struct Point { x: isize, y: isize } trait Methods { fn impurem(&self); fn blockm(&self, f: F) where F: FnOnce(); } impl Methods for Point { fn impurem(&self) { } fn blockm(&self, f: F) where F: FnOnce() { f() } } fn a() { let mut p = Point {x: 3, y: 4}; // Here: it's ok to call even though receiver is mutable, because we // can loan it out. p.impurem(); // But in this case we do not honor the loan: p.blockm(|| { //~ ERROR cannot borrow `p` as mutable p.x = 10; }) } fn b() { let mut p = Point {x: 3, y: 4}; // Here I create an outstanding loan and check that we get conflicts: let l = &mut p; p.impurem(); //~ ERROR cannot borrow l.x += 1; } fn main() { }