Stabilize `num_midpoint_signed` feature This PR proposes that we stabilize the signed variants of [`iN::midpoint`](https://github.com/rust-lang/rust/issues/110840#issue-1684506201), the operation is equivalent to doing `(a + b) / 2` in a sufficiently large number. The stabilized API surface would be: ```rust /// Calculates the middle point of `self` and `rhs`. /// /// `midpoint(a, b)` is `(a + b) / 2` as if it were performed in a /// sufficiently-large signed integer type. This implies that the result is /// always rounded towards zero and that no overflow will ever occur. impl i{8,16,32,64,128,size} { pub const fn midpoint(self, rhs: Self) -> Self; } ``` T-libs-api previously stabilized the unsigned (and float) variants in #131784, the signed variants were left out because of the rounding that should be used in case of negative midpoint. This stabilization proposal proposes that we round towards zero because: - it makes the obvious `(a + b) / 2` in a sufficiently-large number always true - using another rounding for the positive result would be inconsistent with the unsigned variants - it makes `midpoint(-a, -b)` == `-midpoint(a, b)` always true - it is consistent with `midpoint(a as f64, b as f64) as i64` - it makes it possible to always suggest `midpoint` as a replacement for `(a + b) / 2` expressions *(which we may want to do as a future work given the 21.2k hits on [GitHub Search](https://github.com/search?q=lang%3Arust+%2F%5C%28%5Ba-zA-Z_%5D*+%5C%2B+%5Ba-zA-Z_%5D*%5C%29+%5C%2F+2%2F&type=code&p=1))* `@scottmcm` mentioned a drawback in https://github.com/rust-lang/rust/pull/132191#issuecomment-2439891200: > I'm torn, because rounding towards zero makes it "wider" than other values, which `>> 1` avoids -- `(a + b) >> 1` has the nice behaviour that `midpoint(a, b) + 2 == midpoint(a + 2, b + 2)`. > > But I guess overall sticking with `(a + b) / 2` makes sense as well, and I do like the negation property 🤷 Which I think is outweigh by the advantages cited above. Closes #110840 cc `@rust-lang/libs-api` cc `@scottmcm` r? `@dtolnay`
194 lines
5 KiB
Rust
194 lines
5 KiB
Rust
// tidy-alphabetical-start
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#![cfg_attr(target_has_atomic = "128", feature(integer_atomics))]
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#![cfg_attr(test, feature(cfg_match))]
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#![feature(alloc_layout_extra)]
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#![feature(array_chunks)]
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#![feature(array_ptr_get)]
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#![feature(array_try_from_fn)]
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#![feature(array_windows)]
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#![feature(ascii_char)]
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#![feature(ascii_char_variants)]
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#![feature(async_iter_from_iter)]
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#![feature(async_iterator)]
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#![feature(bigint_helper_methods)]
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#![feature(bstr)]
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#![feature(cell_update)]
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#![feature(char_max_len)]
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#![feature(clone_to_uninit)]
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#![feature(const_eval_select)]
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#![feature(const_swap_nonoverlapping)]
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#![feature(const_trait_impl)]
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#![feature(core_intrinsics)]
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#![feature(core_intrinsics_fallbacks)]
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#![feature(core_io_borrowed_buf)]
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#![feature(core_private_bignum)]
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#![feature(core_private_diy_float)]
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#![feature(dec2flt)]
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#![feature(duration_constants)]
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#![feature(duration_constructors)]
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#![feature(error_generic_member_access)]
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#![feature(exact_size_is_empty)]
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#![feature(extend_one)]
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#![feature(extern_types)]
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#![feature(float_minimum_maximum)]
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#![feature(flt2dec)]
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#![feature(fmt_internals)]
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#![feature(formatting_options)]
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#![feature(freeze)]
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#![feature(future_join)]
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#![feature(generic_assert_internals)]
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#![feature(hasher_prefixfree_extras)]
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#![feature(hashmap_internals)]
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#![feature(inline_const_pat)]
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#![feature(int_roundings)]
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#![feature(ip)]
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#![feature(ip_from)]
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#![feature(is_ascii_octdigit)]
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#![feature(iter_advance_by)]
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#![feature(iter_array_chunks)]
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#![feature(iter_chain)]
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#![feature(iter_collect_into)]
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#![feature(iter_intersperse)]
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#![feature(iter_is_partitioned)]
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#![feature(iter_map_windows)]
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#![feature(iter_next_chunk)]
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#![feature(iter_order_by)]
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#![feature(iter_partition_in_place)]
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#![feature(iterator_try_collect)]
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#![feature(iterator_try_reduce)]
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#![feature(layout_for_ptr)]
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#![feature(lazy_get)]
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#![feature(maybe_uninit_fill)]
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#![feature(maybe_uninit_uninit_array_transpose)]
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#![feature(maybe_uninit_write_slice)]
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#![feature(min_specialization)]
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#![feature(never_type)]
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#![feature(numfmt)]
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#![feature(pattern)]
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#![feature(pointer_is_aligned_to)]
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#![feature(portable_simd)]
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#![feature(ptr_metadata)]
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#![feature(slice_from_ptr_range)]
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#![feature(slice_internals)]
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#![feature(slice_partition_dedup)]
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#![feature(slice_split_once)]
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#![feature(slice_take)]
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#![feature(split_array)]
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#![feature(split_as_slice)]
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#![feature(std_internals)]
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#![feature(step_trait)]
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#![feature(str_internals)]
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#![feature(strict_provenance_atomic_ptr)]
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#![feature(strict_provenance_lints)]
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#![feature(test)]
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#![feature(trusted_len)]
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#![feature(trusted_random_access)]
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#![feature(try_blocks)]
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#![feature(try_find)]
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#![feature(try_trait_v2)]
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#![feature(unsigned_is_multiple_of)]
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#![feature(unsize)]
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#![feature(unsized_tuple_coercion)]
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#![feature(unwrap_infallible)]
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// tidy-alphabetical-end
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#![allow(internal_features)]
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#![deny(fuzzy_provenance_casts)]
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#![deny(unsafe_op_in_unsafe_fn)]
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/// Version of `assert_matches` that ignores fancy runtime printing in const context and uses structural equality.
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macro_rules! assert_eq_const_safe {
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($left:expr, $right:expr) => {
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assert_eq_const_safe!($left, $right, concat!(stringify!($left), " == ", stringify!($right)));
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};
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($left:expr, $right:expr$(, $($arg:tt)+)?) => {
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{
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fn runtime() {
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assert_eq!($left, $right, $($($arg)*),*);
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}
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const fn compiletime() {
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assert!(matches!($left, const { $right }));
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}
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core::intrinsics::const_eval_select((), compiletime, runtime)
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}
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};
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}
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/// Creates a test for runtime and a test for constant-time.
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macro_rules! test_runtime_and_compiletime {
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($(
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$(#[$attr:meta])*
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fn $test:ident() $block:block
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)*) => {
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$(
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$(#[$attr])*
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#[test]
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fn $test() $block
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$(#[$attr])*
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const _: () = $block;
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)*
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}
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}
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mod alloc;
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mod any;
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mod array;
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mod ascii;
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mod ascii_char;
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mod asserting;
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mod async_iter;
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mod atomic;
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mod bool;
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mod bstr;
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mod cell;
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mod char;
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mod clone;
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mod cmp;
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mod const_ptr;
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mod convert;
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mod ffi;
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mod fmt;
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mod future;
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mod hash;
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mod intrinsics;
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mod io;
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mod iter;
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mod lazy;
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#[cfg(not(bootstrap))]
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mod macros;
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#[cfg(bootstrap)]
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mod macros_bootstrap;
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mod manually_drop;
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mod mem;
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mod net;
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mod nonzero;
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mod num;
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mod ops;
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mod option;
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mod panic;
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mod pattern;
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mod pin;
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mod pin_macro;
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mod ptr;
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mod result;
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mod simd;
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mod slice;
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mod str;
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mod str_lossy;
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mod task;
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mod time;
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mod tuple;
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mod unicode;
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mod waker;
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/// Copied from `std::test_helpers::test_rng`, see that function for rationale.
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#[track_caller]
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#[allow(dead_code)] // Not used in all configurations.
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pub(crate) fn test_rng() -> rand_xorshift::XorShiftRng {
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use core::hash::{BuildHasher, Hash, Hasher};
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let mut hasher = std::hash::RandomState::new().build_hasher();
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core::panic::Location::caller().hash(&mut hasher);
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let hc64 = hasher.finish();
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let seed_vec = hc64.to_le_bytes().into_iter().chain(0u8..8).collect::<Vec<u8>>();
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let seed: [u8; 16] = seed_vec.as_slice().try_into().unwrap();
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rand::SeedableRng::from_seed(seed)
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}
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