Set mapping keys to "decoded" when custom unmarshaler is used

[mvo5]

decode: set all keys under a mapping with custom marshaler
decoded

This is a naive fix for the issue of custom unmarshal and marking
keys as "decoded". It simply assumes that if there was a custom
unmarshal for a mapping type all keys got handlded by the custom
unmarshal code.

This might be too naive but it seems reasonable and it also seems
we would need a richer interface for a custom unmarshal that gives
access to MetaData if we want to be more fine grained.

---

decode: add (failing) test for undecoded fields in a struct

This commit adds a failing test for the scenario decribed in issue
#425

[arp242]

This seems like a reasonable change.

Could you add a more extensive test though? At the very least something like:

a   = 1
arr = [2]

[tbl]
b = 3

inline = {c = 4}

Just so that keys in different locations are tested: array, table, inline table. Maybe a few more? I'm not sure if these are different code-paths off-hand, but doesn't hurt to test it.

[mvo5]

Thanks a lot for your feedback! I updated the code and it now includes the more complex testcase that you suggested and that uncovered missing handling of nested which I added now too. I am happy to add more tests, but I need to think a bit what else to check for (I'm not super familiar with the toml details). Ideas welcome here of course :)

Please also let me know if I should squash or rebase my commits, I did them linearly mostly to make it easy to review but happy to do whatever is needed/preferred.

[mvo5]

Friendly ping, anything I can do or improve to help this along?

[arp242]

I'd like to carefully look at it and play around with it a bit. Not terrible time-consuming, but I don't have a lot of bandwidth for that right now. It's "on the list" and not forgotten, but it might be a bit before I have a chance to properly look at it.

[mvo5]

Thank you so much! ❤️