'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/librustc_typeck/check/method/suggest.rs:550:46

[sunny-g]

I tried code very similar to this:

use serde::de::*;

pub trait IpldVisitorExt<'de>: Visitor<'de> {
    fn visit_link<E>(self, _cid: Cid) -> Result<<Self as Visitor<'de>>::Value, E>
    where
        E: Error;
}

struct JsonVisitor<V>(V);

impl<'de, V: IpldVisitorExt<'de>> Visitor<'de> for JsonVisitor<V> {
    type Value = V::Value;

    #[inline]
    fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
        formatter.write_str("a JSON map, link object or byte object")
    }

    #[inline]
    fn visit_map<A>(self, map: A) -> Result<Self::Value, A::Error>
    where
        A: MapAccess<'de>,
    {
        let deserializer = value::MapAccessDeserializer::<A>::new(map);
        
        // uncommenting the following line triggers the error
        // let temp: value::MapAccessDeserializer<A> = deserializer.clone();

        Err(Error::custom(""))
    }
}

and it produces some type of type checking error when uncommenting the line beginning with let temp.

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/librustc_typeck/check/method/suggest.rs:550:46
...
query stack during panic:
#0 [typeck_tables_of] type-checking `<codec::dag_json::JsonVisitor<V> as serde::de::Visitor<'de>>::visit_map`
#1 [typeck_item_bodies] type-checking all item bodies
#2 [analysis] running analysis passes on this crate
end of query stack

Meta

rustc --version --verbose:

rustc 1.43.0-nightly (4ad624882 2020-03-03)
binary: rustc
commit-hash: 4ad62488258972bdb0e2df225d100f99ef58dbad
commit-date: 2020-03-03
host: x86_64-unknown-linux-gnu
release: 1.43.0-nightly
LLVM version: 9.0

Backtrace:

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/librustc_typeck/check/method/suggest.rs:550:46
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1053
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1428
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: rustc_driver::report_ice
  11: <alloc::boxed::Box<F> as core::ops::function::Fn<A>>::call
             at /rustc/4ad62488258972bdb0e2df225d100f99ef58dbad/src/liballoc/boxed.rs:1031
  12: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::enter::{{closure}}::{{closure}}
             at /rustc/4ad62488258972bdb0e2df225d100f99ef58dbad/src/libproc_macro/bridge/client.rs:305
  13: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
  14: rust_begin_unwind
             at src/libstd/panicking.rs:378
  15: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  16: core::panicking::panic
             at src/libcore/panicking.rs:52
  17: core::ops::function::impls::<impl core::ops::function::FnMut<A> for &mut F>::call_mut
  18: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  19: rustc_typeck::check::method::suggest::<impl rustc_typeck::check::FnCtxt>::report_method_error
  20: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  21: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  22: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_coercable_to_type
  23: rustc_typeck::check::FnCtxt::check_decl_local
  24: rustc_typeck::check::FnCtxt::check_stmt
  25: rustc_typeck::check::FnCtxt::check_block_with_expected
  26: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  27: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  28: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr
  29: rustc_typeck::check::check_fn
  30: rustc::ty::context::GlobalCtxt::enter_local
  31: rustc_typeck::check::typeck_tables_of
  32: rustc::ty::query::__query_compute::typeck_tables_of
  33: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  34: rustc::dep_graph::graph::DepGraph::with_task_impl
  35: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  36: rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners
  37: rustc_typeck::check::typeck_item_bodies
  38: rustc::ty::query::__query_compute::typeck_item_bodies
  39: rustc::dep_graph::graph::DepGraph::with_task_impl
  40: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  41: rustc_typeck::check_crate
  42: rustc_interface::passes::analysis
  43: rustc::ty::query::__query_compute::analysis
  44: rustc::dep_graph::graph::DepGraph::with_task_impl
  45: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  46: rustc::ty::context::tls::enter_global
  47: rustc_interface::interface::run_compiler_in_existing_thread_pool
  48: rustc_ast::attr::with_globals
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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.43.0-nightly (4ad624882 2020-03-03) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type lib

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

query stack during panic:
#0 [typeck_tables_of] type-checking `<codec::dag_json::JsonVisitor<V> as serde::de::Visitor<'de>>::visit_map`
#1 [typeck_item_bodies] type-checking all item bodies
#2 [analysis] running analysis passes on this crate
end of query stack

[sunny-g]

The error is likely due to the fact that the trait bounds on visit_map don't include Clone for A: MapAccess<'de>, though the compiler/type checker crashing is still likely a bug.

[JohnTitor]

That code was introduced in #69255, cc @estebank. Submitted #69727 as a quick fix.

[hellow554]

I can't get it to work in the same crate, just with two crates, where the first one depends on the other:

src/main.rs

use other_crate::Struct;

fn crash<A>() {
    let _ = Struct::<A>::new().clone();
}

fn main() {}

other_crate/src/lib.rs

#[derive(Clone)]
pub struct Struct<A>(A);

impl<A> Struct<A> {
    pub fn new() -> Self {
        todo!()
    }
}

@rustbot modify labels: -E-needs-mcve

[JohnTitor]

Thanks @hellow554! I included a test in the PR.