Rollup merge of #143907 - Kijewski:pr-inline-split_at_unchecked, r=Mark-Simulacrum
core: make `str::split_at_unchecked()` inline
This PR adds `#[inline]` to the method `str::split_at_unchecked()`. This is done for two reasons:
1. The method is tiny, e.g. on AMD-64 (<https://godbolt.org/z/ba68fdfxn>):
```asm
movq %rdi, %rax
subq %rcx, %rdx
movq %rsi, (%rdi)
addq %rcx, %rsi
movq %rcx, 8(%rdi)
movq %rsi, 16(%rdi)
movq %rdx, 24(%rdi)
retq
```
2. More importantly, inlining the method enables further automatic optimizations. E.g. if you split at index 3, then in the compiler (rustc, llvm or both) knows that this code cannot fail, and the panicking path is omitted in the generated code:
```rust
pub fn punctuation(i: &str) -> Result<(), ()> {
const THREE_CHARS: &[[u8; 3]] = &[*b"<<=", *b">>=", *b"...", *b"..="];
if let Some((head, _)) = i.split_at_checked(3)
&& THREE_CHARS.contains(&head.as_bytes().try_into().unwrap())
{
Ok(())
} else {
Err(())
}
}
```
<details>
<summary>Without PR</summary>
<https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=0234de8158f467eebd73286f20d6e27a>
```asm
playground::punctuation:
subq $40, %rsp
movq %rsi, %rdx
movq %rdi, %rsi
movb $1, %al
cmpq $3, %rdx
ja .LBB2_2
je .LBB2_3
.LBB2_11:
addq $40, %rsp
retq
.LBB2_2:
cmpb $-64, 3(%rsi)
jl .LBB2_11
.LBB2_3:
leaq 8(%rsp), %rdi
movl $3, %ecx
callq *core::str::<impl str>::split_at_unchecked@GOTPCREL(%rip)
movq 8(%rsp), %rcx
movb $1, %al
testq %rcx, %rcx
je .LBB2_11
cmpq $3, 16(%rsp)
jne .LBB2_12
movzwl (%rcx), %edx
movzbl 2(%rcx), %ecx
shll $16, %ecx
orl %edx, %ecx
cmpl $4013115, %ecx
jg .LBB2_8
cmpl $3026478, %ecx
je .LBB2_10
cmpl $4009518, %ecx
je .LBB2_10
jmp .LBB2_11
.LBB2_8:
cmpl $4013630, %ecx
je .LBB2_10
cmpl $4013116, %ecx
jne .LBB2_11
.LBB2_10:
xorl %eax, %eax
addq $40, %rsp
retq
.LBB2_12:
leaq .Lanon.d98a7fbb86d10a97c24516e267466134.2(%rip), %rdi
leaq .Lanon.d98a7fbb86d10a97c24516e267466134.1(%rip), %rcx
leaq .Lanon.d98a7fbb86d10a97c24516e267466134.6(%rip), %r8
leaq 7(%rsp), %rdx
movl $43, %esi
callq *core::result::unwrap_failed@GOTPCREL(%rip)
```
</details>
<details>
<summary>With PR</summary>
<https://play.rust-lang.org/?version=stable&mode=release&edition=2024&gist=5d4058c79ce0f6cb1a434190427d2055>
```asm
playground::punctuation:
movb $1, %al
cmpq $3, %rsi
ja .LBB0_2
je .LBB0_3
.LBB0_9:
retq
.LBB0_2:
cmpb $-64, 3(%rdi)
jl .LBB0_9
.LBB0_3:
movzwl (%rdi), %eax
movzbl 2(%rdi), %ecx
shll $16, %ecx
orl %eax, %ecx
movb $1, %al
cmpl $4013115, %ecx
jg .LBB0_6
cmpl $3026478, %ecx
je .LBB0_8
cmpl $4009518, %ecx
je .LBB0_8
jmp .LBB0_9
.LBB0_6:
cmpl $4013630, %ecx
je .LBB0_8
cmpl $4013116, %ecx
jne .LBB0_9
.LBB0_8:
xorl %eax, %eax
retq
```
</details>
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