thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value'

[DDR0]

The following rust code crashes rustc 1.39.0 (4560ea788 2019-11-04) running on x86_64-unknown-linux-gnu.

extern crate wasm_bindgen;
use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub extern fn third(a: Vec(u32)) -> u32 {
    return a[3]
}

(Note, changing Vec(u32) to Vec<u32> fixes the crash.)

Meta

Running with RUST_BACKTRACE, I get the following error message.

   Compiling wasm_sample v0.1.0 (/home/david/Documents/rust/wasm_sample_2)
thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:378:21
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.37/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.37/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:76
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:60
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1030
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1412
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:64
   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:196
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:210
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:477
  12: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:380
  13: rust_begin_unwind
             at src/libstd/panicking.rs:307
  14: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  15: core::panicking::panic
             at src/libcore/panicking.rs:49
  16: rustc::hir::lowering::LoweringContext::lower_path_segment
  17: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  18: <rustc::hir::ptr::P<[T]> as core::iter::traits::collect::FromIterator<T>>::from_iter
  19: rustc::hir::lowering::LoweringContext::lower_qpath
  20: rustc::hir::lowering::LoweringContext::lower_path_ty
  21: rustc::hir::lowering::LoweringContext::lower_ty_direct
  22: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  23: <rustc::hir::ptr::P<[T]> as core::iter::traits::collect::FromIterator<T>>::from_iter
  24: rustc::hir::lowering::LoweringContext::lower_fn_decl
  25: rustc::hir::lowering::item::<impl rustc::hir::lowering::LoweringContext>::lower_item
  26: rustc::hir::lowering::LoweringContext::with_hir_id_owner
  27: <rustc::hir::lowering::item::ItemLowerer as syntax::visit::Visitor>::visit_mod
  28: rustc::hir::lowering::lower_crate
  29: rustc::util::common::time
  30: rustc_interface::passes::BoxedResolver::access::{{closure}}
  31: rustc_interface::passes::configure_and_expand::{{closure}}
  32: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::access
  33: rustc_interface::queries::Query<T>::compute
  34: rustc_interface::queries::Query<T>::compute
  35: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::prepare_outputs
  36: rustc_interface::interface::run_compiler_in_existing_thread_pool
  37: std::thread::local::LocalKey<T>::with
  38: scoped_tls::ScopedKey<T>::set
  39: syntax::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 1.39.0 (4560ea788 2019-11-04) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
end of query stack
error: could not compile `wasm_sample`.

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

rustc --version --verbose yields

rustc 1.39.0 (4560ea788 2019-11-04)
binary: rustc
commit-hash: 4560ea788cb760f0a34127156c78e2552949f734
commit-date: 2019-11-04
host: x86_64-unknown-linux-gnu
release: 1.39.0
LLVM version: 9.0

Thank you.

[hellow554]

I reduced the proc macro to the following:

use wasm_bindgen_macro::wasm_bindgen;

#[wasm_bindgen]
pub extern fn third(_: Vec(u32)) -> u32 {
    0   
}

extern crate proc_macro;

use proc_macro::TokenStream;
use quote::ToTokens;

#[proc_macro_attribute]
pub fn wasm_bindgen(attr: TokenStream, input: TokenStream) -> TokenStream {
    let mut tokens = proc_macro2::TokenStream::new();
    let item = syn::parse::<syn::Item>(input).expect("parse");

    if let syn::Item::Fn(f) = item {
        f.to_tokens(&mut tokens);
    }

    tokens.into()
}

with tokens being:

[wmacro/src/lib.rs:15] &tokens = TokenStream [
    Ident {
        ident: "pub",
        span: #0 bytes(0..0),
    },
    Ident {
        ident: "extern",
        span: #0 bytes(0..0),
    },
    Literal { lit: Lit { kind: Str, symbol: C, suffix: None }, span: Span { lo: BytePos(0), hi: BytePos(0), ctxt: #0 } },
    Ident {
        ident: "fn",
        span: #0 bytes(0..0),
    },
    Ident {
        ident: "third",
        span: #0 bytes(0..0),
    },
    Group {
        delimiter: Parenthesis,
        stream: TokenStream [
            Ident {
                ident: "_",
                span: #0 bytes(0..0),
            },
            Punct {
                ch: ':',
                spacing: Alone,
                span: #0 bytes(0..0),
            },
            Ident {
                ident: "Vec",
                span: #0 bytes(0..0),
            },
            Group {
                delimiter: Parenthesis,
                stream: TokenStream [
                    Ident {
                        ident: "u32",
                        span: #0 bytes(0..0),
                    },
                ],
                span: #0 bytes(0..0),
            },
        ],
        span: #0 bytes(0..0),
    },
    Punct {
        ch: '-',
        spacing: Joint,
        span: #0 bytes(0..0),
    },
    Punct {
        ch: '>',
        spacing: Alone,
        span: #0 bytes(0..0),
    },
    Ident {
        ident: "u32",
        span: #0 bytes(0..0),
    },
    Group {
        delimiter: Brace,
        stream: TokenStream [
            Literal { lit: Lit { kind: Integer, symbol: 0, suffix: None }, span: Span { lo: BytePos(0), hi: BytePos(0), ctxt: #0 } },
        ],
        span: #0 bytes(0..0),
    },
]

[csmoe]

rust/src/librustc/hir/lowering.rs

Line 1863 in 9124f7a

let split = snippet.find('(').unwrap();

panic due to not found ( inside snippet while trying to suggest Vec(T) to Vec<T>.
cc @estebank

[pnkfelix]

triage: P-high. Removing I-nominated.