The more I thought about the interior references part the more questions I had! For example:

• The actual characters in a String belong to a dynamic memory allocation rather than to the String object itself, so the lifetime of &str references into there is "until any operation that might change the size of the allocation". Since that level of detail doesn't seem visible to to the type system even with the discussed addition, I guess it would reduce just to disallowing any mutable reference to the string so that its content cannot possibly move to a new allocation while the internal references are live. 🤔
• I also thought about the idea that a reference whose lifetime is related to another field in the same object could be represented as an offset from the object's address rather than an absolute pointer and then generate relative accesses when dereferencing, but that would mean that the referents would need to always live inside the object itself, and not in a dynamic allocation as would be the case for &str into a String.

So, with all of that said, I'd love to read an article with more details on that part!