Builders that take &mut self should panic when used a second time

[dtolnay]

let builder = Builder::new().a().b();
let _ = builder.build();
builder.c(); // panic
let _ = builder.build(); // panic

If you want to support this behavior, provide Clone for the builder.

let builder = Builder::new().a().b();
let _ = builder.clone().build();
let _ = builder.c().build();

In general, the user should not be allowed to rely on a builder being in any particular state after calling the terminal method (build or similar). If the ergonomics of doing so weren't awful, the terminal method would take self by value instead of &mut self.

[Kixunil]

Why not just consume builder instead? Compile-time checking FTW.

[dtolnay]

Because then you can't write Builder::new().a().b().build().

[Kixunil]

Ah, I see. In that case I'd say consume everything.