Enforce tail call type is related to body return type in borrowck

[compiler-errors]

Like all call terminators, tail call terminators instantiate the binder of the callee signature with region variables and equate the arg operand types with that signature's args to ensure that the call is valid.

However, unlike normal call terminators, we were forgetting to also relate the return type of the call terminator to anything. In the case of tail call terminators, the correct thing is to relate it to the return type of the caller function (or in other words, the return local _0).

This meant that if the caller's return type had some lifetime constraint, then that constraint wouldn't flow through the signature and affect the args.

This is what's happening in the example test I committed:

fn link(x: &str) -> &'static str {
    become passthrough(x);
}

fn passthrough<T>(t: T) -> T { t }

fn main() {
    let x = String::from("hello, world");
    let s = link(&x);
    drop(x);
    println!("{s}");
}

Specifically, the type x is '?0 str, where '?0 is some universal arg. The type of passthrough is fn(&'?1 str) -> &'?1 str. Equating the args sets '?0 = '?1. However, we need to also equate the return type &'?1 str to &'static str so that we eventually require that '?0 = 'static, which is a borrowck error!

---

Look at the first commit for the functional change, and the second commit is just a refactor because we don't need to pass Option<BasicBlock> to check_call_dest, but just whether or not the terminator is expected to be diverging (i.e. if the return type is !).

Fixes #144916

[rustbot]

r? @lcnr

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[compiler-errors]

r? @WaffleLapkin though lcnr can review it too if they want, this change should be pretty straightforward.

[rustbot]

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[compiler-errors]

I don't really care about the ordering here but the true branch seemed easier to put first

[lcnr]

@bors r+ rollup

[bors]

📌 Commit a573fd9 has been approved by lcnr

It is now in the queue for this repository.