x/tools/cmd/eg: take better care with pointers, maintain addressability of expressions

[josharian]

Given this package:

package egbug

var Imap = map[*T]int{}

type T struct {
    I int
}

func main() {
    var t T
    p := &t.I
    _ = p
}

and this template:

package p

import "egbug"

func before(t *egbug.T) int { return t.I }
func after(t *egbug.T) int  { return egbug.Imap[t] }

applying eg generates uncompilable code:

package egbug

var Imap = map[*T]int{}

type T struct {
    I int
}

func main() {
    var t T
    p := &Imap[t]
    _ = p
}

$ go build
# egbug
./x.go:11: cannot use t (type T) as type *T in map index
./x.go:11: cannot take the address of Imap[t]

There are two separate bugs here--T vs *T and breaking addressability.

This is a real life example, from attempting to use eg to instrument the gc compiler to find correlations in Node field usage.