Make discarded-qualifiers compilation error in meson CI

[user202729]

Avoid errors of the form fixed in the dependencies below.

We only do this in the CI instead of modifying meson.build so that if the user's compile somehow doesn't have the flag or is otherwise stricter etc., the compilation does not fail for the user.

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⌛ Dependencies

#41311

#41314

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[tobiasdiez]

if the user's compile somehow doesn't have the flag or is otherwise stricter etc., the compilation does not fail for the user.

Can you explain this in more detail? Why is https://mesonbuild.com/Adding-arguments.html not working to always upgrade the warning to an error? I think it would be handy to have the local compilation fail iff the CI fails.

[user202729]

there are several things that may differ from user's machine and CI:

• compiler used (not everyone use gcc, they may use say tcc, zig cc, etc. which may not have this warning flag)
• compiler version (maybe some future gcc version is stricter and can diagnose more such cases)
• dependency version (maybe some dependency causes such a warning, or make a breaking change of the form changing a non-const function argument to const, creating a new warning on the caller)

unless we stress test all compilers/versions in existence, better be safe.

[tobiasdiez]

there are several things that may differ from user's machine and CI:

* compiler used (not everyone use gcc, they may use say tcc, zig cc, etc. which may not have this warning flag)

You can easily test for compiler features in meson.

[user202729]

but not the other 2 points.

[cxzhong]

Can you rebase this, because #40980 edit this

[tobiasdiez]

but not the other 2 points.

Sure, but couldn't you apply these arguments to every element related to the compilation process? An the inversion of those points is also true: maybe now the meson-ci will complain about a discarded qualifier but there is no way to locally reproduce this using a different compiler.

I would say if you add it to meson and all CI is green (on all platforms that we test), then there is a high chance that it will work on other configs as well. But it's not super important and I'll not insist that you have to add the flag in meson instead of in the CI.