build error in carnix 0.4.6 with rustc 1.21.0

error.txt

$ nix-env -iA nixos.carnix
... build crates ...
Building src/main.rs (carnix)
Running rustc --crate-name carnix src/main.rs --crate-type bin -C opt-level=3  --out-dir target/bin --emit=dep-info,link -L dependency=target/deps -L native=/nix/store/gplwjf38jzylm1kfxgyd9z5a1lkvhlzv-rust_backtrace-sys-0.1.16/backtrace-sys.out/.libs -L native=/nix/store/z650nric4bzz9h0i1dj54dgx2gbzyyp4-sqlite-3.21.0/lib -l sqlite3 -l static=backtrace   --extern clap=/nix/store/da54ljzhdj1w9lnjany9nhpmfaf13i9j-rust_clap-2.27.1/libclap-0d0f70b4fe.rlib  --extern env_logger=/nix/store/4fjlf2fs9fjpj88b8yzcmv1yxanzrwgr-rust_env_logger-0.4.3/libenv_logger-44fd8c2574.rlib  --extern error_chain=/nix/store/b2s6m89igvcp7igp95pv5pqzlwnrkrnk-rust_error-chain-0.11.0/liberror_chain-e3c9b7a037.rlib  --extern log=/nix/store/zi0py5c94qzzdqsvd9nsffx3clr7pxq2-rust_log-0.3.8/liblog-4ed683d9af.rlib  --extern nom=/nix/store/c2bymx136ba85j1k0k9pzimdfi5y8ymp-rust_nom-3.2.1/libnom-ab93402e0b.rlib  --extern regex=/nix/store/wxnlnhqimhwa3dz2npp5v0bb8g0gwi41-rust_regex-0.2.2/libregex-179326db63.rlib  --extern rusqlite=/nix/store/1xi62b9zapmjzc3pg3m7509kx9qfa1aa-rust_rusqlite-0.12.0/librusqlite-d9b4a7a10c.rlib  --extern serde=/nix/store/i0m6pzjds35nc0szsxlylpq72g7ywzyf-rust_serde-1.0.19/libserde-64d6ac33f7.rlib  --extern serde_derive=/nix/store/dsnz2hcbfc9c8ba5bglm2icy8v7x17vx-rust_serde_derive-1.0.19/libserde_derive-67578e02e0.so  --extern serde_json=/nix/store/gqzsbrkhpyyiiz9zj0wlkwz00rfivgmq-rust_serde_json-1.0.5/libserde_json-16c81cb81c.rlib  --extern tempdir=/nix/store/nkj73jzs0a23f9z36zq60jqyw6w7pwh2-rust_tempdir-0.3.5/libtempdir-9342f90e62.rlib  --extern toml=/nix/store/9ikzxlw0dhkl3ah5db566pbm29vxi062-rust_toml-0.4.5/libtoml-2735bf4a6a.rlib --cap-lints allow  -L native=/nix/store/gplwjf38jzylm1kfxgyd9z5a1lkvhlzv-rust_backtrace-sys-0.1.16/backtrace-sys.out/.libs -L native=/nix/store/z650nric4bzz9h0i1dj54dgx2gbzyyp4-sqlite-3.21.0/lib -l sqlite3 -l static=backtrace
error: expected ident, found |
   --> src/cfg.rs:103:46
    |
103 |         v: delimited!(tag!("\""), take_till!(|c| c == b'"'), tag!("\"")) >>
    |                                              ^
builder for ‘/nix/store/gz1qjl1xw13mqdk7kcbc3x7853nnnpkd-rust_carnix-0.4.6.drv’ failed with exit code 101
$ rustc --version
rustc 1.21.0 (3b72af97e 2017-10-09)