Auto merge of #137908 - scottmcm:another-size-hint-attempt, r= · rust-lang/rust@272c07f (original) (raw)

`@@ -22,21 +22,39 @@ where

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`// empty, but the loop in extend_desugared() is not going to see the

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`// vector being full in the few subsequent loop iterations.

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`// So we get better branch prediction.

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let mut vector = match iterator.next() {

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None => return Vec::new(),

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Some(element) => {

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let (lower, _) = iterator.size_hint();

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let initial_capacity =

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cmp::max(RawVec::::MIN_NON_ZERO_CAP, lower.saturating_add(1));

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let mut vector = Vec::with_capacity(initial_capacity);

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unsafe {

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// SAFETY: We requested capacity at least 1

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ptr::write(vector.as_mut_ptr(), element);

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vector.set_len(1);

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}

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vector

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}

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let (low, high) = iterator.size_hint();

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let Some(first) = iterator.next() else {

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return Vec::new();

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`};

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`` +

// push's growth strategy is (currently) to double the capacity if

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// there's no space available, so it can have up to 50% "wasted" space.

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// Thus if the upper-bound on the size_hint also wouldn't waste more

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// than that, just allocate it from the start. (After all, it's silly

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`` +

// to allocate 254 for a hint of (254, Some(255).)

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let initial_capacity = {

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// This is written like this to not overflow on any well-behaved iterator,

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`` +

// even things like repeat_n(val, isize::MAX as usize + 10)

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`` +

// where low * 2 would need checking.

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`` +

// A bad (but safe) iterator might have low > high, but if so it'll

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`` +

// produce a huge extra that'll probably fail the following check.

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let hint = if let Some(high) = high

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&& let extra = high - low

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&& extra < low

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{

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high

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} else {

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low

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};

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cmp::max(RawVec::::MIN_NON_ZERO_CAP, hint)

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};

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let mut vector = Vec::with_capacity(initial_capacity);

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// SAFETY: We requested capacity at least MIN_NON_ZERO_CAP, which

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// is never zero, so there's space for at least one element.

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unsafe {

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ptr::write(vector.as_mut_ptr(), first);

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vector.set_len(1);

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}

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+

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`// must delegate to spec_extend() since extend() itself delegates

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`// to spec_from for empty Vecs

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` <Vec as SpecExtend<T, I>>::spec_extend(&mut vector, iterator);