feat(serde_with_macros): proper handling of cfg_attr and schemars

[sivizius]

With this commit, cfg_attr will be handled properly. The current implementation does not accept e.g.:

#[serde_as]
#[derive(JsonSchema, Serialize)]
struct Test {
    #[serde_as(as = "DisplayFromStr")]
    #[cfg_attr(any(), arbitrary("some" |weird| syntax::<bool, 2>()))]
    #[cfg_attr(any(), schemars(rename = "foo"), schemars(range(min = 0)))]
    custom: i32,
}

Because due to https://github.com/jonasbb/serde_with/blob/master/serde_with_macros/src/utils.rs#L91, the parser does not accept cfg_attr with multiple arguments, which is allowed, see e.g. https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg_attr-attribute. Another issue I’ve found with arbitrary where some unexpected errors due to this custom parser: https://github.com/jonasbb/serde_with/blob/master/serde_with_macros/src/utils.rs#L191-L218 in combination with this line: https://github.com/jonasbb/serde_with/blob/master/serde_with_macros/src/utils.rs#L240.

Line 240 will only continue, if the outer-most meta if neither cfg_attr or schemars. If however the attribute is cfg_attr, the code after the match-block is reached and the parser will just assume, that the inner meta is schemars and thus fail to parse any meta with some weird syntax.

Even #[schemars(length(min = 1.0))] will fail, because the parser expects all schemars-attributes to be a list of key-value-pairs (foo = bar), thus failing for length(min = 1.0), even though this is perfectly legal (https://graham.cool/schemars/deriving/attributes/#length).

I introduced the type LazyBool to simplify dealing with various conditions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.34%. Comparing base (378fa24) to head (2d70b70).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #782      +/-   ##
==========================================
- Coverage   67.45%   67.34%   -0.11%     
==========================================
  Files          40       40              
  Lines        2464     2468       +4     
==========================================
  Hits         1662     1662              
- Misses        802      806       +4     

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[jonasbb]

Thanks for the PR. In my opinion, it is a bit unfortunate that it is necessary to parse the cfg_attr in the first place, but without that you loose some usability. Since the PR is not so big and this fixes some issues I am fine to merge this. I didn't really check the code in detail, but it looks reasonable overall. Let's hope the test cover enough.