ICE: `Result::unwrap()` on an `Err` value: Utf8Error

[CrystalGamma]

Compiler crashes while compiling https://github.com/CrystalGamma/rusttype/tree/bugrep0 using Cargo.

Cargo version: cargo 0.6.0-nightly (18e496a 2015-10-17)
rustc version: 1.3.0

 thread 'rustc' panicked at 'called `Result::unwrap()` on an `Err` value: Utf8Error { valid_up_to: 502 }', src/libcore/result.rs:732

stack backtrace:
   1:     0x7fc6f8f4766e - sys::backtrace::write::ha67a6a2049d22fadIvs
   2:     0x7fc6f8f4fd55 - panicking::on_panic::hfa4bf0215983f759lzx
   3:     0x7fc6f8f1094e - rt::unwind::begin_unwind_inner::ha1b5da93bdf7e8ffC0w
   4:     0x7fc6f8f116b6 - rt::unwind::begin_unwind_fmt::hac9e09b20e42a51cIZw
   5:     0x7fc6f8f4f8f6 - rust_begin_unwind
   6:     0x7fc6f8f96b64 - panicking::panic_fmt::hd75b6cee85f3afe4SgC
   7:     0x7fc6f7cf58cd - back::link::link_natively::hed2bd12732711d7db0b
   8:     0x7fc6f7cecded - back::link::link_binary::h7266bf96d64e0248uvb
   9:     0x7fc6f94ba8cd - driver::phase_6_link_output::h0d7d5071f34ef57fiRa
  10:     0x7fc6f948ce44 - driver::compile_input::h52c7dafd49963360Tba
  11:     0x7fc6f9572a63 - run_compiler::h473c62e00f865fa9A7b
  12:     0x7fc6f95704de - boxed::F.FnBox<A>::call_box::h10713159552398332324
  13:     0x7fc6f956fe09 - rt::unwind::try::try_fn::h16887904248528792425
  14:     0x7fc6f8f4f84d - __rust_try
  15:     0x7fc6f8f3a8f7 - rt::unwind::try::inner_try::hafffff77ddacfa5fvWw
  16:     0x7fc6f9570028 - boxed::F.FnBox<A>::call_box::h5417278073590676257
  17:     0x7fc6f8f4e991 - sys::thread::Thread::new::thread_start::h29dca2a2cf2294b535v
  18:     0x7fc6f37c64a3 - start_thread
  19:     0x7fc6f8ba813c - clone
  20:                0x0 - <unknown>

[wthrowe]

It looks like this comes from the linker producing output (normal or error) that is not valid UTF-8. Fixing this in any other way than just using a lossy conversion is going to be really painful because it goes through the error logging infrastructure, which all works with strs. That's certainly not ideal, because it would be best to relay other programs' messages to the user unchanged, but it's probably the only realistic way to handle this in the near future.

[arielb1]

@wthrowe

Just dump the bytes as hexadecimal? I am not sure what is going on here.

[wthrowe]

My first thought was that the path was probably not UTF-8, but because of the way cargo invokes it that crashes rustc long before it tries to link anything, so that can't be it. I'm not sure why else the message would be non-UTF-8. (I can't reproduce the ICE on that crate (I just get a normal link error), but I can reproduce the ICE by constructing a situation where the linker prints non-UTF-8 messages.)

But yeah, some kind of lossy conversion with hex escapes is probably the way to go. I'm not aware of an existing function to do that.

[wthrowe]

Actually, mapping with ascii::escape_default is probably good enough.

[wthrowe]

note: zzz\xff/libfoo.so: undefined reference to foo'\ncollect2: error: ld returned 1 exit status\n`

Not great, but not an ICE and probably good enough until we have a dedicated function to do this sort of thing since this should be a very rare error.