serde_derive fail involving conditional compilation

[alexbool]

Cargo.toml:

[package]
name = "serde-fail"
version = "0.1.0"
authors = ["Alexander Bulaev <alexbool@yandex-team.ru>"]

[dependencies]
serde = { version = "*", optional = true }
serde_derive = { version = "*", optional = true }

[features]
serialize = ["serde", "serde_derive"]

src/lib.rs:

#![cfg_attr(feature = "serialize", feature(rustc_macro))]

#[cfg(feature = "serialize")]
extern crate serde;
#[cfg_attr(feature = "serialize", macro_use)]
#[cfg(feature = "serialize")]
extern crate serde_derive;

#[derive(Copy, Clone)]
#[cfg_attr(feature = "serialize", derive(Serialize, Deserialize))]
pub struct MyStruct {
    value: i64,
}

alexbool@alexbool-osx ~/D/I/serde-fail> cargo build --features "serialize"
   Compiling serde-fail v0.1.0 (file:///Users/alexbool/Documents/IdeaProjects/serde-fail)
error: internal implementation detail (see issue #29642)
  --> src/lib.rs:11:1
   |
11 | pub struct MyStruct {
   | ^
   |
   = help: add #![feature(rustc_attrs)] to the crate attributes to enable

error: aborting due to previous error

error: Could not compile `serde-fail`.

To learn more, run the command again with --verbose.

Meta:

alexbool@alexbool-osx ~/D/I/serde-fail> rustc -vV
rustc 1.14.0-nightly (3210fd5c2 2016-10-05)
binary: rustc
commit-hash: 3210fd5c20ffc6da420eb00e60bdc8704577fd3b
commit-date: 2016-10-05
host: x86_64-apple-darwin
release: 1.14.0-nightly

[dtolnay]

Thanks for the detailed report! This is a problem with the current Macros 1.1 implementation in Rust. I commented in the tracking issue so we will see whether it is something they can fix.

For now the workaround is to always put derives that include custom derives above any other attributes:

#[cfg_attr(feature = "serialize", derive(Serialize, Deserialize))]
#[derive(Copy, Clone)]
pub struct MyStruct {
    value: i64,
}

[alexbool]

Thanks! The workaround works :)

[dtolnay]

This will be fixed by rust-lang/rust#37067.

[dtolnay]

This has been fixed as of rustc 1.14.0-nightly (e0111758e 2016-10-17).