failure with non-UTF8 formatting leads to hard-to-debug error #16877
Description
struct Foo;
impl std::fmt::Show for Foo {
fn fmt(&self, fmtr:&mut std::fmt::Formatter) -> std::fmt::Result {
fmtr.write(&[0xFF])
}
}
fn main() {
fail!("{}", Foo)
}task '<main>' failed at 'called `Result::unwrap()` on an `Err` value: [255]', /build/rust-git/src/rust/src/libcore/result.rs:548
playpen: application terminated with error code 101
At the very least it could mention the failure was caused by bad formatting while trying to unwind, or could use from_utf8_lossy.