Comment 1:

Struct tags are nice but I think this overuses them a bit.
Custom JSON marshalling can be done by implementing
the Marshaler interface already. Why not
type User struct { /* whatever */ }
type APIUser User
func (u APIUser) MarshalJSON() ([]byte, error) {
    // Manually marshal only the fields you want to expose. 
}
u := User{}
json.Marshal(u) // for all fields
json.Marshal(APIUser(u)) // just some fields

Comment 2:

I don't believe this is a road I want to go down. Yes it's useful but it's getting to be
a bit much in the tags.

Status changed to WorkingAsIntended.

Comment 3 by beatgammit:

What about adding a field "Tag string" to Decoder such that I can choose which tag gets
used for marshalling? The current behavior uses the magic string "json", and it would be
nice to be able to change this. This would give the nice behavior without promoting use
of multiple tags by having it be on an exported function.
The problem with the prior suggestion of using MarshalJSON() is that *every* type that
embeds a type that implements it would have to implement it as well:
http://play.golang.org/p/B5W8uMs3bA
I understand the opposition to polluting tags, but perhaps there's a way to go mid-way?
If this idea isn't acceptable either, perhaps an interactive marshal would work (kind of
like flag's Visit)? This would allow doing something like MarshalJSON, but on a
per-field basis.

Comment 4 by google@0xc0dedbad.com:

> What about adding a field "Tag string" to Decoder such that I can choose which tag
gets used for marshalling? The current behavior uses the magic string "json", and it
would be nice to be able to change this. This would give the nice behavior without
promoting use of multiple tags by having it be on an exported function.
I'd really love to be able to pass a tag string to the encoder/decoder to make it easy
to input/output different representations of a struct.  It keeps all representations in
a single authoritative location, and allows use of a consistent, easily understood
method for doing so, for a pretty trivial change.