how do we keep the original '-' meanings?

This is a substantial change to the json package and should be a proposal: https://github.com/golang/proposal

Having said that, a couple of ways to do this without adding features come to mind:

• Have multiple versions of the same struct for marshaling and unmarshalilng, with different json field tags. Converting values between these is permitted. Will require some boilerplate if the fields in question are nested, though.
• Manually ignore the fields. If one wants to ignore the field when marshaling, set it as omitempty and temporarily zero it. If one wants to ignore the field when unmarshaling, reset the field to its previous value.

@Ctriple I think - is stay the same as before.

@mvdan :

1. Have multiple versions of the same struct for marshaling and unmarshalilng, with different json field tags is tedious.
2. Manually ignore the fields is tedious too.

Most of the changes that would make the language more complex are to avoid some degree of code verbosity, so that's not a point that stands on its own. The advantages of this proposal must outweigh the disadvantages. To do that, this issue should be a proposal, like I suggested above.

Alternative syntax choices

Another syntax to achieve this, could be to introduce two new struct-tag prefixes, e.g."jsonMarshal" and "jsonUnmarshal". I.e.:

type User struct {
	Name     string `json:"name"`
	Password string `jsonMarshal:"password" jsonUnmarshal:"-"``
}

This even allows using different values for encode a decode, when desirable.

Yet another alternative, would be to allow ,nomarshal and ,nounmarshal keywords (or some similar wording):

type User struct {
	Name     string `json:"name"`
	Password string `json:"password,nounmarshal"`
}

This is less flexible then multiple struct tags, but faster and easier to write, and more in line with current struct-tags like ,omitempty (which is already already affecting only marshal?).

Use-case

With either method, this would be even more useful when using embedded structs as a mechanism for including fields into models on the client side of an API where the server has defined read-only fields. In particular so, because custom marshal / unmarshal methods for structs used as embedded fields, does not have the desired effect.

Example:

package modelprofile

type Model struct {
	ID   string `json:"id,nomarshal"`
	ETag string `json:"_etag,nomarshal"`
}

type WriteInfo struct {
	CreatedBy string     `json:"createdBy,nomarshal"`
	UpdatedBy string     `json:"updatedBy,nomarshal"`
	CreatedAt *time.Time `json:"createdAt,nomarshal"`
	UpdatedAt *time.Time `json:"updatedAt,nomarshal"`
}

type User struct {
	modelprofile.Model
	modelprofile.WriteInfo

 	Name string      `json:"name"`
 	Password string `json:"password,nounmarshal"`
}