Add configuration option to specify tag value that indicates squash

[triarius]

When decoding, a nested struct could be squashed with the mapstructure:",squash" struct tag. A similar functionality appears when marshalling JSON into structs, but it is called inline instead of squash.

Given that a DecoderConfig field allows specifying a different name than mapstructure for the struct tag, I think users would want to be able to specify a different value to indicate the "squashing" of nested structs.

Indeed, such functionality would allow a more satisfactory resolution to spf13/viper #1050. While it remains true that users may specify a struct tag of `mapstructure:",squash", it is sometimes not possible to do this because the structs are not owned by the current package. If this is merged, viper users will be able to correctly parse configuration that contains foreign types that have nested structs that need to be inlined with something like:

cfg := SomeStructWithNestedStructs{}
viper.Unmarshal(&cfg, func(c *mapstructure.DecoderConfig) {
	c.TagName = "json"
	c.SquashTagOption = "inline"
})

Note: I originally made this PR on upstream, but I soon realised that this is the blessed successor. It's somewhat of a co-incidence that the main use case is in viper itself!

[sagikazarmark]

Thanks for the PR @triarius!

I'm a little bit reluctant to accept this change, because it makes it more easy to misconfigure the library.

I'm also a bit worried that JSON's ,inline and mapstructure's ,squash may not always work 100% the same way in which case we might just sent people on a wild goose chase with this change.

However, you make a strong case about being able to change the tag name, but not the option and how that may be a limitation, so I think we are better off with this change after all.

Would you mind taking a look at my comment?

Thanks!

[sagikazarmark]

AFAIK these are called options or tag options, so something like the following would be a better name IMO:

• CustomSquashTagOption
• SquashTagOptionValue
• SquashTagOptionName
• SquashTagOption

SquashTagOption may be the better one as it's short.

[triarius]

Sounds good, I've updated to use SquashTagOption