missing specialization: `<serialize::json::Encoder as SpecializedEncoder<syntax_pos::symbol::Ident>>::specialized_encode` not overridden

[tesuji]

The following code when compiled with -Z ast-json causes ICE:

pub fn main() {
    let a = ["1", "lol", "3", "NaN", "5"];

    let element: Option<i32> = a.iter().filter_map(|s| s.parse().ok()).next();
    assert_eq!(element, Some(1));
}

Error:

% RUST_BACKTRACE=1 cargo rustc -- -Z ast-json
   Compiling check v0.1.0 (/home/lzutao/forked/rust/check)
thread 'rustc' panicked at 'missing specialization: `<serialize::json::Encoder as SpecializedEncoder<syntax_pos::symbol::Ident>>::specialized_encode` not overridden', src/libserialize/serialize.rs:851:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.35/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.35/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:200
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:214
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:481
   8: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:384
   9: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:339
  10: <E as serialize::serialize::SpecializationError>::not_found
  11: <serialize::json::Encoder as serialize::serialize::Encoder>::emit_struct
  12: <serialize::json::Encoder as serialize::serialize::Encoder>::emit_seq
  13: <serialize::json::Encoder as serialize::serialize::Encoder>::emit_struct
  14: <serialize::json::Encoder as serialize::serialize::Encoder>::emit_struct
  15: <serialize::json::AsJson<T> as core::fmt::Display>::fmt
  16: core::fmt::write
             at src/libcore/fmt/mod.rs:1029
  17: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1399
  18: <std::io::stdio::Stdout as std::io::Write>::write_fmt
             at src/libstd/io/stdio.rs:533
  19: std::io::stdio::print_to::{{closure}}
             at src/libstd/io/stdio.rs:786
  20: std::thread::local::LocalKey<T>::try_with
             at src/libstd/thread/local.rs:262
  21: std::io::stdio::print_to
             at src/libstd/io/stdio.rs:780
  22: std::io::stdio::_print
             at src/libstd/io/stdio.rs:802
  23: rustc_interface::passes::configure_and_expand_inner
  24: rustc_interface::passes::configure_and_expand::{{closure}}
  25: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::new
  26: rustc_interface::passes::configure_and_expand
  27: rustc_interface::queries::Query<T>::compute
  28: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::expansion
  29: rustc_interface::interface::run_compiler_in_existing_thread_pool
  30: std::thread::local::LocalKey<T>::with
  31: scoped_tls::ScopedKey<T>::set
  32: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
end of query stack

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 1.39.0-nightly (c1b08dd26 2019-08-19) running on x86_64-unknown-linux-gnu

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

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

{"module":{"inner":{"lo":0,"hi":174},"items":[{"ident":
error: Could not compile `check`.

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