Should not derive Debug, Copy, Clone for struct without definition

Input C/C++ Header

typedef struct foo bar;

Bindgen Invocation

$ bindgen input.h 

Actual Results

/* automatically generated by rust-bindgen 0.56.0 */

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct foo {
    _unused: [u8; 0],
}
pub type bar = foo;

Expected Results

Debug, Copy, Clone should not be derived if bindgen does not know the true size or layout of struct foo and is just using a placeholder. Obviously I can use no_copy, no_debug, no_defaultto suppress it, but bindgen should never derive these traits on such a struct no matter how these flags are set.

Reproduce the reported input.h and bindgen invocation first, then inspect the code path that emits derives for placeholder structs. Done means generated bindings for typedef struct foo bar; omit Debug, Copy, and Clone while preserving the placeholder and alias behavior.