut self) -> [Self::Item; N] {
         let result = self.inner_array();
         assert!(self.next().is_none());
         result
@@ -34,5 +34,5 @@ where
 
 fn main() {
     let mut foos = [0u64; 9].iter().cloned();
-    let _bar: [u64; 9] = foos.custom_collect_array::<9_usize>();
+    let _bar: [u64; 9] = foos.collect_array::<9_usize>();
 }

tests/ui/mir/mir-inlining/ice-issue-77564.rs

@@ -29,11 +29,10 @@ where
 
 fn main() {
     assert_eq!(
-        CollectArray::collect_array(
-            &mut [[1, 2], [3, 4]]
+        [[1, 2], [3, 4]]
             .iter()
-            .map(|row| CollectArray::collect_array(&mut row.iter()))
-        ),
+            .map(|row| row.iter().collect_array())
+            .collect_array(),
         [[&1, &2], [&3, &4]]
     );
 }