fix(cc): move user provided -isystem paths to front of command

[The-Briel-Deal]

Problem: Currently if you pass a -isystem argument to zig cc, the include path will be searched after all of the libc, libcpp, and native system paths. However if you run the same command against clang and gcc you'll see that any user provided -isystem paths are at the very front.

Solution: Pull out user provided -isystem from mod.cc_argv and comp.global_cc_argv at the start of addCCArgs(), append these paths as -isystem args just before we append the native system paths. Finally we append the rest of the cc args in the same place.

Fixes: #24243

[alexrp]

This doesn't seem quite correct; with this change, C flags on modules will no longer be able to override any default flags that we add in addCCArgs().

[andrewrk]

That's right - those are at the end on purpose so they can override everything else.

The proper fix is to adjust the parsing in the CLI to account for isystem args without piggybacking on the general last resort overriding flag set.

[The-Briel-Deal]

That's right - those are at the end on purpose so they can override everything else.

The proper fix is to adjust the parsing in the CLI to account for isystem args without piggybacking on the general last resort overriding flag set.

I think I understand! I believe I saw that some flags are parsed out earlier and are pulled out of cc_argv. That makes sense and is a fair concern. Let me spend some time reworking this change and I'll mark it as ready for review again.

Thanks for the super quick feedback! I appreciate it!

Side Note: I'd really like to add a test for this as well, originally I wanted to perform a test call to zig cc to ensure that the right header is included, then when I couldn't find a good way to do that I tried just testing Compilation.addCCArgs() but it ended up being a ton of code to get a fake Compilation struct setup. How would you guys test this sort of change?

[alexrp]

Putting a test somewhere in test/standalone would make sense.

[The-Briel-Deal]

Hey @alexrp, so I fixed this by just pulling out the -isystem paths from the global and module cc_argv, then appending the paths just before native paths are appended.

Let me know what you think. Part of me was wondering if I should pull this out in buildOutputType(), but I decided against that because isystem paths should pretty much exclusively be used for C compilation I think.

Let me know if I should tweak anything else.

Also, I added a unit test against addCCArgs() in order to validate that order is correct now.