cargo build-dir-new-layout breaks feature detection

[illicitonion]

This feature detection code no longer works correctly on nightly as of rust-lang/rust@83709ee which pulls in rust-lang/cargo#17258 which changes the target directory layout. All features are now treated as disabled.

[epage]

CARGO_CFG_FEATURE is exposed to build scripts but that would require every library using trybuild to have a build script and then forward that to trybuild.

I've talked about the idea of bypassing the need for build scripts for reading --cfgs by having cfg_value!("...") / cfg_values!("feature") macros to pair with cfg!.

[epage]

We did do a crater run but I assume that just caused things to be skipped, rather than fail.

[illicitonion]

I'm not sure how crater runs work - my guess is it only runs crates with default features? If so, I suspect it wouldn't pick up this kind of breakage very often, e.g. my crate which hit this only hit this issue when tests are being run with non-default features enabled (see CI config).

[ranger-ross]

Went ahead and raised #338 with a near term fix. (updated the directory logic to also handle the new build-dir layout)

Also documented this usecase in rust-lang/cargo#17287 in hopes that Cargo could eventually provide a stable way to do without many traversing the build-dir tree or relying on the fingerprint file.