Minimal example of problem with borrowck, mutability, and trait objects.

minimal_borrowck.rs

// traitfail.rs

trait Slide {
	fn new(x: int) -> Self;
	fn x_mut<'r> (&'r mut self) -> &'r mut int;
	fn x(&self) -> int;
}

struct Bead {
	x: int
}

impl Slide for Bead {
	pub fn new(x: int) -> Bead {
		Bead { x: x}
	}
	
	pub fn x(&self) -> int {
		self.x
	}
	
	pub fn x_mut<'r>(&'r mut self) -> &'r mut int {
		&'r mut self.x
	}
}

fn main() {
	let slide = &mut Slide::new::<Bead>(5) as &mut Slide;
	
	let x = slide.x();
	
	let x_mut = slide.x_mut();
	*x_mut += 2;
}

// Compiler output

traitfail.rs:30:13: 30:18 error: cannot borrow immutable local variable as mutable
traitfail.rs:30 	let x_mut = slide.x_mut();
                	            ^~~~~
error: aborting due to previous error