-Zno-link is broken

[bjorn3]

I tried this command:

$ echo "fn main() {}" | rustc -Zno-link -

I expected to see this happen: An .rlink file is generated.

Instead, this happened:

thread 'rustc' panicked at 'cannot encode `DefIndex` with `rustc_serialize::json::Encoder`', /rustc/381b445ff5751f9f39ec672b623372dff09c276e/compiler/rustc_span/src/def_id.rs:135:9
stack backtrace:
   0: rust_begin_unwind
             at /rustc/381b445ff5751f9f39ec672b623372dff09c276e/library/std/src/panicking.rs:483
   1: std::panicking::begin_panic_fmt
             at /rustc/381b445ff5751f9f39ec672b623372dff09c276e/library/std/src/panicking.rs:437
   2: <rustc_span::def_id::DefIndex as rustc_serialize::serialize::Encodable<E>>::encode
   3: <rustc_serialize::json::Encoder as rustc_serialize::serialize::Encoder>::emit_struct
   4: <rustc_serialize::json::Encoder as rustc_serialize::serialize::Encoder>::emit_struct
   5: <rustc_serialize::json::Encoder as rustc_serialize::serialize::Encoder>::emit_seq
   6: <rustc_serialize::json::Encoder as rustc_serialize::serialize::Encoder>::emit_map
   7: <rustc_serialize::json::Encoder as rustc_serialize::serialize::Encoder>::emit_struct
   8: <rustc_serialize::json::Encoder as rustc_serialize::serialize::Encoder>::emit_struct
   9: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::link
  10: rustc_interface::queries::Linker::link
  11: rustc_span::with_source_map
  12: rustc_interface::interface::create_compiler_and_run
  13: scoped_tls::ScopedKey<T>::set

Meta

rustc --version --verbose:

rustc 1.48.0-nightly (381b445ff 2020-09-29)
binary: rustc
commit-hash: 381b445ff5751f9f39ec672b623372dff09c276e
commit-date: 2020-09-29
host: x86_64-unknown-linux-gnu
release: 1.48.0-nightly
LLVM version: 11.0

Originally posted by @bjorn3 in #77795 (comment)

cc @0dvictor (original implementer of -Zno-link

[0dvictor]

PR #73851 changed the encode/decode method of DefId to call the encode/decode method of its field pub index: DefIndex; however, DefIndex is only encodable in metadata. It panics if encoded anywhere else.

When no-link was implemented, the encode/decode of DefId was purposely crafted to avoid calling the encode/decode method of DefIndex. The most straightforward fix is to implement Encodable/Decodable for DefIndex to allow it be serialized outside of metadata.

After that, may be now it is the time to re-think the way we store data in the rlink file.

What do you think @bjorn3 ?

[bjorn3]

I think DefId should completely be avoided in the input of the linker function. One of the uses if NativeLib in it's foreign_module field. This field doesn't seem to be used by the linking code.

[0dvictor]

Good suggestion, let me look into it.

[bjorn3]

@0dvictor are you still working on this?

[0dvictor]

Thank you for reminding and apologize for the delay. I did create a prototype but haven't got a chance polish it yet. I'll create the PR this week.

[0dvictor]

Sorry again about the delay. I've created PR #80187, which fixes this issue.