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Fix tail calls to `#[track_caller]` functions We want `#[track_caller]` to be semver independent, i.e. it should not be a breaking change to add or remove it. Since it changes ABI of a function (adding an additional argument) we have to be careful to preserve this property when adding tail calls. The only way to achieve this that I can see is: - we forbid tail calls in functions which are marked with `#[track_caller]` (already implemented) - tail-calling a `#[track_caller]` marked function downgrades the tail-call to a normal call (or equivalently tail-calls the shim made by fn def to fn ptr cast) (this pr) Ideally the downgrade would be performed by a MIR pass, but that requires post mono MIR opts (cc ```@saethlin,``` rust-lang/rust#131650). For now I've changed code in cg_ssa to accomodate this behaviour (+ added a hack to mono collector so that the shim is actually generated) Additionally I added a lint, although I don't think it's strictly necessary. Alternative to rust-lang/rust#144762 (and thus closes rust-lang/rust#144762) Fixes https://github.com/rust-lang/rust/issues/144755