#[deriving] and unsafe pointers are easy to get wrong

[huonw]

In the following, the deriving does the wrong thing: it shallow copies the pointer when the semantics mean it should be making a new allocation and copying the data; there's no indication of this until runtime double-frees/segfaults.

/// Wrapper around an owned allocation
#[deriving(Clone)]
struct Struct {
     owned: *mut int
}

impl Struct {
     fn new(x: int) { Struct { owned: unsafe {cast::transmute(~x)} } }
}
impl Drop for Struct {
    fn drop(&mut self) {
         let _: ~int = unsafe {cast::transmute(self.owned)};
    }
}

Some possible resolutions:

• do nothing: unsafe code is unsafe
• have lints for deriving + unsafe pointers
• have deriving completely disabled on unsafe pointers
• remove the trait impls for raw pointers

[nikomatsakis]

I favor a lint.

[huonw]

(I'm happy to mentor this.)

[pongad]

I'm interested in doing this. I'll find @huonw on IRC.