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x/tools/cmd/eg: take better care with pointers, maintain addressability of expressions #10326

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josharian opened this issue Apr 3, 2015 · 0 comments
Open

x/tools/cmd/eg: take better care with pointers, maintain addressability of expressions #10326

josharian opened this issue Apr 3, 2015 · 0 comments
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Unreleased

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@josharian
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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.
@rsc rsc added this to the Unplanned milestone Apr 10, 2015
@rsc rsc changed the title cmd/eg: take better care with pointers, maintain addressability of expressions x/tools/cmd/eg: take better care with pointers, maintain addressability of expressions Apr 14, 2015
@rsc rsc modified the milestones: Unreleased, Unplanned Apr 14, 2015
@rsc rsc removed the repo-tools label Apr 14, 2015
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@josharian @rsc @alandonovan