go/src/cmd/go/internal/imports/tags.go

// Tags returns a set of build tags that are true for the target platform.
// It includes GOOS, GOARCH, the compiler, possibly "cgo",
// release tags like "go1.13", and user-specified build tags.
func Tags() map[string]bool { … }

var loadTagsOnce …

func loadTags() map[string]bool { … }

// AnyTags returns a special set of build tags that satisfy nearly all
// build tag expressions. Only "ignore" and malformed build tag requirements
// are considered false.
func AnyTags() map[string]bool { … }

var anyTagsOnce …