thread 'rustc' panicked at 'assertion failed: !self.substitutions.is_empty()', compiler/rustc_errors/src/lib.rs:189:9

[szunami]

thread 'rustc' panicked at 'assertion failed: !self.substitutions.is_empty()', compiler/rustc_errors/src/lib.rs:189:9
stack backtrace:
   0: rust_begin_unwind
             at /rustc/e9920ef7749d11fc71cc32ca4ba055bcfeaab945/library/std/src/panicking.rs:493:5
   1: core::panicking::panic_fmt
             at /rustc/e9920ef7749d11fc71cc32ca4ba055bcfeaab945/library/core/src/panicking.rs:92:14
   2: core::panicking::panic
             at /rustc/e9920ef7749d11fc71cc32ca4ba055bcfeaab945/library/core/src/panicking.rs:50:5
   3: rustc_errors::CodeSuggestion::splice_lines
   4: <rustc_errors::emitter::EmitterWriter as rustc_errors::emitter::Emitter>::emit_diagnostic
   5: rustc_errors::json::Diagnostic::from_errors_diagnostic
   6: <rustc_errors::json::JsonEmitter as rustc_errors::emitter::Emitter>::emit_diagnostic
   7: rustc_errors::HandlerInner::emit_diagnostic
   8: rustc_errors::Handler::emit_diagnostic
   9: rustc_mir::borrow_check::do_mir_borrowck
  10: rustc_infer::infer::InferCtxtBuilder::enter
  11: rustc_mir::borrow_check::mir_borrowck
  12: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_borrowck>::compute
  13: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  14: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  15: rustc_data_structures::stack::ensure_sufficient_stack
  16: rustc_query_system::query::plumbing::force_query_with_job
  17: rustc_query_system::query::plumbing::get_query_impl
  18: rustc_query_system::query::plumbing::ensure_query_impl
  19: rustc_interface::passes::analysis
  20: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  21: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  22: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  23: rustc_data_structures::stack::ensure_sufficient_stack
  24: rustc_query_system::query::plumbing::force_query_with_job
  25: rustc_query_system::query::plumbing::get_query_impl
  26: rustc_interface::passes::QueryContext::enter
  27: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  28: rustc_span::with_source_map
  29: rustc_interface::interface::create_compiler_and_run
  30: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.52.0-nightly (e9920ef77 2021-02-11) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z share-generics=y -C embed-bitcode=no -C debuginfo=2 -C linker=/usr/bin/clang -C incremental -C link-arg=-fuse-ld=lld --crate-type bin

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

query stack during panic:
#0 [mir_borrowck] borrow-checking `falling`
#1 [analysis] running analysis passes on this crate
end of query stack
warning: 5 warnings emitted

For more information about this error, try `rustc --explain E0594`.
error: could not compile `staircases`

Code that generated this error is available here:

szunami/staircases@cc74cec

I am happy to spend some time trying to create a smaller repro but it will take me a few days. This error is reproducible on my machine.

[szunami]

FYI, with szunami/staircases@a0dfe83 this code compiled.

[tmiasko]

Minimized:

fn main() {
    for v in Query.iter_mut() {
        *v -= 1;
    }
}

pub struct Query;
pub struct QueryIter<'a>(&'a i32);

impl Query {
    pub fn iter_mut<'a>(&'a mut self) -> QueryIter<'a> {
        todo!();
    }
}

impl<'a> Iterator for QueryIter<'a> {
    type Item = &'a i32;

    fn next(&mut self) -> Option<Self::Item> {
        todo!();
    }
}

Similar error was reported in #83292.

The issue is in code that attempts to suggest a different method, that might potentially return &mut items. In that particular case there are no other suggestions to make, but code will attempt to emit diagnostic anyway, eventually failing the assertion:

rust/compiler/rustc_mir/src/borrow_check/diagnostics/mutability_errors.rs

Lines 627 to 656 in 41b315a

	let opt_suggestions = path_segment
	.hir_id
	.map(|path_hir_id| self.infcx.tcx.typeck(path_hir_id.owner))
	.and_then(|typeck| typeck.type_dependent_def_id(*hir_id))
	.and_then(|def_id| self.infcx.tcx.impl_of_method(def_id))
	.map(|def_id| self.infcx.tcx.associated_items(def_id))
	.map(|assoc_items| {
	assoc_items
	.in_definition_order()
	.map(|assoc_item_def| assoc_item_def.ident)
	.filter(|&ident| {
	let original_method_ident = path_segment.ident;
	original_method_ident != ident
	&& ident
	.as_str()
	.starts_with(&original_method_ident.name.to_string())
	})
	.map(|ident| format!("{}()", ident))
	});
	if let Some(suggestions) = opt_suggestions {
	err.span_suggestions(
	path_segment.ident.span,
	&format!("use mutable method"),
	suggestions,
	Applicability::MaybeIncorrect,
	);
	}
	}
	};

@rustbot label: +E-easy

[fee1-dead]

@rustbot claim

[tmiasko]

@rustbot label +regression-from-stable-to-beta