zfs.NilBool: document its purpose and move it to its own package 'nodefault'

util/nodefault/nodefault.go

@@ -0,0 +1,32 @@
+// Package nodefault provides newtypes around builtin Go types
+// so that if the newtype is used as a field in a struct and that field
+// is zero-initialized (https://golang.org/ref/spec#The_zero_value),
+// accessing the field will cause a null pointer deref panic.
+// Or in other terms: It soft-enforces that the caller sets the field
+// explicitly when constructing the struct.
+//
+// Example:
+//
+//  type Config struct {
+//      // This field must be set to a non-nil value,
+//      // forcing the caller to make their mind up
+//      // about this field.
+//      CriticalSetting   *nodefault.Bool
+//  }
+//
+// An function that takes such a Config should _not_ check for nil-ness:
+// and instead unconditionally dereference:
+//
+//  func f(c Config) {
+//      if (c.CriticalSetting) { }
+//  }
+//
+// If the caller of f forgot to specify the .CriticalSetting
+// field, the Go runtime will issue a nil-pointer deref panic
+// and it'll be clear that the caller did not read the docs of Config.
+//
+//  f(Config{}) // crashes
+//
+//  f Config{ CriticalSetting: &nodefault.Bool{B: false}} // doesn't crash
+//
+package nodefault

util/nodefault/nodefault_bool.go

@@ -0,0 +1,19 @@
+package nodefault
+
+import "fmt"
+
+type Bool struct{ B bool }
+
+func (n *Bool) ValidateNoDefault() error {
+	if n == nil {
+		return fmt.Errorf("must explicitly set `true` or `false`")
+	}
+	return nil
+}
+
+func (n *Bool) String() string {
+	if n == nil {
+		return "unset"
+	}
+	return fmt.Sprintf("%v", n.B)
+}