--sysroot= flag works now

[dschuff]

... and that's probably not a good thing.

I found this trying to reproduce a partner's build, which used CMake's -DCMAKE_SYSROOT flag, which results in --sysroot= being passed to the compiler. This was a bug, because I don't think emscripten has ever supported that flag "correctly". However, before #12863 I believe we just ignored it (or I guess more correctly it was overridden by our use of -nostdsysteminc), but now it actually has an effect. In this case the correct thing to do was not to use a sysroot (since the sysroot they were using was for x86_64), but since the above NFC-intended change actually is user-visible, we should probably do one or more of the following:

1. Actually make sure that -sysroot does what it should. Probably this is what we want eventually but based on my interpretatation of Run clang with --sysroot. NFC #12863 I don't think we are quite there yet.
2. (assuming we don't do 1 yet) Explicitly ignore (or error out?) -sysroot and related flags that don't work.
3. Add to the release notes if and when we make any user-visible changes.

[dschuff]

/cc @sbc100

[sbc100]

I think we should explicitly ignore it with a warning for now.

In the future we can explicitly allow it, once we have a true sysroot ourselves that we might want people to be able choose a different location for.

Presumably the intermediate for for them is to just stop passing -DCMAKE_SYSROOT?

[dschuff]

Yeah, ignoring it sounds good. And yes, stop passing DCMAKE_SYSROOT is correct anyway in this case.

[stale]

This issue has been automatically marked as stale because there has been no activity in the past year. It will be closed automatically if no further activity occurs in the next 30 days. Feel free to re-open at any time if this issue is still relevant.

[dschuff]

Does #13090 mean this works "correctly" now, or that we ignore user-supplied --sysroot? or is this still broken in the way OP describes?

[sbc100]

We currently honor --sysroot due to the fact that we add our implicit --sysroot on the command line before any user flags.

I think this is a reasonable outcome. If a user passes --sysroot we should probably respect that and assume they have a sysroot they want to use.

[dschuff]

Yeah, that makes sense to me, thanks for confirming. So that's option 1 above, and that means this bug is fixed.