Improve handling of compile_data with mixed sources

[martingms]

I'm not very well versed in starlark nor the rules_rust codebase, so feel free to ignore this and address the issue in a more fitting way, but this fixes #3171 and a related issue for me.

[sam-mccall]

I was looking at generated sources recently, this change makes sense to me.

Needs an owner, @krasimirgg we were talking about this very issue a couple of days ago :-)

[sam-mccall]

do we need to worry about compile_data being None?

[martingms]

afaict this shouldn't be possible, attr.label_list defaults to [], and we set compile_data explicitly to a depset based on that in all codepaths, but I can add this check back if you think it's better to be defensive. It was a bit confusing to me, which is why I removed it!

[krasimirgg]

Looks good! Please re-format the file that the Buildifier workflow is complaining about.

[krasimirgg]

Please reformat according to https://buildkite.com/bazel/rules-rust-rustlang/builds/13739#01945f02-1578-4657-8c29-aeaef49853d9/127-128

[martingms]

Sorry, missed buildifier and one test affected by the last commit, pushed an updated patch now 👍

[UebelAndre]

This has lead to a regression for targets which have compile data from completely other packages. See repro at (#3193)

ERROR: /Users/user/Code/rules_rust/test/unit/compile_data/src/BUILD.bazel:11:13: in rust_library rule //test/unit/compile_data/src:lib:
Traceback (most recent call last):
	File "/Users/user/Code/rules_rust/rust/private/rust.bzl", line 87, column 32, in _rust_library_impl
		return _rust_library_common(ctx, "rlib")
	File "/Users/user/Code/rules_rust/rust/private/rust.bzl", line 152, column 55, in _rust_library_common
		srcs, compile_data, crate_root = transform_sources(ctx, ctx.files.srcs, ctx.files.compile_data, crate_root)
	File "/Users/user/Code/rules_rust/rust/private/utils.bzl", line 825, column 64, in transform_sources
		generated_compile_data = [_symlink_for_non_generated_source(ctx, src, package_root) for src in compile_data]
	File "/Users/user/Code/rules_rust/rust/private/utils.bzl", line 870, column 64, in _symlink_for_non_generated_source
		src_symlink = ctx.actions.declare_file(paths.relativize(src_short_path, package_root))
	File "/private/var/tmp/_bazel_user/76282c66b0dfe3c5cb9a230bdc913a52/external/bazel_skylib~/lib/paths.bzl", line 247, column 17, in _relativize
		fail("Path '%s' is not beneath '%s'" % (path, start))
Error in fail: Path 'test/unit/compile_data/data/data.txt' is not beneath 'test/unit/compile_data/src'
ERROR: /Users/user/Code/rules_rust/test/unit/compile_data/src/BUILD.bazel:11:13: Analysis of target '//test/unit/compile_data/src:lib' failed

I don't quite know how to solve for this as I'm not sure what the symlink path should be for this foreign path.

[krasimirgg]

Happy if we rolled this back until we have a full fix for this.

[martingms]

@krasimirgg The crash should be fixed by #3196 or similar, but it's unclear to me how the repro in #3193 could have worked (as in the data.txt file is actually available during compile) even before this (#3176) PR was merged, as generated sources were built in the out tree even before. I'm probably missing something! Do you have any pointers to what the situation is in the real world failures?

Fine with me to roll it back too obviously.