I propose adding a new struct tag to the json/encoding package that tells the unmarshaller to skip a field when it is unmarshalling, and leave the existing value as-is.
This mirrors the behaviour the "-" option in the marshal operation.

An example use case of this is a RESTful API service that serves JSON data, and also accepts JSON data from clients to update the data. The endpoint is represented as a struct:

type Endpoint struct {
	VariableField string `json:"variable"`
	FixedField    string `json:"fixed"`
}

func NewEndpoint() *Endpoint {
	var ep *Endpoint
	ep.FixedField = "/should/never/change"
	return ep
}

In this case the client might want to GET the endpoint, modify the data and then POST it directly back. However, if we unmarshal this directly into an Endpoint struct, the client could accidentally (or purposefully) overwrite our fixed field. ´To fix this, lets add a semantic to the struct tag to express that this field should be skipped by the unmarshaller:

type Endpoint struct {
	VariableField string `json:"variable,skip"`
	FixedField    string `json:"fixed"`
}

I used the term "skip" here as an example, perhaps we can use some clearer terminology in the actual implementation.

In this trivial example it is simple to add some logic that ignores this field in the request handler. However, in larger structs it becomes clearer and less error-prone to have a declarative way to express that a value should not be modified by the unmarshalling process.