[Measter]

> There are also related restrictions about function call barriers. AFAIK the Rust compiler cannot "peek into" called function bodies to figure out what's actually going on inside those functions (and that information would be very valuable for fine-grained borrow checking), it can only work with the information in the function signature.

I don't think it's so much "can not" as it is "will not". Allowing the function signature to be determined by the body can lead to accidentally breaking callers by changing the body.

That, at least, is consistent with other parts of the signature: the input/output types and how the lifetimes of input/output references are related.