move /showIncludes from always_args to ninja_compile_args

[peterh]

The /showIncludes flag is required by ninja, but not by any of the other backends. Since this is a ninja-specific requirement, it makes sense to include it with the ninja backend.

Fixes a compile performance regression with the vs backend when using Visual Studio 2026 version 18.7

Fixes #15959

[dcbaker]

We have lots of operations in the Compiler classes that exist for only one backend. What issue are you solving by moving this to the ninja backend instead of being a property of the compiler?

[peterh]

What issue are you solving by moving this to the ninja backend instead of being a property of the compiler?

Short version: Compiles take 40% longer than they should with backend=vs.

Long version: See #15959

[dcbaker]

I'd prefer if we added a new method to the Compiler class for that, implemented it as return [] for the base compiler, and then specialized it in the VisualStudioLikeMixin, then the ninja backend can call that method to get /showIncludes and the vs and xcode backends can just not call it.

[bonzini]

Oh indeed that's the better implementation! Sorry for missing the obvious

[dcbaker]

the CI failure is one of those sporadic failures. I've restarted that check, but this looks good to me.

@bonzini, any more comments?