Suggest -Zrun-dsymutil-no for macOS fast compiles

[lberrymage]

This keeps original object files around after compilation in the event that debug information is needed.

Resolves #527

[CleanCut]

This is amazing! By far the single-most effective incremental compilation speedup I've ever seen in debug mode!

I tested this out on a 16-core Mac Pro and incremental compilation in debug-mode decreased from an average of 7.74s to 2.16s. The difference is HUGE! Over 3x faster!

Clean compile time in debug mode decreased from an average of 53.83s to 46.71s. Looks like about the same absolute time savings, but much more modest percentage-wise here.

Given the time savings is fixed, I expect slower machines will see a smaller percentage savings, but still it's by far the biggest single improvement I've seen.

--

It is interesting to note that this has no measurable affect in release mode, either for clean compiles or incremental compiles. Even more interesting, incremental compiles in release mode averaged 1.82s (with or without the setting). Up until now I had assumed release mode would have slower incremental compiles.

Clean compiles in release mode are somewhat slower (62s), but that's to be expected.

[CleanCut]

Suggested change

	- [Another fast compile flag for macOS][552]
	- [Another fast compile flag for macOS that gives incremental compilation a big boost!][552]

[CleanCut]

I went ahead and updated the config in bevy_template based off of this. Though, I'm going to be running my stuff with --release most of the time now, so ironically this config won't help me much. 😛

@cart - this one's worth merging.

[lberrymage]

Thanks a bunch! But give bjorn3 all the credit for this one; I just saw his issue, looked into it, and whipped up a PR. Glad to see it makes a big difference!

[bjorn3]

Even more interesting, incremental compiles in release mode averaged 1.82s (with or without the setting). Up until now I had assumed release mode would have slower incremental compiles.

What if you compile only dependencies with optimizations and without debuginfo? You can use [profile.dev.package.crate_name] to chnage the profile for just a single crate. See for example https://github.com/bjorn3/rustc_codegen_cranelift/blob/e5b2b1ba8101bc9d5da6fb05e669fc8f9d784368/Cargo.toml#L41

[cart]

Oooh this is really nice. im going to merge this as its a clear win, but feel free to follow up on @bjorn3's comment with another pr if it pans out.

[CleanCut]

Using the following in my Cargo.toml to compile dependencies with full optimizations and no debug symbols, but compile the project itself with no optimizations and with debug symbols decreases incremental time further to 1.38s! I don't see any "static" way to configure this, since the stanza re-enabling debug mode for the project itself needs to contain the project name (punchball, in this case), but I plan to add support for configuring this to bevy_template later today.

# Compile all the *dependencies* in optimized release mode even if `--release` is not passed in
[profile.dev]
opt-level = 3
debug = false

# But compile the project itself in debug mode if `--release` is not passed in
[profile.dev.package.punchball]
opt-level = 0
debug = true

[CleanCut]

...but I plan to add support for configuring this to bevy_template later today.

✅ CleanCut/bevy_template#2