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Insurance policy in case iter.size_hint() lies. #65749

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Oct 25, 2019
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Insurance policy in case iter.size_hint() lies. #65749

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6 changes: 5 additions & 1 deletion src/librustc/ty/context.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2930,14 +2930,18 @@ impl<T, R, E> InternIteratorElement<T, R> for Result<T, E> {
// lower bounds from `size_hint` agree they are correct.
Ok(match iter.size_hint() {
(1, Some(1)) => {
f(&[iter.next().unwrap()?])
let t0 = iter.next().unwrap()?;
assert!(iter.next().is_none());
f(&[t0])
}
(2, Some(2)) => {
let t0 = iter.next().unwrap()?;
let t1 = iter.next().unwrap()?;
assert!(iter.next().is_none());
f(&[t0, t1])
}
(0, Some(0)) => {
assert!(iter.next().is_none());
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If it matters this could be written without unwrap's or asserts.

let a = match iter.next() {
    None => return Ok(f(&[])),
    Some(a) => a?,
};
let b = match iter.next() {
    None => return Ok(f(&[a])),
    Some(b) => b?,
};
let c = match iter.next() {
    None => return Ok(f(&[a, b])),
    Some(c) => c?,
};
let mut vec: SmallVec<[_; 8]> = SmallVec::with_capacity(iter.size_hint().0 + 3);
vec.push(a);
vec.push(b);
vec.push(c);
for result in iter {
    vec.push(result?);
}
Ok(f(&vec))

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Nice! OTOH, the original order of the match was chosen by @nnethercote to reflect the frequency of the cases.

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@RalfJung Should we perf test this variant?

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No opinion. @nnethercote what do you think?

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The vec construction could be more concise, e.g. using extend. And I would use t0, t1, t2 instead of a, b, c. And the comment needs updating. Otherwise, fine by me to use this form if the perf isn't hurt.

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extend would be better but there isn't a way in std to extend from a Result iterator so you need a workaround like https://github.com/mitsuhiko/redis-rs/blob/418512bf9171589b814da4c5ecb500e8747781e0/src/parser.rs#L21-L54

f(&[])
}
_ => {
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