ICE: Defn conflict

[GuillaumeGomez]

Rust version:

> ./build/x86_64-apple-darwin/stage1/bin/rustc --version
rustc 1.17.0-dev (5ac7a035a 2017-02-26)

The code:

macro_rules! make {
    ($n:expr) => {
        pub trait T<X> {
            fn ignore(_: &X) {}
            const C: X;
            const D: i32 = ($n * $n);
        }

        impl U<Vec<i32>> for S {
            const C: Vec<i32> = vec![0; $n];
        }
    }
}

make!(4);

Which gives:

error: internal compiler error: unexpected panic

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 'adding a def'n for node-id NodeId(3) and data Initializer but a previous def'n exists: DefKey { parent: Some(DefIndex(0)), disambiguated_data: DisambiguatedDefPathData { data: TypeNs("std"), disambiguator: 0 } }', src/librustc/hir/map/definitions.rs:335
stack backtrace:
   1:        0x111baabdc - std::sys::imp::backtrace::tracing::imp::write::h2e3ef1dc0e51ba99
   2:        0x111bb763e - std::panicking::default_hook::{{closure}}::haa60669a0a8b43e3
   3:        0x111bb71e3 - std::panicking::default_hook::h0a42e1acc48a315c
   4:        0x111bb7af7 - std::panicking::rust_panic_with_hook::h78a15ea95e294ff1
   5:        0x111bb79a4 - std::panicking::begin_panic::hba8a9f7990fef69b
   6:        0x111bb78c2 - std::panicking::begin_panic_fmt::hc74ec0af4c54fbab
   7:        0x10ebf43bc - rustc::hir::map::definitions::Definitions::create_def_with_parent::hbb102500d73fd9c1
   8:        0x10ebf11f2 - <rustc::hir::map::def_collector::DefCollector<'a> as syntax::visit::Visitor<'a>>::visit_impl_item::h81eb5dc485f72f1c
   9:        0x10eb6bb11 - syntax::visit::walk_item::h1a65815c3ec0cd25
  10:        0x10ebf0c53 - <rustc::hir::map::def_collector::DefCollector<'a> as syntax::visit::Visitor<'a>>::visit_item::{{closure}}::ha54e917d31c5446f
  11:        0x10ebf059c - <rustc::hir::map::def_collector::DefCollector<'a> as syntax::visit::Visitor<'a>>::visit_item::hdd7947b3b5779933
  12:        0x10ea711de - rustc_resolve::macros::<impl syntax::ext::base::Resolver for rustc_resolve::Resolver<'a>>::visit_expansion::h2d197847c9c689f6
  13:        0x111690d6f - syntax::ext::expand::MacroExpander::collect_invocations::hccfea5820a6cd13e
  14:        0x11168dd2b - syntax::ext::expand::MacroExpander::expand::h03bd73f7ebe53885
  15:        0x11168c61f - syntax::ext::expand::MacroExpander::expand_crate::hc1e13e0d2bbc2981
  16:        0x10d7e2881 - rustc_driver::driver::phase_2_configure_and_expand::{{closure}}::h5f58f7c0b9aa3518
  17:        0x10d7d13d0 - rustc_driver::driver::phase_2_configure_and_expand::h403f66a4c4ce608a
  18:        0x10d8a0a34 - rustdoc::core::run_core::h20b285afcad21f3a
  19:        0x10d7f0d02 - std::panicking::try::do_call::hf08ba2b971ea2e3a
  20:        0x111bba70a - __rust_maybe_catch_panic
  21:        0x10d81815a - <F as alloc::boxed::FnBox<A>>::call_box::h4d43130e36f88422
  22:        0x111bb6764 - std::sys::imp::thread::Thread::new::thread_start::h3187989cf33213df
  23:     0x7fffb9824aba - _pthread_body
  24:     0x7fffb9824a06 - _pthread_start

[TimNN]

A bit minified:

macro_rules! make {
    ($n:expr) => {
        impl Foo for Bar {
            const C: Vec<i32> = vec![0; $n];
        }
    }
}

make!(4);

Also, technically this is a regression from stable to stable (error -> ice).

[nikomatsakis]

cc @eddyb -- looks related to in-demand maybe?

[eddyb]

I'd think @jseyfried might know something in the macro refactors that can cause this.

[jseyfried]

Further minified:

#![feature(associated_consts)]

macro_rules! m { () => { 0 } }

struct S;
impl S {
    const C: i32 = m!(); // Replacing `m!()` with `0` compiles
}

I suspect this was caused by my macro modularization groundwork (probably one of the "miscellaneous groundwork" PRs from #35896 (comment)) -- I'll investigate ASAP.

[jseyfried]

This was introduced in #36601, fixed in #40272.