Custom attributes parser

[sphynx]

Closes #178

I've added a new module: parse.rs, which contains an intermediate representation for structopt attributes. This representation reflects domain-specific attributes of structopt and is used instead of generic syn.Meta, which played a similar role in the previous version of the code. This allows parsing arbitrary expressions instead of String literals in attribute values. I tried to keep those intermediate structures low-level, i.e. closer to the token level, so that it's easy to write Parse trait implementations for them. Later, they are converted to more high-level representation in attrs module.

I didn't change the parsing of doc-comments, since they are not affected by the introduction of arbitrary tokens in attributes in Rust 1.34. So syn.Meta is still used there.

Please take a look at the code: if you like the approach, I'll add some notes to the documentation and the ChangeLog.

Thank you!

[sphynx]

My newly added tests obviously can't be compiled on Rust 1.21.0, because the feature in question is supported in 1.34.0. I'm not sure how to proceed with those tests in the best way.

The issue is similar to arg_enum! from clap which is present in examples, but it's a test, not an example. I'm not sure how to disable a single test module (can't find anything useful in cargo test --help).

Perhaps I can [ignore] those tests by default and turn them on in all the cases except Rust 1.21.0? It's not ideal though. Or I can try to introduce a custom build attribute and use it with cfg?

[TeXitoi]

We can just skip the tests on rust 1.21. If it compils, and tests are passing using latest stable rust, there is no reason that it would not work with 1.21 for the usable features.

[TeXitoi]

I may take a bit of time to review this PR as it's quite long.

[sphynx]

We can just skip the tests on rust 1.21. If it compils, and tests are passing using latest stable rust, there is no reason that it would not work with 1.21 for the usable features.

Ok, so we should update .travis.yml for that, right? I'm not sure how to do it, my guess is that overriding script for 1.21.0 can work:

    - rust: 1.21.0
      # just build the code for 1.21.0, since some of the tests use features
      # which are unsupported in 1.21.0
      script: cargo build

Is it the right way to do it in Travis? If so, I can add that change to this PR.

[sphynx]

Is it the right way to do it in Travis? If so, I can add that change to this PR.

Ok, I've changed .travis.yaml, we'll see how it goes :)

[TeXitoi]

Yep, it's working.

[sphynx]

Note that 'full' version of syn is needed for this change to support parsing of more complicated expressions like &[AppSettings::Foo, AppSetting::Bar].

A quote from syn documentation:

full — Data structures for representing the syntax tree of all valid Rust source code, including items and expressions.

[TeXitoi]

As noted in #200, v0.2.17 was a breaking change version. Thus, I've yanked the version, and updated the minimal rust version to 1.34. The plan is to put the new version in v0.3. v0.3 "need" a edition 2018 before publishing.

[TeXitoi]

Great work! It will be a big improvement!

[sphynx]

I've implemented the requested changes, could you please take a look again?

Thank you!

[sphynx]

I've improved the error messages from parse.rs, deleted obsoleted trybuild test cases and rebased several others to contain updated error messages.

As a side note: this pull request is getting longer and longer (already at 18 commits!) and it's becoming harder to maintain: to rebase it or to merge the other changes in...

[TeXitoi]

Yes, sorry, that's the priority now. Will try to review it soon. No other PR will be merged before this one.

[TeXitoi]

Really great!

Just a small test to remove, and I merge this PR.

Thanks again!