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Preallocate the vector containing predicates in decode_predicates #55534

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Preallocate the vector containing predicates in decode_predicates #55534

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34 changes: 19 additions & 15 deletions src/librustc/ty/codec.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -175,22 +175,26 @@ pub fn decode_predicates<'a, 'tcx, D>(decoder: &mut D)
where D: TyDecoder<'a, 'tcx>,
'tcx: 'a,
{
let parent = Decodable::decode(decoder)?;
let decoder_len = decoder.read_usize()?;
let mut predicates = Vec::with_capacity(decoder_len);
for _ in 0..decoder_len {
// Handle shorthands first, if we have an usize > 0x80.
let predicate = if decoder.positioned_at_shorthand() {
let pos = decoder.read_usize()?;
assert!(pos >= SHORTHAND_OFFSET);
let shorthand = pos - SHORTHAND_OFFSET;

decoder.with_position(shorthand, ty::Predicate::decode)
} else {
ty::Predicate::decode(decoder)
}?;
predicates.push((predicate, Decodable::decode(decoder)?))
}

Ok(ty::GenericPredicates {
parent: Decodable::decode(decoder)?,
predicates: (0..decoder.read_usize()?).map(|_| {
// Handle shorthands first, if we have an usize > 0x80.
let predicate = if decoder.positioned_at_shorthand() {
let pos = decoder.read_usize()?;
assert!(pos >= SHORTHAND_OFFSET);
let shorthand = pos - SHORTHAND_OFFSET;

decoder.with_position(shorthand, ty::Predicate::decode)
} else {
ty::Predicate::decode(decoder)
}?;
Ok((predicate, Decodable::decode(decoder)?))
})
.collect::<Result<Vec<_>, _>>()?,
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Unless I'm mistaken, the iterator will already pre-allocate a vector of the right size here, so this doesn't really buy us anything.

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@ljedrz ljedrz Nov 14, 2018 •
edited
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The lower bound of size_hint for the implementation of FromIterator for Result is equal to zero and the implementation of from_iter for Vec uses it to calculate the capacity. I don't think this is a case of TrustedLen (due to non-conforming size_hint), otherwise this PR (where the number of elements iterated over is known) wouldn't benefit from such a change either.

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Ah, I see, ok.

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I also see I missed a lot of discussion on this topic already 😊

parent,
predicates
})
}

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