encoding/xml: field tag cleanup

[rsc]

The field tags in package xml were one of the earliest uses of tags
and need to be cleaned up, like json was cleaned up.

Specifically, the field lookup should look for an identical match,
not a case-insensitive or 'strip the invalid characters' match.
Any custom matching should be done with field tags, not heuristics.
The field tags should use attributes like json does instead of 
overloading certain names.  For example it is impossible today
to unmarshal an XML tag <attr> because the "attr" tag means
'make this field an attribute'.  It is also impossible to choose a
name for that attribute.

Specific changes needed:
   * Fields match only if the field name matches the element name
     exactly (for field with no tag name) or matches the tag name
     exactly.
   * Field with "-" tag is ignored completely.
   * Attributes are marked by ",attr" or "x,attr" tag, not just "attr".

[rogpeppe]

Comment 1:

Tags aren't symmetrical either. The following code panics:
package main
import (
    "fmt"
    "encoding/xml"
    "bytes"
)
type Foo struct {
    XMLName   xml.Name `xml:"foo"`
    X int  `xml:"y"`
}
func main() {
    var b bytes.Buffer
    xml.Marshal(&b, &Foo{
        X: 56,
    })
    var x Foo
    err := xml.Unmarshal(&b, &x)
    if err != nil {
        fmt.Printf("error: %v\n", err)
        return
    }
    if x.X != 56 {
        panic("unexpected unmarshalled value")
    }
}

[nsf]

Comment 2:

I think I'll just put it here. A small feature request for future "xml" package versions.
I had a need to filter xml elements by their namespace, consider this example:
<package>
  <include name="GObject"/>
  <include name="Gtk"/>
  <c:include name="glib-object.h"/>
  <c:include name="gtk/gtk.h"/>
</package>
So, I would like to unmarshal that into a struct with two arrays for each namespace:
type Package struct {
  Includes []Include
  CIncludes []Include
}
And currently such thing is impossible as far as I can see. 
Also attributes may have namespaces as well. Parser understands that, but there is no
way to use attribute namespaces with struct tags. E.g.:
<method name="get_comments" c:identifier="gtk_about_dialog_get_comments"
version="2.6">
  ...
</method>
Maybe though attributes cannot have matching local names in xml, I don't know.
P.S. It's a shame that there is so much real world data in this perverted format.

[rsc]

Comment 3:

Labels changed: added priority-later, removed priority-medium.

[rsc]

Comment 4:

Labels changed: added priority-go1.

[niemeyer]

Comment 5:

Owner changed to @niemeyer.

Status changed to Started.

[niemeyer]

Comment 6:

This issue was closed by revision 1627b46.

Status changed to Fixed.