$ cat yes.rs 
fn main() { loop { println!("y"); } }
$ rustc yes.rs && ./yes | head -n1
y
thread 'main' panicked at 'failed printing to stdout: Broken pipe (os error 32)', src/libstd/io/stdio.rs:692:8
note: Run with `RUST_BACKTRACE=1` for a backtrace.
$ yes | head -n1
y

This was originally filed here but @sfackler determined the cause:

This is due to println! panicking on errors:

rust/src/libstd/io/stdio.rs

Line 671 in f1ea23e

fn print_to<T>(args: fmt::Arguments,

.

C-based programs typically just get killed off with a SIGPIPE, but Rust ignores that signal.

Note that to see the backtrace, the data being piped has to be large enough to overflow the kernel pipe buffer.