Replace runtime `cfg!(target_os = ...` with macros for compile-time

[n0toose]

Hi, I spent a bit of time looking through Anki's source code and realized that there were many excerpts that check for the operating system that Anki is being currently run on, e.g.:

anki/qt/launcher/src/main.rs

Line 291 in 9679923

if cfg!(target_os = "macos") {

In many cases, like the one linked above, such checks can be done at compile-time and don't have to take place during the runtime, for example see here:

anki/qt/launcher/src/main.rs

Line 187 in 9679923

#[cfg(target_os = "macos")]

There are a couple of cases where this turns up: https://github.com/search?q=repo%3Aankitects%2Fanki+cfg%21%28target_&type=code

The if-else cases can be substituted e.g. with #[cfg(not(target_os = "macos"))] or similar.

[abdnh]

My understanding is that there's not much difference and both are compile-time checks. The compiler should be smart enough to remove dead branches: https://doc.rust-lang.org/rust-by-example/attribute/cfg.html

[n0toose]

* one shouldn't have to hint to the compiler for this is what I intended to say, this was grounded in consistent code style standards in line with the rest of the community's

[abdnh]

Replacing cfg! can hurt readability in some places for no benefit. It's mostly used in build scripts and tests anyway, so even if the compiler fails to optimize it (which I doubt), it won't matter much.

[dae]

Unless it's a hot code path, runtime checks are generally preferable, as they're easier to read, and the code gets checked/compiled on all platforms. Code that is conditionally excluded on certain platforms is harder to edit, as the IDE won't compile it/allow you to click on references in it/etc unless you're on that platform.