struct tags can't be built from string constants

[bradfitz]

This is a little unfortunate:

    http://play.golang.org/p/AaMZm0pI6g

I'd like to be able to do:

  type schemaBlob struct {
    ClaimDate string `json:"` + fieldClaimDate + `"`
  }

If we ever permit tags on things other than struct fields, I'd like to keep this in mind.

[rsc]

Comment 1:

I believe we discussed this when we added tags and explicitly decided to keep them
simple literals, the same way that import paths are.

Status changed to WorkingAsIntended.