[nickitolas]

It's not just about that. What rust sees is you are calling two functions: the len() function with a shared reference, and the indexing operator implementation for the Vec's [] (Which is just a function) with a mutable reference, which returns a mutable reference. The compiler does not look at the actual implementation of these 2 functions to figure things out, it only looks at the signatures (If it did look at the implementations, it would probably be slower, and what's worse, the caller's code could stop compiling if you change the implementation! That would be specially bad across a library boundary). The implementations are irrelevant AIUI to the borrow checker, only the signatures matter.