Function entry 

pub fn entry(
    out: &mut Func,
    block: Block,
    params: &[Type],
    conv: &CallRegs,
    names: &mut Interner,
) -> Result<Vec<Reg>, (usize, Missing)>

Binds a function’s parameters to the registers the convention says they arrive in.

The registers come back in the order the parameters were given, so the caller can bind each IR parameter to the one at its position.

Errors

The first parameter this cannot bring in, and why. A function with one is reported rather than compiled, because the alternative is a function that reads an argument from wherever the last one happened to leave a register.