Replace cfg_if! macro with cfg_select! fallback

[clarfonthey]

The main benefit this gives is rustfmt support; cfg_select! is properly formatted, unlike cfg_if!. Additionally, it makes migrating when we eventually bump MSRV easier.

[clarfonthey]

We actually never used this branch, but I guess we wanted it for completeness.

[xtqqczze]

Perhaps this implementation might work better with _ fallback syntax:

https://github.com/rust-lang/compiler-builtins/blob/9c7d3e8d064e331144dbe3426a0bf5311399a857/libm/src/math/support/macros.rs#L5

[clarfonthey]

That sure is a simpler implementation. I'll try that one out.

[xtqqczze]

Perhaps someone could publish a cfg_select crate, since this has come up elsewhere: rusqlite/rusqlite#1850 (comment)

[clarfonthey]

We explicitly avoid depending on cfg-if to avoid increasing dependencies, so, unlikely we'd add a new dependency here. But if someone wants to maintain that, it might be helpful.

[xtqqczze]

One limitation of the current implementation is that branch bodies must be enclosed in braces. For example, the following is not supported:

cfg_select! {
    unix => foo()
    windows => bar()
}

[xtqqczze]

I’d still include a link to the compiler-builtins implementation in a comment, as it may help keep the two implementations in sync over time:

https://github.com/rust-lang/compiler-builtins/blob/9c7d3e8d064e331144dbe3426a0bf5311399a857/libm/src/math/support/macros.rs

[clarfonthey]

That seems pretty unnecessary IMHO. This satisfies our needs until we can update the MSRV to remove it.