ICE

[truchi]

Code

pub struct Color<T>(pub T);

pub struct Cell<Fg, Bg = Fg> {
    foreground: Color<Fg>,
    background: Color<Bg>,
}

pub trait Over<Bottom, Output> {
    fn over(self, bottom: Bottom) -> Output;
}

// Cell: Over<Color, Cell>
impl<C, Fg, Bg, NewFg, NewBg> Over<Color<C>, Cell<NewFg, NewBg>> for Cell<Fg, Bg>
where
    Fg: Over<C, NewFg>,
    Bg: Over<C, NewBg>,
{
    fn over(self, _: Color<C>) -> Cell<NewFg, NewBg> {
        todo!();
    }
}

// Cell: Over<Cell, Cell>
impl<TopFg, TopBg, BottomFg, BottomBg, NewFg, NewBg>
    Over<Cell<BottomFg, BottomBg>, Cell<NewFg, NewBg>> for Cell<TopFg, TopBg>
where
    Self: Over<Color<BottomBg>, Cell<NewFg, NewBg>>,
{
    fn over(self, bottom: Cell<BottomFg, BottomBg>) -> Cell<NewFg, NewBg> {
        self.over(bottom.background);
        todo!();
    }
}

// Cell: Over<&mut Cell, ()>
impl<'b, TopFg, TopBg, BottomFg, BottomBg> Over<&'b mut Cell<BottomFg, BottomBg>, ()>
    for Cell<TopFg, TopBg>
where
    Cell<TopFg, TopBg>: Over<Cell<BottomFg, BottomBg>, Cell<BottomFg, BottomBg>>,
{
    fn over(self, _: &'b mut Cell<BottomFg, BottomBg>) {
        todo!();
    }
}

// &Cell: Over<&mut Cell, ()>
impl<'t, 'b, TopFg, TopBg, BottomFg, BottomBg> Over<&'b mut Cell<BottomFg, BottomBg>, ()>
    for &'t Cell<TopFg, TopBg>
where
    Cell<TopFg, TopBg>: Over<Cell<BottomFg, BottomBg>, Cell<BottomFg, BottomBg>>,
{
    fn over(self, _: &'b mut Cell<BottomFg, BottomBg>) {
        todo!();
    }
}

Meta

rustc --version --verbose:

rustc 1.53.0-nightly (42816d61e 2021-04-24)
binary: rustc
commit-hash: 42816d61ead7e46d462df997958ccfd514f8c21c
commit-date: 2021-04-24
host: x86_64-unknown-linux-gnu
release: 1.53.0-nightly
LLVM version: 12.0.0

Same in 1.52.0-nightly, same in 1.51.0.

Error output

error: internal compiler error: compiler/rustc_trait_selection/src/traits/select/confirmation.rs:201:17: Where clause `Binder(<style::color::Color<TopBg> as style::Over<style::cell::Cell<BottomFg, BottomBg>, style::cell::Cell<NewFg, NewBg>>>, [])` was applicable to `Obligation(predicate=Binder(TraitPredicate(<style::color::Color<TopBg> as style::Over<style::cell::Cell<_, _>, style::cell::Cell<_, _>>>), []), depth=2)` but now is not

thread 'rustc' panicked at 'Box<Any>', /rustc/42816d61ead7e46d462df997958ccfd514f8c21c/library/std/src/panic.rs:59:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.53.0-nightly (42816d61e 2021-04-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `style::cell::Cell<TopFg, TopBg>: style::Over<^1_0, ^1_1>`
#1 [typeck] type-checking `style::cell::<impl at src/style/cell.rs:74:1: 91:2>::over`
end of query stack
error: aborting due to previous error

error: could not compile `tender`

To learn more, run the command again with --verbose.

RUST_BACKTRACE=1 cargo build:

Backtrace

error: internal compiler error: compiler/rustc_trait_selection/src/traits/select/confirmation.rs:201:17: Where clause `Binder(<style::color::Color<TopBg> as style::Over<style::cell::Cell<BottomFg, BottomBg>, style::cell::Cell<NewFg, NewBg>>>, [])` was applicable to `Obligation(predicate=Binder(TraitPredicate(<style::color::Color<TopBg> as style::Over<style::cell::Cell<_, _>, style::cell::Cell<_, _>>>), []), depth=2)` but now is not

thread 'rustc' panicked at 'Box<Any>', /rustc/42816d61ead7e46d462df997958ccfd514f8c21c/library/std/src/panic.rs:59:5
stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::bug_fmt
   7: rustc_trait_selection::traits::select::confirmation::<impl rustc_trait_selection::traits::select::SelectionContext>::confirm_candidate
   8: rustc_infer::infer::InferCtxt::probe
   9: rustc_trait_selection::traits::select::SelectionContext::evaluate_candidate
  10: rustc_trait_selection::traits::select::SelectionContext::evaluate_stack
  11: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  12: rustc_query_system::dep_graph::graph::DepGraph<K>::with_anon_task
  13: rustc_trait_selection::traits::select::SelectionContext::evaluate_trait_predicate_recursively
  14: rustc_trait_selection::traits::select::SelectionContext::evaluate_predicate_recursively::{{closure}}
  15: rustc_trait_selection::traits::select::SelectionContext::evaluate_predicate_recursively
  16: rustc_trait_selection::traits::select::SelectionContext::evaluate_predicates_recursively
  17: rustc_infer::infer::InferCtxt::probe
  18: rustc_trait_selection::traits::select::SelectionContext::evaluate_candidate
  19: rustc_trait_selection::traits::select::SelectionContext::evaluate_stack
  20: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  21: rustc_query_system::dep_graph::graph::DepGraph<K>::with_anon_task
  22: rustc_trait_selection::traits::select::SelectionContext::evaluate_trait_predicate_recursively
  23: rustc_trait_selection::traits::select::SelectionContext::evaluate_predicate_recursively::{{closure}}
  24: rustc_trait_selection::traits::select::SelectionContext::evaluate_predicate_recursively
  25: rustc_trait_selection::traits::select::SelectionContext::evaluate_predicates_recursively
  26: rustc_infer::infer::InferCtxt::probe
  27: rustc_trait_selection::traits::select::SelectionContext::evaluate_candidate
  28: <core::iter::adapters::ResultShunt<I,E> as core::iter::traits::iterator::Iterator>::next
  29: <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter
  30: rustc_trait_selection::traits::select::candidate_assembly::<impl rustc_trait_selection::traits::select::SelectionContext>::candidate_from_obligation_no_cache
  31: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  32: rustc_query_system::dep_graph::graph::DepGraph<K>::with_anon_task
  33: rustc_trait_selection::traits::select::candidate_assembly::<impl rustc_trait_selection::traits::select::SelectionContext>::candidate_from_obligation
  34: rustc_trait_selection::traits::select::SelectionContext::evaluate_stack
  35: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  36: rustc_query_system::dep_graph::graph::DepGraph<K>::with_anon_task
  37: rustc_trait_selection::traits::select::SelectionContext::evaluate_trait_predicate_recursively
  38: rustc_trait_selection::traits::select::SelectionContext::evaluate_predicate_recursively::{{closure}}
  39: rustc_trait_selection::traits::select::SelectionContext::evaluate_predicate_recursively
  40: rustc_infer::infer::InferCtxt::probe
  41: rustc_trait_selection::traits::select::SelectionContext::evaluate_root_obligation
  42: rustc_infer::infer::InferCtxtBuilder::enter_with_canonical
  43: rustc_traits::evaluate_obligation::evaluate_obligation
  44: rustc_query_impl::<impl rustc_query_system::query::config::QueryAccessors<rustc_query_impl::plumbing::QueryCtxt> for rustc_query_impl::queries::evaluate_obligation>::compute
  45: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  46: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  47: rustc_data_structures::stack::ensure_sufficient_stack
  48: rustc_query_system::query::plumbing::force_query_with_job
  49: rustc_query_system::query::plumbing::get_query_impl
  50: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::evaluate_obligation
  51: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt>::evaluate_obligation
  52: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt>::evaluate_obligation_no_overflow
  53: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt>::predicate_may_hold
  54: rustc_infer::infer::InferCtxt::probe
  55: <alloc::vec::Vec<T> as alloc::vec::spec_from_iter::SpecFromIter<T,I>>::from_iter
  56: rustc_typeck::check::method::probe::ProbeContext::pick_method
  57: rustc_typeck::check::method::probe::ProbeContext::pick_core
  58: rustc_typeck::check::method::probe::ProbeContext::pick
  59: rustc_infer::infer::InferCtxt::probe
  60: rustc_typeck::check::method::probe::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::probe_for_name
  61: rustc_typeck::check::method::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::lookup_probe
  62: rustc_typeck::check::method::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::lookup_method
  63: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  64: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  65: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_block_with_expected
  66: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  67: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  68: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  69: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  70: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  71: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_block_with_expected
  72: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  73: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_return_expr
  74: rustc_typeck::check::check::check_fn
  75: rustc_infer::infer::InferCtxtBuilder::enter
  76: rustc_typeck::check::typeck
  77: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  78: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  79: rustc_data_structures::stack::ensure_sufficient_stack
  80: rustc_query_system::query::plumbing::force_query_with_job
  81: rustc_query_system::query::plumbing::get_query_impl
  82: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::typeck
  83: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::par_body_owners
  84: rustc_typeck::check::typeck_item_bodies
  85: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  86: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  87: rustc_data_structures::stack::ensure_sufficient_stack
  88: rustc_query_system::query::plumbing::force_query_with_job
  89: rustc_query_system::query::plumbing::get_query_impl
  90: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::typeck_item_bodies
  91: rustc_session::utils::<impl rustc_session::session::Session>::time
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.53.0-nightly (42816d61e 2021-04-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `style::cell::Cell<TopFg, TopBg>: style::Over<^1_0, ^1_1>`
#1 [typeck] type-checking `style::cell::<impl at src/style/cell.rs:74:1: 91:2>::over`
#2 [typeck_item_bodies] type-checking all item bodies
#3 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

error: could not compile `tender`

To learn more, run the command again with --verbose.

Good luck!
<3

[truchi]

This happens on top of too much code for me to take the time to minimalize this issue, sorry.
Though I'll try to file the fixing patch.

[truchi]

Wait! I made it! @JohnTitor

playground

pub struct Color<T>(pub T);

pub struct Cell<Fg, Bg = Fg> {
    foreground: Color<Fg>,
    background: Color<Bg>,
}

pub trait Over<Bottom, Output> {
    fn over(self, bottom: Bottom) -> Output;
}

// Cell: Over<Color, Cell>
impl<C, Fg, Bg, NewFg, NewBg> Over<Color<C>, Cell<NewFg, NewBg>> for Cell<Fg, Bg>
where
    Fg: Over<C, NewFg>,
    Bg: Over<C, NewBg>,
{
    fn over(self, _: Color<C>) -> Cell<NewFg, NewBg> {
        todo!();
    }
}

// Cell: Over<Cell, Cell>
impl<TopFg, TopBg, BottomFg, BottomBg, NewFg, NewBg>
    Over<Cell<BottomFg, BottomBg>, Cell<NewFg, NewBg>> for Cell<TopFg, TopBg>
where
    Self: Over<Color<BottomBg>, Cell<NewFg, NewBg>>,
{
    fn over(self, bottom: Cell<BottomFg, BottomBg>) -> Cell<NewFg, NewBg> {
        self.over(bottom.background);
        todo!();
    }
}

// Cell: Over<&mut Cell, ()>
impl<'b, TopFg, TopBg, BottomFg, BottomBg> Over<&'b mut Cell<BottomFg, BottomBg>, ()>
    for Cell<TopFg, TopBg>
where
    Cell<TopFg, TopBg>: Over<Cell<BottomFg, BottomBg>, Cell<BottomFg, BottomBg>>,
{
    fn over(self, _: &'b mut Cell<BottomFg, BottomBg>) {
        todo!();
    }
}

// &Cell: Over<&mut Cell, ()>
impl<'t, 'b, TopFg, TopBg, BottomFg, BottomBg> Over<&'b mut Cell<BottomFg, BottomBg>, ()>
    for &'t Cell<TopFg, TopBg>
where
    Cell<TopFg, TopBg>: Over<Cell<BottomFg, BottomBg>, Cell<BottomFg, BottomBg>>,
{
    fn over(self, _: &'b mut Cell<BottomFg, BottomBg>) {
        todo!();
    }
}

This issue revolves around L.31. Comment to see the ice go away.
Interestingly, comment L.40 (or the whole impl L.37-45) to see ice go away as well.
Commenting L.47-56 does not make ice go away.
Commenting L.13-22 should raise another error (as L.31 is supposed to go there), but doesn't.

Actually, the real fix is to replace L.31 with:

<_ as Over<_, Cell<NewFg, NewBg>>>::over(self, bottom.background)

playground

Have fun!
💖

[langston-barrett]

Minimized with Icemelter, then a bit by hand:

struct Cell<Fg, Bg = Fg> {
    foreground: Color<Fg>,
    background: Color<Bg>,
}

trait Over<Bottom, Output> {
    fn over(self) -> Output;
}

impl<TopFg, TopBg, BottomFg, BottomBg, NewFg, NewBg>
    Over<Cell<BottomFg, BottomBg>, Cell<NewFg, NewBg>> for Cell<TopFg, TopBg>
where
    Self: Over<Color<BottomBg>, Cell<NewFg>>,
{
    fn over(self) -> Cell<NewFg> {
        self.over();
    }
}

impl<'b, TopFg, TopBg, BottomFg, BottomBg> Over<&Cell<BottomFg, BottomBg>, ()>
    for Cell<TopFg, TopBg>
where
    Cell<TopFg, TopBg>: Over<Cell<BottomFg>, Cell<BottomFg>>,
{
    fn over(self) -> Cell<NewBg> {
        self.over();
    }
}

Details

Icemelter version: v0.2.0

@rustbot label +S-bug-has-mcve