slog::warn!

macro_rules! warn {
    ($logger:ident, $msg:expr) => { ... };
    ($logger:ident, $msg : expr, $($k:expr => $v:expr),*) => { ... };
}

Log a warning level logging record

#[macro_use]
extern crate slog;
fn main() {
    let root = root_logger!("key" => "value");
    warn!(root, "something happened", "counter" => 1);
}