source(from) could imply source(true)

[tjkirch]

In #109, when considering the field attributes that can work together, there was some agreement that source(true) and source(from) make sense together on a field. source(true) indicates that a field is the source even though it can't be named "source". source(from(...)) tells Snafu how to transform another type into the type indicated in the field. (See https://docs.rs/snafu/0.4.3/snafu/guide/attributes/index.html#transforming-the-source). There's no conflict in these intentions.

Moreover, if you specify source(from) on a field, you almost certainly mean for that to imply source(true) - you've already said "source" once, and it's a conflict to specify source(false) and source(from). It's not the case today, though - the source(from) attribute would be ignored if there weren't some other signal that the field is a source.

It'd reduce line count and cognitive overhead to have source(from) imply that the field is a source. source(true) on the same field would still be valid, just redundant.

[tjkirch]

This may be trivial to add with #109 while I'm finishing that up, but it's not strictly related to the error handling changes so I wanted to represent it as a separate issue.

[shepmaster]

source(true) would still be valid, just unnecessary.

To be pedantic, source(true, from(...)) would be redundant, but source(true) by itself would still be important in the case where the source field isn't named source.

[tjkirch]

To be pedantic, source(true, from(...)) would be redundant, but source(true) by itself would still be important in the case where the source field isn't named source.

That's indeed what I meant - updated the description!

I have a commit (auto-linked above) that accomplishes this, and adds the updated compile-fail cases, a unit test, and a doc update. It's based on #109 so I was waiting for that to get merged before making an official PR here, so that I didn't need to do the dance of changing base branch. Feel free to assign this issue to me and remove the help wanted, if you like!