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It's basically just a less capable version of `SmallVec`.
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Extra allocations are a significant cost of NLL, and the most common
ones come from within `Canonicalizer`. In particular, `canonical_var()`
contains this code:
indices
.entry(kind)
.or_insert_with(|| {
let cvar1 = variables.push(info);
let cvar2 = var_values.push(kind);
assert_eq!(cvar1, cvar2);
cvar1
})
.clone()
`variables` and `var_values` are `Vec`s. `indices` is a `HashMap` used
to track what elements have been inserted into `var_values`. If `kind`
hasn't been seen before, `indices`, `variables` and `var_values` all get
a new element. (The number of elements in each container is always the
same.) This results in lots of allocations.
In practice, most of the time these containers only end up holding a few
elements. This PR changes them to avoid heap allocations in the common
case, by changing the `Vec`s to `SmallVec`s and only using `indices`
once enough elements are present. (When the number of elements is small,
a direct linear search of `var_values` is as good or better than a
hashmap lookup.)
The changes to `variables` are straightforward and contained within
`Canonicalizer`. The changes to `indices` are more complex but also
contained within `Canonicalizer`. The changes to `var_values` are more
intrusive because they require defining a new type
`SmallCanonicalVarValues` -- which is to `CanonicalVarValues` as
`SmallVec` is to `Vec -- and passing stack-allocated values of that type
in from outside.
All this speeds up a number of NLL "check" builds, the best by 2%.
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These unstable items are deprecated:
* The `std::collections::range::RangeArgument` reexport
* The `std::collections::range` module.
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This reverts commit 143206d54d7558c2326212df99efc98110904fdb.
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Move it and Bound to core::ops while we're at it.
Closes #30877
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Its nowhere used (if it had been used used, the rust stack would have overflown
due to the recursion). Its presence was confusing for mrustc.
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You can now call .drain(..) on SmallVec, AccumulateVec and ArrayVec
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data_structures::SmallVec.
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AccumulateVec is generic over the Array trait, which is currently only
implemented for [T; 8].
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