"error: target type not found for const cast" triggered by the position of a mod item or by adding #[deriving(Show)]

[japaric]

STR

// main.rs
extern crate libc;

#[cfg(error)]
mod trigger;

const CSTOPB_: u32 = 64;
const CREAD_: u32 = 128;

const CSTOPB: u32 = CSTOPB_;
// This cast to `c_uint` is necessary
const CREAD: u32 = CREAD_ as ::libc::c_uint;

#[cfg(not(error))]
mod trigger;

fn main() {}

// trigger.rs
#[repr(u32)]
enum Bit {
    CSTOPB = super::CSTOPB,
    CREAD = super::CREAD,
}

Output

rustc --cfg error main.rs generates the following compiler error:

main.rs:12:30: 12:44 error: target type not found for const cast
main.rs:12 const CREAD: u32 = CREAD_ as ::libc::c_uint;

rustc main.rs compiles successfully.

Version

rustc 0.13.0-dev (93e589c87 2014-10-17 09:22:14 +0000)

I can trace this behavior back to 0.11

---

I thought that the position of the items in a crate (except for use imports and extern crates which must go at the beginning) didn't affect the compiler output. That makes me think this is a bug.

[japaric]

Found another way to generate the same error, this time only by adding a #[deriving(Show)]

STR

extern crate libc;

pub mod raw {
    pub use self::ffi::tcflag_t;

    #[allow(dead_code, non_camel_case_types)]
    mod ffi {
        pub type tcflag_t = ::libc::c_uint;

        pub const CSTOPB_: ::libc::c_uint = 64;
        pub const CREAD_: ::libc::c_uint = 128;
    }

    pub const CSTOPB: tcflag_t = self::ffi::CSTOPB_ as tcflag_t;
    pub const CREAD: tcflag_t = self::ffi::CREAD_ as tcflag_t;
}

pub mod control {
    use raw;

    #[cfg(error)]
    #[repr(u32)]
    #[deriving(Show)]
    pub enum Bit {
        CSTOPB = raw::CSTOPB,
        CREAD = raw::CREAD,
    }

    #[cfg(not(error))]
    #[repr(u32)]
    pub enum Bit {
        CSTOPB = raw::CSTOPB,
        CREAD = raw::CREAD,
    }
}

fn main() {}

Output

main.rs:14:56: 14:64 error: target type not found for const cast
main.rs:14     pub const CSTOPB: tcflag_t = self::ffi::CSTOPB_ as tcflag_t;

Version

rustc 0.13.0-dev (93e589c87 2014-10-17 09:22:14 +0000)

0.12 generates the same error, but 0.11 compiles successfully

---

It seems there's some problem around the intersection of const and casting.

cc @alexcrichton

[steveklabnik]

Both of these examples compile for me today.