LLVM assertion

collections.rs

#![feature(lang_items)]

#![crate_type = "rlib"]
#![no_std]

// This core crate has an `Option` that uses unboxed closures (see #19467)
extern crate core;

use core::prelude::{Iterator, IteratorExt, Option, None};
use core::slice::SlicePrelude;

struct ElementSwaps {
    sdir: &'static [uint],
}

impl Iterator<()> for ElementSwaps {
    // This attribute triggers the assertion
    #[inline]
    fn next(&mut self) -> Option<()> {
        self.sdir.
            iter().
            max_by(|&sd| sd);

        None
    }
}

how-to-reproduce.md

1. Build a rustc that includes rust-lang/rust#19449

2. Checkout rust-lang/rust#19467

3. Bootstrap (2) using (1) as stage-0 compiler, pass RUSTLFLAGS=-Ccodegen-units=2, you should hit an LLVM assertion (see below) while building stage0/libcollections.

4. Alternatively compile the collections.rs (see above) file using the rustc from (1) and passing the -Ccodegen-units=2 flag.

output

$ LD_LIBRARY_PATH=/home/japaric/tmp/rust/build/x86_64-unknown-linux-gnu/stage0/lib \
  /home/japaric/tmp/rust/build/x86_64-unknown-linux-gnu/stage0/bin/rustc \
  -Ccodegen_units=2 \
  collections.rs
collections.rs:9:1: 11:2 warning: struct is never used: `ElementSwaps`, #[warn(dead_code)] on by default
collections.rs:9 struct ElementSwaps {
collections.rs:10     sdir: &'static [uint],
collections.rs:11 }
rustc: /var/tmp/paludis/build/dev-lang-rust-scm/work/rust-scm/src/llvm/lib/IR/Constants.cpp:1609: static llvm::Constant* llvm::ConstantExpr::getPointerCast(llvm::Constant*, llvm::Type*): Assertion `S->getType()->isPtrOrPtrVectorTy() && "Invalid cast"' failed.
./reduce.sh: line 6:  9793 Aborted                 (core dumped) LD_LIBRARY_PATH=/home/japaric/tmp/rust/build/x86_64-unknown-linux-gnu/stage0/lib /home/japaric/tmp/rust/build/x86_64-unknown-linux-gnu/stage0/bin/rustc -Ccodegen_units=2 collections.rs 2>&1

I was able to reduce to this

#![feature(lang_items, unboxed_closures)]

#![crate_type = "rlib"]
#![no_std]

// This core crate has an `Option` that uses unboxed closures (see #19467)
extern crate core;

use core::prelude::{Iterator, IteratorExt, Option, None};
use core::slice::SlicePrelude;

static x: &'static [uint] = &[];

#[inline]
fn next() -> Option<()> {
    x.iter().max_by(|_| 0i);
    None
}

further reduction:

#![feature(lang_items, unboxed_closures)]

#![crate_type = "rlib"]
#![no_std]
#![allow(dead_code)]

// This core crate has an `Option` that uses unboxed closures (see #19467)
extern crate core;

use core::prelude::{IteratorExt};
use core::slice::SlicePrelude;

static X: &'static [uint] = &[];

#[inline]
fn next() {
    X.iter().max_by(|_| 0i);
}