JSON should sanity-check tags

[robpike]

The JSON package uses structure tags to allow the user to define names for the JSON
fields that differ from those in the Go struct.  The protocol buffer uses tags for a
different purpose.  If one wants to use JSON to marshal a value achieved from a protocol
buffer, things go south; one gets badly formed names.

The protocol buffer code introduced the format "PB(...)" for its tags.  We
could extend that convention with JSON, saying (perhaps) that "JSON(XXX)" is
the way to define a renaming.

In any case, JSON should certainly not use a tag that isn't an identifier.  This would
be a simple step that avoids the most obvious collision.

[gopherbot]

Comment 1 by cw@f00f.org:

Small note:
Right now I have JSON performance issues that could be further exacerbated by making the
tags even more complex.

[robpike]

Comment 2:

They don't need to be more complex.  All I'm asking, at least for now, is a sanity
check, which should be very cheap.
JSON's implementation could be smarter, too. It's heavily reflection-driven but there
are ways to do better if it becomes important.

[rsc]

Comment 3:

This issue was closed by revision f80d002.

Status changed to Fixed.