#[deriving(Anything)] fails silently

[kornelski]

I've got bitten by:

#[deriving(Copy)]
struct Foo;

The compiler silently accepts this code, no warning at all, but nothing is derived.

(rustc 1.0.0-nightly (44a287e 2015-01-08 17:03:40 -0800))

[japaric]

I get two warnings with a more recent nightly (rustc 1.0.0-nightly (3d0d9bb6f 2015-01-12 22:56:20 +0000)):

issue-21166.rs:4:1: 4:16 warning: type could implement `Copy`; consider adding `impl Copy`, #[warn(missing_copy_implementations)] on by default
issue-21166.rs:4 pub struct Foo;
                 ^~~~~~~~~~~~~~~
issue-21166.rs:3:1: 3:18 warning: unused attribute, #[warn(unused_attributes)] on by default
issue-21166.rs:3 #[deriving(Copy)]
                 ^~~~~~~~~~~~~~~~~

[bombless]

I tried playpen just now:

#[deriving(Copy)]
struct Foo;
fn main() {
    let _ = Foo;
}

And the output gives:

<anon>:1:1: 1:18 warning: unused attribute, #[warn(unused_attributes)] on by default
<anon>:1 #[deriving(Copy)]
         ^~~~~~~~~~~~~~~~~

I think it is a clear warning which is good enough.

[huonw]

If one uses the type in a place that needs the derived trait, one will get type errors before any of those warnings appear. E.g.

#[deriving(Copy)]
struct Foo;
fn main() {
    let x = Foo;
    // use it twice:
    (x, x);
}

<anon>:6:9: 6:10 error: use of moved value: `x`
<anon>:6     (x, x);
                 ^
<anon>:6:6: 6:7 note: `x` moved here because it has type `Foo`, which is non-copyable
<anon>:6     (x, x);
              ^
error: aborting due to previous error

[kornelski]

I've updated to rustc 1.0.0-nightly (170c439 2015-01-14 00:41:55 +0000). I'm getting same error as @huonw.

Even though it shows an error, I think this is a problem, because it shows a wrong, misleading error — the problem isn't in the line where value is copied. It's in the line which has been ignored.

Because of that I've been pulling my hair out and tripple-checking all borrows in my program, documentation etc. and wondering why correctly-looking code does't work.