proposal: cmd/cover: ignore empty functions in coverage report

[abemedia]

Proposal Details

A common way to implement union types in Go is with an interface e.g.

type Record struct {
	Title string
	Value valueType
}

type valueType interface {
	isValue()
}

type ValueFoo struct {
	Foo string
}

func (ValueFoo) isValue() {}

type ValueBar struct {
	Bar string
}

func (ValueBar) isValue() {}

This method is amongst others employed by protobuf. The problem is that these empty functions negatively affect coverage, so in projects where coverage is enforced it can lead to having to write nonsensical tests like this:

func TestValueType(*testing.T) {
	tests := []valueType{
		ValueFoo{},
		ValueBar{},
	}

	for _, tt := range tests {
		tt.isValue()
	}
}

To avoid this it would be great if empty functions were excluded from the coverage report.

[ianlancetaylor]

I don't see how it would be correct in general to ignore empty functions. Some people will want coverage for them, some will not.

In general we don't aim for 100% coverage as a goal.

[seankhliao]

#46825 looks like the opposite case where empty functions should be marked as uncovered

[earthboundkid]

This is another special case of #53271.