Somewhat. It is due to auto-deref: deriving(Clone) essentially expands to
fn clone(&self) -> List<'a> {
match *self {
Nil => Nil,
Next(ref x) => Next(x.clone())
}
}
`x` is of type `&&List<'a>`, but the `x.clone()` call auto-derefs
through both layers of & to be calling List's clone directly (returning
a `List<'a>`), rather than duplicating the reference.
This will be fixed with UFCS, which will allow deriving to expand to
something like `Next(Clone::clone(x))` and this does not undergo auto-deref.
You can work around this by writing a Clone implementation by hand. In
this case, List is Copy, so the implementation can be written as
impl<'a> Clone for List<'a> {
fn clone(&self) -> List<'a> {
*self
}
}
(Clone for more "interesting" List types (which aren't Copy, in general)
will likely need to be implemented with a match and some internal Clones.)
Huon
On 03/06/14 19:59, Igor Bukanov wrote:
Consider the following enum:
#[deriving(Clone)]
enum List<'a> {
Nil,
Next(&'a List<'a>)
}
It generates en error:
<anon>:4:10: 4:22 error: mismatched types: expected `&List<>` but
found `List<>` (expected &-ptr but found enum List)
<anon>:4 Next(&'a List<'a>)
^~~~~~~~~~~~
note: in expansion of #[deriving]
<anon>:1:1: 2:5 note: expansion site
error: aborting due to previous error
Is it a bug in #[deriving] ?
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