[feature] may not be used on the stable release channe

[ccinoo]

#cargo build --release
Compiling proc-macro2 v0.1.4
$&lt;2&gt;error[E0554]$&lt;2>$<2>: #![feature] may not be used on the stable release channel$<2>
$&lt;2&gt;$&lt;2>--> $&lt;2&gt;/root/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-0.1.4/src/lib.rs:22:35$&lt;2>
$&lt;2&gt;$&lt;2>|$<2>
$&lt;2&gt;22$&lt;2> $&lt;2&gt;$&lt;2>| $&lt;2&gt;#![cfg_attr(feature = "unstable", feature(proc_macro))]$&lt;2>
$&lt;2&gt;$&lt;2>| $&lt;2&gt; $&lt;2>$<2>^^^^^^^^^^^^^^^^^^^^$<2>

$&lt;2&gt;error$&lt;2>$<2>: aborting due to previous error$<2>

error: Could not compile proc-macro2.

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

[ccinoo]

Compiling proc-macro2 v0.1.3
Running rustc --crate-name proc_macro2 /root/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-0.1.3/src/lib.rs --crate-type lib --emit=dep-info,link -C debuginfo=2 --cfg 'feature="unstable"' -C metadata=a35ea6711ad5263a -C extra-filename=-a35ea6711ad5263a --out-dir /root/rssbot-1.4.1/target/debug/deps -L dependency=/root/rssbot-1.4.1/target/debug/deps --extern unicode_xid=/root/rssbot-1.4.1/target/debug/deps/libunicode_xid-5ab8279c3be8661d.rlib --cap-lints allow
error[E0277]: the trait bound proc_macro::Span: std::default::Default is not satisfied
--> /root/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-0.1.3/src/unstable.rs:163:17
|
163 | pub struct Span(proc_macro::Span);
| ^^^^^^^^^^^^^^^^^ the trait std::default::Default is not implemented for proc_macro::Span
|
= note: required by std::default::Default::default

error: aborting due to previous error

For more information about this error, try rustc --explain E0277.
error: Could not compile proc-macro2.

[iovxw]

cargo +nightly build --release

[ccinoo]

Thank you very much, the problem has been solved.