auxiliary files in compile-test aren't compatible with `extern_prelude` feature

[nikomatsakis]

When we use the "auxiliary crate" feature in compile-test, it adds a -L <aux-path> flag that points rustc at the directory where to find those crates. This means that when rustc sees an extern crate it then searches those directories. It's good to have the ability to test that behavior, but it's not the way that cargo drives rustc -- and as a result it's not compatible with the new extern-prelude feature set, requiring awkward run-make tests like extern-prelude.

It would probably be better for compiletest, by default, to add a --extern foo=<path-to-foo> flag for each auxiliary crate. We could maybe keep a way to use the "auxdir" option.

cc @petrochenkov @zackmdavis @nrc @qmx

[nikomatsakis]

Here is where to find the current code around auxiliary crates. The idea, for review, is that tests can have a // aux-build:foo.rs annotation like this one:

rust/src/test/pretty/attr-derive.rs

Line 11 in 3d5753f

// aux-build:derive-foo.rs

When we see such an annotation, we will build the crate auxiliary/foo.rs. This is done in this loop here:

rust/src/tools/compiletest/src/runtest.rs

Line 1490 in 3d5753f

for rel_ab in &self.props.aux_builds {

We then later add a -L flag with the directory. Here is one such place but there seem to be several:

rust/src/tools/compiletest/src/runtest.rs

Line 1311 in 3d5753f

rustc.arg("-L").arg(&self.aux_output_dir_name());

The ideal seems to me to be to instead be adding --extern flags, but it'll take a bit of digging to see just the best place to do it.

[ehuss]

I think this is now essentially resolved via #67074 which added the aux-crate header which uses --extern instead of -L.

It's not a huge benefit because aux crates still default to 2015, and you can't refer to these crates in use statements, unless you change the edition in the aux crate.

[jieyouxu]

compiletest triage: indeed, closing as resolved.