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[Experiment] Replace unreachable_unchecked() with uninit().assume_init() in unwrap_unchecked() #124737

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[Experiment] Replace unreachable_unchecked() with uninit().assume_init() in unwrap_unchecked() #124737

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3d67c90
Replace `unreachable_unchecked` with `uninit().assume_init()`
alion02 May 5, 2024
b0eca0b
Fix build
alion02 May 5, 2024
7456fd1
Really fix build (I think)
workingjubilee May 5, 2024
fd98988
Really really fix build
workingjubilee May 5, 2024
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2 changes: 1 addition & 1 deletion library/core/src/option.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1037,7 +1037,7 @@ impl<T> Option<T> {
match self {
Some(val) => val,
// SAFETY: the safety contract must be upheld by the caller.
None => unsafe { hint::unreachable_unchecked() },
None => unsafe { mem::MaybeUninit::uninit().assume_init() },
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is this really better? that's disappointing.. how about using offset_of and accessing the value with directly with a pointer offset, entirely removing the condition?

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I don't know if it's better in general - I just know it's better in at least one case. I think it's worth running a benchmark for it at least though. (which I can't do 🥺)

TIL about offset_of. uninit seems to be chucked pretty readily, but offset_of might be better in low opt level builds etc.

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@Nilstrieb It doesn't seem to have the desired effect:
https://godbolt.org/z/z5fv4as4s

@alion02 Please make your new libcore compile.

}
}

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4 changes: 2 additions & 2 deletions library/core/src/result.rs
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Expand Up @@ -1460,7 +1460,7 @@ impl<T, E> Result<T, E> {
match self {
Ok(t) => t,
// SAFETY: the safety contract must be upheld by the caller.
Err(_) => unsafe { hint::unreachable_unchecked() },
Err(_) => unsafe { mem::MaybeUninit::uninit().assume_init() },
}
}

Expand Down Expand Up @@ -1491,7 +1491,7 @@ impl<T, E> Result<T, E> {
debug_assert!(self.is_err());
match self {
// SAFETY: the safety contract must be upheld by the caller.
Ok(_) => unsafe { hint::unreachable_unchecked() },
Ok(_) => unsafe { mem::MaybeUninit::uninit().assume_init() },
Err(e) => e,
}
}
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