https://go-review.googlesource.com/38409 broke field tracking somehow. Field tracking has no test and the only documentation I can find is internal to Google, so here's repro instructions.

First, build a toolchain with GOEXPERIMENT=fieldtrack. This is important and easy to miss. Then compile the following:

package main

import “fmt”

type T struct {
	X int `go:”track”`
	Y int `go:”track”`
}

func (t *T) String() string {
	return fmt.Sprintf(“T(X=%d)”, t.X)
}

func (t *T) Sum() int {
	return t.X + t.Y
}

func Run() {
	t := new(T)
	fmt.Println(t)
}

func main() {
	Run()
}

The resulting object should have undefined symbols:

         U go.track.%22%22.T.X
         U go.track.%22%22.T.Y

which would then be used by the linker to build the used field list. After https://go-review.googlesource.com/38409, they aren't there.

If it's useful, the full command I used for bisecting was

(cd src; GOEXPERIMENT=fieldtrack ./make.bash); bin/go tool compile -o fieldtrack.o fieldtrack.go; bin/go tool nm fieldtrack.o | grep go.track || exit 1

CC @josharian @mdempsky