Go: cmd/go: include files and lines in import-cycle errors

[willfaught]

The error can be more helpful if it pinpoints the files/lines that cause the cycle.

Example:

$ go build
can't load package: import cycle not allowed
package foo
    imports foo
import cycle not allowed
package foo
    imports foo

That could be anywhere in foo.

[ianlancetaylor]

I think it's a fine idea, but the cycle can't be anywhere in foo: it has to be an import statement, and import statements have to be near the start of a file.

[willfaught]

Yes, but it could be in any file.

[willfaught]

Also, I don't understand why it prints the message and cycle twice.

[VojtechVitek]

@ianlancetaylor

It be nice to have file:line in the error message.

Instead of

import cycle not allowed
package github.com/pressly/api/cmd/api
        imports github.com/pressly/api
        imports github.com/pressly/api/data
        imports github.com/pressly/api/data/presenter
        imports github.com/pressly/api/data

I'd propose

import cycle not allowed
package github.com/pressly/api/cmd/api
        imports github.com/pressly/api
                at cmd/main.go:12
        imports github.com/pressly/api/data
                at api.go:8
        imports github.com/pressly/api/data/presenter
                at data/user.go:14
        imports github.com/pressly/api/data
                at data/presenter/user.go:6

I can implement this if you guys like the idea.

[ianlancetaylor]

Sure, though I would keep it on line ("imports github.com/pressly/api at cmd/main.go:12"). It's too late for Go 1.8 but please send a CL for Go 1.9.