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tracking issue: #112788

After discussion in #144855, I was wrong and it is actually possible to support tail calls with PassMode::Indirect { on_stack: false, .. } arguments.

Normally an indirect argument with on_stack: false would be passed as a pointer into the caller's stack frame. For tail calls, that would be unsound, because the caller's stack frame is overwritten by the callee's stack frame.

Therefore we store the argument for the callee in the corresponding caller's slot. Because guaranteed tail calls demand that the caller's signature matches the callee's, the corresponding slot has the correct type.

To handle cases like the one below, the tail call arguments must first be copied to a temporary, and can only then be copied to the caller's argument slots.

// A struct big enough that it is not passed via registers.
pub struct Big([u64; 4]);

fn swapper(a: Big, b: Big) -> (Big, Big) {
    become swapper_helper(b, a);
}

I'm not really familiar with MIR and what tricks/helpers we have, so I'm just cobbling this together. Hopefully we can arrive at something more elegant.