Print a warning for un-applied commands being dropped from a CommandQueue

[garychia]

Objective

• Fixes Print a warning if a CommandQueue gets dropped with un-applied commands #11125

Solution

Add a warning for un-applied commands to the drop function.

[notmd]

I think we should only warn in debug mode and save an additional check here

[nicopap]

The bevy book recommends compiling bevy with opt-level=3. Disabling in debug mode means people who follow those instructions won't see the warning, making it pretty useless.

I do agree however that we could avoid checks, and additional binary bloat (log statements add a fairly large number of instructions, without counting the log message) But not sure how. I see some alternatives:

• Use info!, so that the log can be disabled at compile-time using the log/max_level_warn feature
• (controversial I think) use the detailed_trace feature
• Add a new feature, enabled by default, something like costly_runtime_checks and use it as a feature gate for the check.

Honestly, I don't prefer any in particular, all have their appeal and tradeoffs, and I don't think it's very important to avoid the check/log here.

[JoJoJet]

The bevy book recommends compiling bevy with opt-level=3. Disabling in debug mode means people who follow those instructions won't see the warning, making it pretty useless.

Debug checks are still included when compiling with optimizations, as long as you don't switch to release mode.

That said, I don't think it's worth spending too much thought on this check. For regular systems, or exclusive systems that properly reuse their SystemState, command queues will only get dropped when the app exit. The only time this check will be called unnecessarily is if users create a new SystemState every time the system runs, which is a performance pitfall of its own that's going to matter a lot more than this check.

[nicopap]

Here is a gist with code to test the feature:

https://gist.github.com/nicopap/cbff7a6dd36df728dd73ec40914bf931

It should show the warn, and when line 38 is uncommented, it shouldn't.