Add support for tracing-flame

[ctron]

This PR is based on #48. Only the last commit belongs to this PR.

This adds support for tracing-flame. (See #47)

This PR works, but might need some work on the documentation. I am happy to follow up with it if you think the approach is good in general.

Here is how this works:

• Enable the tracing-flame feature on test-log (this by itself doesn't to much other than adding the dependency).
• Setting the TEST_LOG_FLAMES env-var to a path when running tests, will write traced out as {module}/{name}.folded.
• Those can be processed with inferno as described by tracing-flames

The result of this looks something like:

[d-e-s-o]

In my opinion this functionality doesn't belong in this crate, or it will become a dumping ground for all sorts of initialization logic. I suggest you create a new crate.

[ctron]

Fair. What about adding some kind of "initializer" where someone could provide some fn that can hook into the process?

[d-e-s-o]

Fair. What about adding some kind of "initializer" where someone could provide some fn that can hook into the process?

Conceptually, yes, that may be okay. That being said, I don't believe that's possible at this point. Perhaps once global attribute support is available on stable Rust that could be made to work, but until then I am not sure how you'd set global state before test-log runs, short of parametrizing the existing per-test attribute, which may end up being racy and doesn't really solve the usability issues, in my opinion.

Do you have a way to go about it?

[ctron]

Without having any concrete idea, I was hoping that I could get the following to work (syntax might vary):

#[test_log::test(init=init_fn)]
fn test() {}

[d-e-s-o]

Without having any concrete idea, I was hoping that I could get the following to work (syntax might vary):

#[test_log::test(init=init_fn)]
fn test() {}

Right, but if init_fn touches anything global this will be inherently racy: some tests set it, others may not. Depending on which runs first all may see the result, or only some. See cbcdb8c

[ctron]

I put this into draft state. I doesn't have a high priority for me. I'll see if I can come up with something reasonable, but don't want to make any promises.