Are there examples where borrow checker stands on your way? Like the mentioned (in comments) way of building a tree. I'm sure there should be really weird edge cases.
Another good example is that the borrow checker is currently based on lexical scope. See all the examples in https://github.com/rust-lang/rust/issues/6393 .
In general, any kind of analysis like the borrow checker will reject some valid programs, as it pays to be extra conservative. You then slowly expand the set of accepted programs, until hopefully, it matches the true set of valid programs, without accepting invalid ones.
What kinds of pet patterns have you seen?
This kind of thing happens any time you transition languages: when I started hacking on Rust, I was trying to figure out how to do the metaprogramming shenanigans that Ruby lets me do. Ironically, Rust's metaprogramming features are my biggest weakness as a Rust programmer, I don't really write my own macros.
I'm wondering, is this possible in principle?
Context: I'm thinking in terms of sound-and-decidable type-checking conservativeness [0], but perhaps that's a bad way of thinking about this? Perhaps borrow-checking is a special-enough case of type-checking that somehow isn't afflicted by this (how?) or gives up the soundness (does it?)?
[0] "A sound type system with decidable type-checking (and possibly decidable type inference) must be conservative." -- http://gallium.inria.fr/~remy/mpri/cours1.pdf
See also: https://books.google.com/books?id=7Uh8XGfJbEIC&pg=PA134