Is .size_hint().1 ever used?

[scottmcm]

I tried to make use of it in this PR, but was unsuccessful:

Try to guess a smarter initial capacity in Vec::from_iter

Out of curiosity, I started looking through rust to see if it’s ever actually used. And I found lots of things like

rust-lang/rust/blob/ab93561b5fa54954159480ddc10bbb69f015e539/src/liballoc/collections/binary_heap.rs#L1180-L1182

rust-lang/rust/blob/ab93561b5fa54954159480ddc10bbb69f015e539/src/librustc_data_structures/small_vec.rs#L172-L174

rust-lang/rust/blob/ab93561b5fa54954159480ddc10bbb69f015e539/src/liballoc/vec.rs#L2639-L2641

But not a single case of something actually using the upper bound. (There were lots of passing it along or validity-checking it, but never doing something useful with it.)

Have you ever found a use for it?

[kornel]

That’s a shame it isn’t used.

Using something like upper_bound.min(lower_bound * 2) as the initial capacity for collect could save a reallocation.

[withoutboats]

The lower bound is used to guide allocations for collecting. I don’t know why this is, I’d guess to avoid massive overallocations when the upper bound is far too vague.

The higher bound is used in TrustedLen, but then, its the same value as the lower bound in that case.

[scottmcm]

@kornel

Using something like upper_bound.min(lower_bound * 2) as the initial capacity for collect could save a reallocation.

Yup, tried that

scottmcm/rust/blob/aa080f40413693e9bb8dd55e645c47984952a517/src/liballoc/vec.rs#L1864-L1868

The perf results weren't a clear win, though.

@withoutboats

The higher bound is used in TrustedLen , but then, its the same value as the lower bound in that case.

Yeah, ExactSizeIterator has a similar check, which of course isn't really much of a use:

rust-lang/rust/blob/c0955a34bcb17f0b31d7b86522a520ebe7fa93ac/src/libcore/iter/traits.rs#L691-L695