Code:
#![crate_type="lib"]
pub fn foo() -> ~str {
    format!("Hello {:s}",
            "world");
} 
Compilation attempt:
% rustc --version
/Users/fklock/opt/rust-dbg/bin/rustc 0.11-pre (0deb16a 2014-04-07 02:26:37 -0700)
host: x86_64-apple-darwin
% rustc /tmp/f.rs
/tmp/f.rs:3:25: 3:25 note: consider removing this semicolon:
/tmp/f.rs:3     format!("Hello {:s}",
                                    ^
note: in expansion of format_args!
...
 
It should be pointing at the semi-colon on the next line, not the comma on this line.