'crate-type = ["proc-macro"]' causes 'error: proc-macro crate not dylib' and ICE when documenting downstream crates

[ExpHP]

Reproduction

Minimal reproduction requires two crates. Here's a script to generate them:

Bash script

#!/usr/bin/env bash

rm -rf dep main

cargo new --lib dep
cargo new --bin main

cat >dep/Cargo.toml <<EOF
[package]
name = "dep"
version = "0.1.0"

[lib]
crate-type = ["proc-macro"]
EOF

cat >main/Cargo.toml <<EOF
[package]
name = "main"
version = "0.1.0"

[dependencies]
dep = { path = "../dep" }
EOF

cat >main/src/main.rs <<EOF
#![allow(unused)]
#[macro_use]
extern crate dep;

fn main() {}
EOF

cd dep
cargo build || exit 1 # ok
cargo doc   || exit 1 # ok
cd ..

cd main
cargo build || exit 1 # ok
cargo doc   || exit 1 # 'error: proc-macro crate not dylib'
cd ..

Output

 Documenting main v0.1.0 (file:///home/lampam/cpp/throwaway/rep/main)                                                                       
thread '<unnamed>' panicked at 'librustc_metadata/creader.rs:524: proc-macro crate not dylib', librustc/session/mod.rs:1297:26              
note: Run with `RUST_BACKTRACE=1` for a 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 1.29.0-nightly (1ecf6929d 2018-07-16) running on x86_64-unknown-linux-gnu

error: Could not document `main`.                                                                                                           

Caused by:
  process didn't exit successfully: `rustdoc --crate-name main src/main.rs -o /home/lampam/cpp/throwaway/rep/main/target/doc -L dependency=/home/lampam/cpp/throwaway/rep/main/target/debug/deps --extern dep=/home/lampam/cpp/throwaway/rep/main/target/debug/deps/libdep-81a747942b9c6cac.rmeta` (exit code: 101)

Stack trace is probably easy to guess and not very interesting, but here you go:

Stack trace

   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:475
   5: std::panicking::begin_panic
   6: rustc::session::opt_span_bug_fmt::{{closure}}
   7: rustc::ty::context::tls::with_opt::{{closure}}
   8: rustc::ty::context::tls::with_context_opt
   9: rustc::ty::context::tls::with_opt
  10: rustc::session::opt_span_bug_fmt
  11: rustc::session::span_bug_fmt
  12: rustc_metadata::creader::CrateLoader::resolve_crate
  13: <rustc_metadata::creader::CrateLoader<'a> as rustc::middle::cstore::CrateLoader>::process_extern_crate
  14: rustc_resolve::build_reduced_graph::<impl rustc_resolve::Resolver<'a>>::build_reduced_graph_for_item
  15: <rustc_resolve::build_reduced_graph::BuildReducedGraphVisitor<'a, 'b> as syntax::visit::Visitor<'a>>::visit_item
  16: syntax::visit::walk_item
  17: <rustc_resolve::build_reduced_graph::BuildReducedGraphVisitor<'a, 'b> as syntax::visit::Visitor<'a>>::visit_item
  18: syntax::ext::expand::AstFragment::visit_with
  19: rustc_resolve::macros::<impl syntax::ext::base::Resolver for rustc_resolve::Resolver<'a>>::visit_ast_fragment_with_placeholders
  20: syntax::ext::expand::MacroExpander::collect_invocations
  21: syntax::ext::expand::MacroExpander::expand_fragment
  22: syntax::ext::expand::MacroExpander::expand_crate
  23: rustc_driver::driver::phase_2_configure_and_expand_inner::{{closure}}
             at /checkout/src/librustc_driver/driver.rs:986
             at /checkout/src/librustc/util/common.rs:166
             at /checkout/src/librustc/util/common.rs:160
             at /checkout/src/librustc_driver/driver.rs:985
  24: rustc::util::common::time
             at /checkout/src/librustc/util/common.rs:166
             at /checkout/src/librustc/util/common.rs:160
  25: rustdoc::core::run_core::{{closure}}
             at /checkout/src/librustc_driver/driver.rs:938
             at librustdoc/core.rs:293
  26: rustc_driver::driver::spawn_thread_pool
             at /checkout/src/librustc_driver/driver.rs:73
             at /cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
             at /checkout/src/librustc_driver/driver.rs:72
  27: rustdoc::core::run_core
             at librustdoc/core.rs:248
  28: <scoped_tls::ScopedKey<T>>::set
             at librustdoc/lib.rs:674
             at /cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
             at /checkout/src/libsyntax/lib.rs:100
             at /cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
  29: syntax::with_globals
             at /checkout/src/libsyntax/lib.rs:99
  30: std::panicking::try::do_call
             at librustdoc/lib.rs:670
             at /checkout/src/librustc_driver/lib.rs:1633
             at /checkout/src/libstd/panic.rs:313
             at /checkout/src/libstd/panicking.rs:310
  31: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:106
  32: rustc_driver::monitor
             at /checkout/src/libstd/panicking.rs:289
             at /checkout/src/libstd/panic.rs:392
             at /checkout/src/librustc_driver/lib.rs:1565
             at /checkout/src/librustc_driver/lib.rs:1632
  33: rustdoc::rust_input
             at librustdoc/lib.rs:670
  34: rustdoc::main_args
             at librustdoc/lib.rs:590
             at librustdoc/lib.rs:545
  35: <scoped_tls::ScopedKey<T>>::set
             at librustdoc/lib.rs:116
             at /checkout/src/libcore/option.rs:414
             at librustdoc/lib.rs:116
             at /cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
             at /checkout/src/libsyntax/lib.rs:100
             at /cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
  36: syntax::with_globals
             at /checkout/src/libsyntax/lib.rs:99
  37: std::panicking::try::do_call
             at /checkout/src/libstd/thread/mod.rs:409
             at /checkout/src/libstd/panic.rs:313
             at /checkout/src/libstd/panicking.rs:310
  38: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:106
  39: <F as alloc::boxed::FnBox<A>>::call_box
             at /checkout/src/libstd/panicking.rs:289
             at /checkout/src/libstd/panic.rs:392
             at /checkout/src/libstd/thread/mod.rs:408
             at /checkout/src/liballoc/boxed.rs:640
  40: std::sys_common::thread::start_thread
             at /checkout/src/liballoc/boxed.rs:650
             at libstd/sys_common/thread.rs:24
  41: std::sys::unix::thread::Thread::new::thread_start
             at libstd/sys/unix/thread.rs:90
  42: start_thread
  43: clone

Description

Basically, crate-type = ["proc-macro"] is a thing that mostly appears to work, even though all documentation says to use proc-macro = true. However, using it causes issues when documenting downstream crates, most notably the above.

See also rust-lang/cargo#5310.

[real-felix]

Thanks for your tip, I replaced the following line:

crate_type = ["proc-macro"]

with

proc-macro = true

and now I can generate the documentation without ICE.

[bjorn3]

Fixed by rust-lang/cargo#6256