[Discussion] Hybrid borrow

[cg909]

This sounds a lot like "downgradable" or "downgrading" borrows that were discussed before:

Pre-RFC: Downgradable mutable borrows

Some months ago I've read the following comment by steffahn and I think a generalized &mut T could be viable. So here's my very first Pre-RFC based on that idea: Summary Generalize &mut with a second, optional lifetime to allow opt-in downgradeable borrows. Motivation Currently coercing or reborrowing a &mut as an immutable reference always extends the unique borrow to the lifetime of the resulting reference. When interior mutability is involved, this behavior is neccessary for correctness …

Allow reading of a mutable reference after it's reborrowed as immutable

struct Foo{ name: String, score: u32 } impl Foo{ fn new(id: String, score: u32) -> Foo{ Foo { name: id, score: score } } fn add_score(&mut self) -> &u32 { self.score += 1; &self.score } fn get_name(&self) -> &String { &self.name } } fn main() { let mut foo = Foo::new("foo".into(), 0); let score = foo.add_score(); let name = foo.get_name(); println!("{}: {}", name, score); } error[E0502]: cannot borrow `foo` as …

The main problem is that it must stay impossible to get a shared borrow from an unique/mutable borrow when interior mutability is involved. So your BorrowHybrid::immut() may not be called when Borrowed has interior mutability.