Get rid of "partition_add_index" feature

It offers nothing over the "nightly" feature and makes testing more
complicated.

Cargo.toml

@@ -23,7 +23,7 @@ appveyor = { repository = "rust-random/rand" }
 [features]
 # Meta-features:
 default = ["std", "std_rng"]
-nightly = ["simd_support", "partition_at_index"] # enables all features requiring nightly rust
+nightly = ["simd_support"] # enables all features requiring nightly rust
 serde1 = ["serde"]
 
 # Option (enabled by default): without "std" rand uses libcore; this option
@@ -45,9 +45,6 @@ std_rng = ["rand_chacha", "rand_hc"]
 # Option: enable SmallRng
 small_rng = ["rand_pcg"]
 
-# Option (requires nightly): better performance of choose_multiple_weighted
-partition_at_index = []
-
 [workspace]
 members = [
     "rand_core",

src/lib.rs

@@ -50,7 +50,7 @@
 #![doc(test(attr(allow(unused_variables), deny(warnings))))]
 #![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(all(feature = "simd_support", feature = "nightly"), feature(stdsimd))]
-#![cfg_attr(all(feature = "partition_at_index", feature = "nightly"), feature(slice_partition_at_index))]
+#![cfg_attr(feature = "nightly", feature(slice_partition_at_index))]
 #![cfg_attr(doc_cfg, feature(doc_cfg))]
 #![allow(
     clippy::excessive_precision,

src/seq/index.rs

@@ -268,7 +268,7 @@ where R: Rng + ?Sized {
 /// an alternative.
 ///
 /// This implementation uses `O(length + amount)` space and `O(length)` time
-/// if the "partition_at_index" feature is enabled, or `O(length)` space and
+/// if the "nightly" feature is enabled, or `O(length)` space and
 /// `O(length + amount * log length)` time otherwise.
 ///
 /// Panics if `amount > length`.
@@ -298,7 +298,7 @@ where
 /// This implementation uses the algorithm described by Efraimidis and Spirakis
 /// in this paper: https://doi.org/10.1016/j.ipl.2005.11.003
 /// It uses `O(length + amount)` space and `O(length)` time if the
-/// "partition_at_index" feature is enabled, or `O(length)` space and `O(length
+/// "nightly" feature is enabled, or `O(length)` space and `O(length
 /// + amount * log length)` time otherwise.
 ///
 /// Panics if `amount > length`.
@@ -342,7 +342,7 @@ where
     }
     impl<N> Eq for Element<N> {}
 
-    #[cfg(feature = "partition_at_index")]
+    #[cfg(feature = "nightly")]
     {
         let mut candidates = Vec::with_capacity(length.as_usize());
         for index in 0..length.as_usize() {
@@ -370,7 +370,7 @@ where
         Ok(IndexVec::from(result))
     }
 
-    #[cfg(not(feature = "partition_at_index"))]
+    #[cfg(not(feature = "nightly"))]
     {
         #[cfg(all(feature = "alloc", not(feature = "std")))]
         use crate::alloc::collections::BinaryHeap;