rust/library/coretests/tests/lib.rs
Jubilee 480a72d601
Rollup merge of #134340 - Urgau:stabilize-num_midpoint_signed, r=scottmcm
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`
2025-02-20 14:58:16 -08:00

194 lines
5 KiB
Rust

// tidy-alphabetical-start
#![cfg_attr(target_has_atomic = "128", feature(integer_atomics))]
#![cfg_attr(test, feature(cfg_match))]
#![feature(alloc_layout_extra)]
#![feature(array_chunks)]
#![feature(array_ptr_get)]
#![feature(array_try_from_fn)]
#![feature(array_windows)]
#![feature(ascii_char)]
#![feature(ascii_char_variants)]
#![feature(async_iter_from_iter)]
#![feature(async_iterator)]
#![feature(bigint_helper_methods)]
#![feature(bstr)]
#![feature(cell_update)]
#![feature(char_max_len)]
#![feature(clone_to_uninit)]
#![feature(const_eval_select)]
#![feature(const_swap_nonoverlapping)]
#![feature(const_trait_impl)]
#![feature(core_intrinsics)]
#![feature(core_intrinsics_fallbacks)]
#![feature(core_io_borrowed_buf)]
#![feature(core_private_bignum)]
#![feature(core_private_diy_float)]
#![feature(dec2flt)]
#![feature(duration_constants)]
#![feature(duration_constructors)]
#![feature(error_generic_member_access)]
#![feature(exact_size_is_empty)]
#![feature(extend_one)]
#![feature(extern_types)]
#![feature(float_minimum_maximum)]
#![feature(flt2dec)]
#![feature(fmt_internals)]
#![feature(formatting_options)]
#![feature(freeze)]
#![feature(future_join)]
#![feature(generic_assert_internals)]
#![feature(hasher_prefixfree_extras)]
#![feature(hashmap_internals)]
#![feature(inline_const_pat)]
#![feature(int_roundings)]
#![feature(ip)]
#![feature(ip_from)]
#![feature(is_ascii_octdigit)]
#![feature(iter_advance_by)]
#![feature(iter_array_chunks)]
#![feature(iter_chain)]
#![feature(iter_collect_into)]
#![feature(iter_intersperse)]
#![feature(iter_is_partitioned)]
#![feature(iter_map_windows)]
#![feature(iter_next_chunk)]
#![feature(iter_order_by)]
#![feature(iter_partition_in_place)]
#![feature(iterator_try_collect)]
#![feature(iterator_try_reduce)]
#![feature(layout_for_ptr)]
#![feature(lazy_get)]
#![feature(maybe_uninit_fill)]
#![feature(maybe_uninit_uninit_array_transpose)]
#![feature(maybe_uninit_write_slice)]
#![feature(min_specialization)]
#![feature(never_type)]
#![feature(numfmt)]
#![feature(pattern)]
#![feature(pointer_is_aligned_to)]
#![feature(portable_simd)]
#![feature(ptr_metadata)]
#![feature(slice_from_ptr_range)]
#![feature(slice_internals)]
#![feature(slice_partition_dedup)]
#![feature(slice_split_once)]
#![feature(slice_take)]
#![feature(split_array)]
#![feature(split_as_slice)]
#![feature(std_internals)]
#![feature(step_trait)]
#![feature(str_internals)]
#![feature(strict_provenance_atomic_ptr)]
#![feature(strict_provenance_lints)]
#![feature(test)]
#![feature(trusted_len)]
#![feature(trusted_random_access)]
#![feature(try_blocks)]
#![feature(try_find)]
#![feature(try_trait_v2)]
#![feature(unsigned_is_multiple_of)]
#![feature(unsize)]
#![feature(unsized_tuple_coercion)]
#![feature(unwrap_infallible)]
// tidy-alphabetical-end
#![allow(internal_features)]
#![deny(fuzzy_provenance_casts)]
#![deny(unsafe_op_in_unsafe_fn)]
/// Version of `assert_matches` that ignores fancy runtime printing in const context and uses structural equality.
macro_rules! assert_eq_const_safe {
($left:expr, $right:expr) => {
assert_eq_const_safe!($left, $right, concat!(stringify!($left), " == ", stringify!($right)));
};
($left:expr, $right:expr$(, $($arg:tt)+)?) => {
{
fn runtime() {
assert_eq!($left, $right, $($($arg)*),*);
}
const fn compiletime() {
assert!(matches!($left, const { $right }));
}
core::intrinsics::const_eval_select((), compiletime, runtime)
}
};
}
/// Creates a test for runtime and a test for constant-time.
macro_rules! test_runtime_and_compiletime {
($(
$(#[$attr:meta])*
fn $test:ident() $block:block
)*) => {
$(
$(#[$attr])*
#[test]
fn $test() $block
$(#[$attr])*
const _: () = $block;
)*
}
}
mod alloc;
mod any;
mod array;
mod ascii;
mod ascii_char;
mod asserting;
mod async_iter;
mod atomic;
mod bool;
mod bstr;
mod cell;
mod char;
mod clone;
mod cmp;
mod const_ptr;
mod convert;
mod ffi;
mod fmt;
mod future;
mod hash;
mod intrinsics;
mod io;
mod iter;
mod lazy;
#[cfg(not(bootstrap))]
mod macros;
#[cfg(bootstrap)]
mod macros_bootstrap;
mod manually_drop;
mod mem;
mod net;
mod nonzero;
mod num;
mod ops;
mod option;
mod panic;
mod pattern;
mod pin;
mod pin_macro;
mod ptr;
mod result;
mod simd;
mod slice;
mod str;
mod str_lossy;
mod task;
mod time;
mod tuple;
mod unicode;
mod waker;
/// Copied from `std::test_helpers::test_rng`, see that function for rationale.
#[track_caller]
#[allow(dead_code)] // Not used in all configurations.
pub(crate) fn test_rng() -> rand_xorshift::XorShiftRng {
use core::hash::{BuildHasher, Hash, Hasher};
let mut hasher = std::hash::RandomState::new().build_hasher();
core::panic::Location::caller().hash(&mut hasher);
let hc64 = hasher.finish();
let seed_vec = hc64.to_le_bytes().into_iter().chain(0u8..8).collect::<Vec<u8>>();
let seed: [u8; 16] = seed_vec.as_slice().try_into().unwrap();
rand::SeedableRng::from_seed(seed)
}