std: Add Vec::from_iter comment

[alexcrichton]

Requested by Niko in #22200 (and is good to have anyway)

[rust-highfive]

r? @gankro

(rust_highfive has picked a reviewer for you, use r? to override)

[brson]

@bors: r+ 42053 rollup y'all

[huonw]

FWIW, I'm not super keen on this fuse approach. In general it adds a check to every next call, to protect against a case that is basically "unspecified behaviour" anyway (the iterator being shorter than its stated size_hint).

I'd prefer adding a check after the loop:

// the iterator ran out of elements before filling the whole vector, so we're done.if vector.len() < vector.capacity(){return vector }

since that is guaranteed to be only a single check and outside the inner loop, even if the optimiser can't remove it.

[alexcrichton]

Good idea! I have updated and added an appropriate comment as well.

[alexcrichton]

It appears this landed in a rollup but then wasn't auto-closed.

re-r? @huonw

[huonw]

s/ear/era/

[huonw]

r=me with the speeling fix.

[alexcrichton]

@bors: r+ 95a28c9