cmd/go: test -coverprofile should ask compiler to drop (incorrect) position information

% cat x.go
package p

func f(int) string

func g() {
	b := f(1)
	collect := func(min, max, stop int) []int {
		return nil
	}
	b := f(2)
	_ = collect
}
% go test -coverprofile=c.out x.go
# command-line-arguments
./x.go:6:2: `b' declared and not used
./x.go:10:35: no new variables on left side of :=

There is no column 35 on the line b := f(2), making the error message very confusing. The problem is that cover has inserted text into the lines. When the compiler did not print column information, that was invisible. Now it's not.

To make it invisible again, the compiler should add a flag to drop position information, and then the go command should use that flag when compiling with test coverage.