1dd515f273
Add slice methods for indexing via an array of indices.
Disclaimer: It's been a while since I contributed to the main Rust repo, apologies in advance if this is large enough already that it should've been an RFC.
---
# Update:
- Based on feedback, removed the `&[T]` variant of this API, and removed the requirements for the indices to be sorted.
# Description
This adds the following slice methods to `core`:
```rust
impl<T> [T] {
pub unsafe fn get_many_unchecked_mut<const N: usize>(&mut self, indices: [usize; N]) -> [&mut T; N];
pub fn get_many_mut<const N: usize>(&mut self, indices: [usize; N]) -> Option<[&mut T; N]>;
}
```
This allows creating multiple mutable references to disjunct positions in a slice, which previously required writing some awkward code with `split_at_mut()` or `iter_mut()`. For the bound-checked variant, the indices are checked against each other and against the bounds of the slice, which requires `N * (N + 1) / 2` comparison operations.
This has a proof-of-concept standalone implementation here: https://crates.io/crates/index_many
Care has been taken that the implementation passes miri borrow checks, and generates straight-forward assembly (though this was only checked on x86_64).
# Example
```rust
let v = &mut [1, 2, 3, 4];
let [a, b] = v.get_many_mut([0, 2]).unwrap();
std::mem::swap(a, b);
*v += 100;
assert_eq!(v, &[3, 2, 101, 4]);
```
# Codegen Examples
<details>
<summary>Click to expand!</summary>
Disclaimer: Taken from local tests with the standalone implementation.
## Unchecked Indexing:
```rust
pub unsafe fn example_unchecked(slice: &mut [usize], indices: [usize; 3]) -> [&mut usize; 3] {
slice.get_many_unchecked_mut(indices)
}
```
```nasm
example_unchecked:
mov rcx, qword, ptr, [r9]
mov r8, qword, ptr, [r9, +, 8]
mov r9, qword, ptr, [r9, +, 16]
lea rcx, [rdx, +, 8*rcx]
lea r8, [rdx, +, 8*r8]
lea rdx, [rdx, +, 8*r9]
mov qword, ptr, [rax], rcx
mov qword, ptr, [rax, +, 8], r8
mov qword, ptr, [rax, +, 16], rdx
ret
```
## Checked Indexing (Option):
```rust
pub unsafe fn example_option(slice: &mut [usize], indices: [usize; 3]) -> Option<[&mut usize; 3]> {
slice.get_many_mut(indices)
}
```
```nasm
mov r10, qword, ptr, [r9, +, 8]
mov rcx, qword, ptr, [r9, +, 16]
cmp rcx, r10
je .LBB0_7
mov r9, qword, ptr, [r9]
cmp rcx, r9
je .LBB0_7
cmp rcx, r8
jae .LBB0_7
cmp r10, r9
je .LBB0_7
cmp r9, r8
jae .LBB0_7
cmp r10, r8
jae .LBB0_7
lea r8, [rdx, +, 8*r9]
lea r9, [rdx, +, 8*r10]
lea rcx, [rdx, +, 8*rcx]
mov qword, ptr, [rax], r8
mov qword, ptr, [rax, +, 8], r9
mov qword, ptr, [rax, +, 16], rcx
ret
.LBB0_7:
mov qword, ptr, [rax], 0
ret
```
## Checked Indexing (Panic):
```rust
pub fn example_panic(slice: &mut [usize], indices: [usize; 3]) -> [&mut usize; 3] {
let len = slice.len();
match slice.get_many_mut(indices) {
Some(s) => s,
None => {
let tmp = indices;
index_many::sorted_bound_check_failed(&tmp, len)
}
}
}
```
```nasm
example_panic:
sub rsp, 56
mov rax, qword, ptr, [r9]
mov r10, qword, ptr, [r9, +, 8]
mov r9, qword, ptr, [r9, +, 16]
cmp r9, r10
je .LBB0_6
cmp r9, rax
je .LBB0_6
cmp r9, r8
jae .LBB0_6
cmp r10, rax
je .LBB0_6
cmp rax, r8
jae .LBB0_6
cmp r10, r8
jae .LBB0_6
lea rax, [rdx, +, 8*rax]
lea r8, [rdx, +, 8*r10]
lea rdx, [rdx, +, 8*r9]
mov qword, ptr, [rcx], rax
mov qword, ptr, [rcx, +, 8], r8
mov qword, ptr, [rcx, +, 16], rdx
mov rax, rcx
add rsp, 56
ret
.LBB0_6:
mov qword, ptr, [rsp, +, 32], rax
mov qword, ptr, [rsp, +, 40], r10
mov qword, ptr, [rsp, +, 48], r9
lea rcx, [rsp, +, 32]
mov edx, 3
call index_many::bound_check_failed
ud2
```
</details>
# Extensions
There are multiple optional extensions to this.
## Indexing With Ranges
This could easily be expanded to allow indexing with `[I; N]` where `I: SliceIndex<Self>`. I wanted to keep the initial implementation simple, so I didn't include it yet.
## Panicking Variant
We could also add this method:
```rust
impl<T> [T] {
fn index_many_mut<const N: usize>(&mut self, indices: [usize; N]) -> [&mut T; N];
}
```
This would work similar to the regular index operator and panic with out-of-bound indices. The advantage would be that we could more easily ensure good codegen with a useful panic message, which is non-trivial with the `Option` variant.
This is implemented in the standalone implementation, and used as basis for the codegen examples here and there.
158 lines
3.8 KiB
Rust
158 lines
3.8 KiB
Rust
#![feature(alloc_layout_extra)]
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#![feature(array_chunks)]
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#![feature(array_methods)]
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#![feature(array_windows)]
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#![feature(bigint_helper_methods)]
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#![feature(cell_update)]
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#![feature(const_align_offset)]
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#![feature(const_assume)]
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#![feature(const_align_of_val_raw)]
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#![feature(const_black_box)]
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#![feature(const_bool_to_option)]
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#![feature(const_caller_location)]
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#![feature(const_cell_into_inner)]
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#![feature(const_convert)]
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#![feature(const_hash)]
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#![feature(const_heap)]
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#![feature(const_maybe_uninit_as_mut_ptr)]
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#![feature(const_maybe_uninit_assume_init_read)]
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#![feature(const_nonnull_new)]
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#![feature(const_num_from_num)]
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#![feature(const_pointer_byte_offsets)]
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#![feature(const_pointer_is_aligned)]
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#![feature(const_ptr_as_ref)]
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#![feature(const_ptr_read)]
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#![feature(const_ptr_write)]
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#![feature(const_trait_impl)]
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#![feature(const_likely)]
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#![feature(const_location_fields)]
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#![feature(core_intrinsics)]
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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(div_duration)]
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#![feature(duration_consts_float)]
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#![feature(duration_constants)]
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#![feature(exact_size_is_empty)]
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#![feature(extern_types)]
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#![feature(flt2dec)]
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#![feature(fmt_internals)]
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#![feature(float_minimum_maximum)]
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#![feature(future_join)]
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#![feature(generic_assert_internals)]
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#![feature(array_try_from_fn)]
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#![feature(hasher_prefixfree_extras)]
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#![feature(hashmap_internals)]
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#![feature(try_find)]
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#![feature(inline_const)]
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#![feature(is_sorted)]
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#![feature(layout_for_ptr)]
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#![feature(pattern)]
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#![feature(pin_macro)]
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#![feature(sort_internals)]
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#![feature(slice_take)]
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#![feature(slice_from_ptr_range)]
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#![feature(split_as_slice)]
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#![feature(maybe_uninit_uninit_array)]
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#![feature(maybe_uninit_write_slice)]
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#![feature(maybe_uninit_uninit_array_transpose)]
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#![feature(min_specialization)]
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#![feature(numfmt)]
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#![feature(step_trait)]
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#![feature(str_internals)]
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#![feature(std_internals)]
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#![feature(test)]
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#![feature(trusted_len)]
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#![feature(try_blocks)]
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#![feature(try_trait_v2)]
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#![feature(slice_internals)]
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#![feature(slice_partition_dedup)]
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#![feature(iter_advance_by)]
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#![feature(iter_array_chunks)]
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#![feature(iter_collect_into)]
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#![feature(iter_partition_in_place)]
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#![feature(iter_intersperse)]
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#![feature(iter_is_partitioned)]
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#![feature(iter_next_chunk)]
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#![feature(iter_order_by)]
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#![feature(iter_repeat_n)]
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#![feature(iterator_try_collect)]
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#![feature(iterator_try_reduce)]
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#![feature(const_mut_refs)]
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#![feature(const_pin)]
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#![feature(const_waker)]
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#![feature(never_type)]
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#![feature(unwrap_infallible)]
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#![feature(pointer_byte_offsets)]
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#![feature(pointer_is_aligned)]
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#![feature(portable_simd)]
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#![feature(ptr_metadata)]
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#![feature(once_cell)]
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#![feature(option_result_contains)]
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#![feature(unsized_tuple_coercion)]
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#![feature(const_option)]
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#![feature(const_option_ext)]
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#![feature(const_result)]
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#![feature(integer_atomics)]
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#![feature(int_roundings)]
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#![feature(slice_group_by)]
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#![feature(split_array)]
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#![feature(strict_provenance)]
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#![feature(strict_provenance_atomic_ptr)]
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#![feature(trusted_random_access)]
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#![feature(unsize)]
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#![feature(const_array_from_ref)]
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#![feature(const_slice_from_ref)]
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#![feature(waker_getters)]
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#![feature(slice_flatten)]
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#![feature(provide_any)]
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#![feature(utf8_chunks)]
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#![feature(is_ascii_octdigit)]
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#![feature(get_many_mut)]
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#![deny(unsafe_op_in_unsafe_fn)]
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#![deny(fuzzy_provenance_casts)]
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extern crate test;
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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 asserting;
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mod atomic;
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mod bool;
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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 fmt;
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mod future;
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mod hash;
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mod intrinsics;
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mod iter;
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mod lazy;
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mod macros;
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mod manually_drop;
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mod mem;
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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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