cat_expr Errd on "fold"

[adinapoli]

Hello folks,

I'm dutifully reporting this as the compiler told me to so. The bug is the same reported already by #24504 and #22897, although it appeared on a different circumstance. I have this simple program (I'm a Rust newbie so It's very likely I'm doing things terribly wrong):

use std::io::*;

const initial_height: u32 = 1;

fn summer_growth(h: u32) -> u32 {
    h + 1
}

fn spring_growth(h: u32) -> u32 {
    h * 2
}

fn utopian_tree(cycles: u32) -> u32 {
    (0..cycles).fold(|acc, x| grow(acc,x), initial_height);
}

fn grow(acc: u32, cycle: u32) -> u32 {
    match cycle % 2 == 0 {
        true => spring_growth(acc),
        false => summer_growth(acc)
    }
}

fn main () {
    let mut buf = String::new();
    stdin().read_line(&mut buf).unwrap();
    let test_cases: u32 = buf.trim().parse().unwrap();
    for _ in 0..test_cases {
        stdin().read_line(&mut buf).unwrap();
        let cycle: u32 = buf.trim().parse().unwrap();
        println!("{}", utopian_tree(cycle));
    }
}

Expected outcome:

• The compile to tell me how wrong I am

Actual outcome:

☁  algorithms  rustc utopian_tree.rs -o utopian_tree
utopian_tree.rs:14:5: 14:59 error: internal compiler error: cat_expr Errd
utopian_tree.rs:14     (0..cycles).fold(|acc, x| grow(acc,x), initial_height);
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/m
aster/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'Box<Any>', /Users/rustbuild/src/rust-buildbot/slave/
beta-dist-rustc-mac/build/src/libsyntax/diagnostic.rs:149

Meta

☁  algorithms  rustc --version --verbose
rustc 1.0.0-beta.2 (e9080ec39 2015-04-16) (built 2015-04-16)
binary: rustc
commit-hash: e9080ec39d9a44eb0773d648b348575b19f655e5
commit-date: 2015-04-16
build-date: 2015-04-16
host: x86_64-apple-darwin
release: 1.0.0-beta.2
☁  algorithms

Shows up with latest beta as well:

☁  algorithms  rustc utopian_tree.rs -o utopian_tree
utopian_tree.rs:14:5: 14:59 error: internal compiler error: cat_expr Errd
utopian_tree.rs:14     (0..cycles).fold(|acc, x| grow(acc,x), initial_height);
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'Box<Any>', /Users/rustbuild/src/rust-buildbot/slave/beta-dist-rustc-mac/build/src/libsyntax/diagnostic.rs:149


☁  algorithms  rustc --verbose --version
rustc 1.0.0-beta.4 (850151a75 2015-04-30) (built 2015-04-30)
binary: rustc
commit-hash: 850151a75709f146addd30bbbf1f23d384f0b381
commit-date: 2015-04-30
build-date: 2015-04-30
host: x86_64-apple-darwin
release: 1.0.0-beta.4
☁  algorithms

[pnkfelix]

cc me

[nagisa]

stack backtrace:
   1:     0x7f290f12bee9 - sys::backtrace::write::h9d474f8e9af32ff4eas
   2:     0x7f290f133a29 - panicking::on_panic::h780bc016e25d560eHOw
   3:     0x7f290f0f4a52 - rt::unwind::begin_unwind_inner::he10b5af763481bfaRtw
   4:     0x7f290c4e999d - rt::unwind::begin_unwind::h10007532221998759728
   5:     0x7f290c4e9932 - diagnostic::SpanHandler::span_bug::hdbc9b25081b2fe3dKWB
   6:     0x7f290d10f83d - session::Session::abort_if_errors::ha2c34f711c028133dJq
   7:     0x7f290e9d6f54 - check_crate::closure.38688
   8:     0x7f290e9d0180 - check_crate::h08a7388cd8cd25829BC
   9:     0x7f290f678bb8 - driver::phase_3_run_analysis_passes::h876ca40ef22f652btGa
  10:     0x7f290f659fec - driver::compile_input::h19dbb81f9d079a10Qba
  11:     0x7f290f712dc1 - run_compiler::h6fdcc5ae4819da2c65b
  12:     0x7f290f710612 - boxed::F.FnBox<A>::call_box::h15867338134671801927
  13:     0x7f290f70fbd9 - rt::unwind::try::try_fn::h10571205104630549427
  14:     0x7f290f1a72f8 - rust_try_inner
  15:     0x7f290f1a72e5 - rust_try
  16:     0x7f290f70fe74 - boxed::F.FnBox<A>::call_box::h11752498478748955554
  17:     0x7f290f1327c1 - sys::thread::Thread::new::thread_start::h9f92b31902592327dAv
  18:     0x7f29091e5373 - start_thread
  19:     0x7f290ed8827c - clone
  20:                0x0 - <unknown>

[arielb1]

Minified:

fn utopian_tree(cycles: u32) {
    <_>::fold((cycles..), ||{}, ());
}

We do write ty_err to node 11 (<_>::fold((cycles..), || { }, ()))). Apparently we somehow don't report an error.

[pnkfelix]

triage: P-medium

[arielb1]

Whoops. There is indeed an error, which is indeed gets to report_selection_error, but when we do resolve_type_vars_if_possible (error_reporting.rs:184), we get an error, and think this one was a duplicate.

[arielb1]

no_std version:

#![feature(no_std,lang_items)]
#![no_std]
#![crate_type="rlib"]

#[lang="sized"] pub trait Sized {}

struct S;

trait InOut<T> { type Out; }

impl MIterator<u32> for S {}

trait MIterator<T> : Sized {
    fn _fold<B, F: InOut<B, Out=B>>(self, init: B, f: F) -> B { loop {} }
}

fn bot<T>() -> T { loop {} }

fn utopian_tree() {
    <S as MIterator<u32>>::_fold(S, bot(), ());
}

fn main() {}

[arielb1]

You don't even need the trait:

struct S;

trait InOut<T> { type Out; }

fn do_fold<B, F: InOut<B, Out=B>>(init: B, f: F)  { }

fn bot<T>() -> T { loop {} }

fn main() {
    do_fold(bot(), ());
}