It feels like this is a poor way to determine if a feature should be enabled. There are just too many reasons it could fail, and default off on compilation failure just seems like a glaring security bug. If this is a common practice, it seems like the community should rethink this choice.
I don't think it fails the build. It's part of a test, trying to compile a bit of code. If it compiles the test is true and a certain feature is enabled. If the compilation fails the the feature is disabled.
This is quite common in build systems. I had such a test produce incorrect results in the kernel build system recently. The problem is that the tests should really look carefully for an expected error message. If the compilation fails with the expected message the test result is false. If the compilation fails for some other reason the build should fail so the developer is forced to investigate.
Disclaimer: I have not studied the xz build system in detail, just what I saw from the diff plus a bit of extrapolation.
Problem is that every compiler/compiler version will have different messages for the same error. And xz is the kind of project that gets built on really really random homegrown compilers. Maybe we need an option to make the compilers output a standard JSON or whatever...
> Maybe we need an option to make the compilers output a standard JSON or whatever...
While I like the idea, I fear this would go the same route as UserAgent strings, with everybody giving carefully crafted lies. The main issue is that bugs are only known in retrospect.
As an example, suppose Compiler A and Compiler B both implement Feature X. Therefore, they both produce JSON output indicating that they support Feature X. A year later, developers for Compiler A find and fix a bug in its implementation of Feature X. Now, how do we handle that case?
* Update the version for Feature X as by Compiler A? Now Compiler B is publishing the same version as the buggy Compiler A did, and would get marked as not having a usable implementation.
* Update the spec, forcing a version update for Feature X in both Compiler A and Compiler B? Now a bugfix in one compiler requires code updates in others.
* In the build system, override the JSON output from Compiler A, such that Feature X is marked as unavailable for the buggy versions? Now the JSON is no longer trustable, as it requires maintaining a list of exceptions.
* Try to trigger the bug that Compiler A just fixed, so that the build system can recognize the buggy versions, regardless of the JSON output? Now we're back to the starting point, with features determined based on attempted compilation.
