cmd/compile: improve error message in initialization loop

[willfaught]

Please answer these questions before submitting your issue. Thanks!

• What version of Go are you using (go version)?

The current play.golang.org version.

• What operating system and processor architecture are you using (go env)?

The current play.golang.org version.

• What did you do?

See https://play.golang.org/p/ISPl1bA6OP:

package main

import (
    "fmt"
)

var x int = y

var y int = x

func main() {
    fmt.Println(x, y)
}

• What did you expect to see?

This:

prog.go:7: initialization loop:
    prog.go:7 x refers to y
    prog.go:9 y refers to x

• What did you see instead?

This:

prog.go:7: initialization loop:
    prog.go:7 x refers to
    prog.go:9 y refers to
    prog.go:7 x

[odeke-em]

Could we update the title of this bug from

cmd/compile:

to something maybe like

cmd/compile: improve error message in initialization loop

because fixers can't easily search for it.

[davecheney]

Done. @adg @broady how about giving @odeke-em the commit bit so he can do this himself?

[bradfitz]

@davecheney, @odeke-em, done.

[odeke-em]

Wow, thanks @davecheney and @bradfitz!!

[griesemer]

FWIW: Not sure making this more verbose helps much. If you read the messages together it's pretty clear, w/o repetition. But I defer to the sentiment of the majority here.

[rsc]

The current formatting is not a mistake.

[willfaught]

The issue isn't the formatting, it's the wording: it clearly doesn't make sense: "prog.go:7: x refers to prog.go:9: y refers to prog.go:7: x". If a cycle has two edges, it doesn't take three lines to describe it.

[griesemer]

@willfaught It does with the current formatting which is less verbose. There's no need to change it.