Nearly every type in Rust has a Default implementation. When you define your own struct that only contains elements that already have a Default, you have the option to derive from Default as well [42]. In the case of enums or complex structs, you can easily write your own implementation of Default instead [55], as there's only one method you have to provide. After this, the struct returned by Default::default() is implicitly inferrable as yours, if you tell the compiler what your type actually is. This is why in line [3] we have to write foo: i32, or else Rust wouldn't know what type the default object actually should become.
If you only want to specify some elements and leave the others at the default, you can use the syntax in line [29]. Keep in mind that you can configure and skip as many values as you want, as shown in lines [33 to 37].