cmd/cover: Avoid importing atomic package if not used

[JayNakrani]

What did you do?

$ cat ~/tmp/test/haha.go
package haha

type A struct{
	i int
}

$ go tool cover -mode=atomic ~/tmp/test/haha.go
package haha

import _cover_atomic_ "sync/atomic"

type A struct {
	i int
}

var _ = _cover_atomic_.AddUint32

var GoCover = struct {
	Count     [0]uint32
	Pos       [3 * 0]uint32
	NumStmt   [0]uint16
} {
	Pos: [3 * 0]uint32{
	},
	NumStmt: [0]uint16{
	},
}

What did you expect to see?

If not needed, sync/atomic shouldn't be imported. Expected to see instrumented code like below

$ go tool cover -mode=atomic ~/tmp/test/haha.go
package haha

type A struct {
	i int
}

var GoCover = struct {
	Count     [0]uint32
	Pos       [3 * 0]uint32
	NumStmt   [0]uint16
} {
	Pos: [3 * 0]uint32{
	},
	NumStmt: [0]uint16{
	},
}

What did you see instead?

sync/atomic was imported, even though it's not used (well, except for blank assignment, but that's to prevent unused compiler errors).

[bradfitz]

Why does it matter to you?

[JayNakrani]

This is not critical, but it's better if we can avoid importing unused dependencies. Looks like it's easier to do: golang.org/cl/32763

[gopherbot]

CL https://golang.org/cl/32763 mentions this issue.

[JayNakrani]

As discussed in the CL, this adds more complexity for less value. Closing the issue.