phantom types for fake mut polymorphism

gistfile1.rs

use std::cast;
use std::unstable::raw;

struct Mutable;
struct Immutable;

struct Foobar<'t, Mutability> {
    /* priv */ ptr: raw::Slice<u8>
}

impl<'t, M> Foobar<'t, M> {
    /* priv */ fn get_immut(&self) -> &'t [u8] {
        unsafe { cast::transmute(self.ptr) }
    }

    pub fn get_first(&self) -> u8 {
        self.get_immut()[0]
    }
}

impl<'t> Foobar<'t, Immutable> {
    pub fn new_immut(vec: &'t [u8]) -> Foobar<'t, Immutable> {
        Foobar { ptr: unsafe { cast::transmute(vec) } }
    }
}

impl<'t> Foobar<'t, Mutable> {
    /* priv */ fn get_mut(&self) -> &'t mut [u8] {
        unsafe { cast::transmute(self.ptr) }
    }

    pub fn new_mut(vec: &'t mut [u8]) -> Foobar<'t, Mutable> {
        Foobar { ptr: unsafe { cast::transmute(vec) } }
    }

    pub fn set_first(&self, x: u8) {
        self.get_mut()[0] = x;
    }
}

<sfackler> kmc: that implementation is unsound; the lifetime parameter isn't used so it'll be ignored
<sfackler> kmc: you just need to add a marker type like http://static.rust-lang.org/doc/master/std/kinds/marker/struct.ContravariantLifetime.html