Use a consistent approach for organising test fixture files

[edmorley]

Previously test fixtures were located under a mixture of:

• <crate>/fixtures/
• <crate>/test-fixtures/

Now for consistency they are all located under:
<crate>/tests/fixtures/

This matches the pattern already used by the Python CNB, Procfile CNB etc.

GUS-W-14161083.

[Malax]

+1 for consistency, I'm not sure about the location though. Cargo will try to compile code it finds in tests, even when in subdirectories below tests. If we ever had a test fixture that Cargo would detect as test code (imagine a test fixture that is an app written in Rust) this location will give us trouble.

[edmorley]

Cargo will try to compile code it finds in tests, even when in subdirectories below tests

That doesn't seem to be the case, when I tried an experiment just now?

With this PR there is already Rust code under one of the tests/fixtures/ directories, so I added a #[test] annotation to a random function, added a typo to that function, ran cargo test in the libcnb workspace root, and I didn't get a test failure (which suggests the fixture wasn't accidentally picked up).

My understanding of Cargo's automatic target discovery (https://doc.rust-lang.org/cargo/reference/cargo-targets.html#target-auto-discovery and https://doc.rust-lang.org/cargo/guide/project-layout.html) is that it only looks for modules (or lib/bin targets) within the top-level tests/ directory. These modules then need to import any nested modules for the nested modules to be picked up. (This is the only reason the "run all integration tests in a single binary" strategy works iirc.)

[Malax]

I tried adding a main.rs with a compile error into tests/fixtures and the tests didn't compile anymore, i.e. cargo test. It would be picked up because it's called main, I assume the same would be true for lib.rs any maybe bin/foo.rs too?

Edit: it seems only main.rs directly in test/fixtures causes issues.

[edmorley]

Edit: it seems only main.rs directly in test/fixtures causes issues.

Yeah anything nested in a further subdirectory isn't picked up. There are already some main.rs files nested further (eg in the test buildpack fixtures).

Since all of our fixtures will be in their own directory, and not loose in the fixtures/ directory (plus the failure mode is pretty clear; it's not hard to debug undefined behaviour) I think this is fine :-)