move implementation to internal/ directory (#828)

internal/util/nodefault/nodefault.go

@@ -0,0 +1,31 @@
+// 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