[Suggestion] E0432 - could it include suggestions to add "lib" in crate-type?

[dannywillems]

Hi,

I am learning Rust and started a playbook.

I encountered the error E0432 while trying to use a binary in a library I have just created.
I spent a bit of time to understand it was because of a missing "lib" in crate-type. I thought it was automatically added as it was in the lib section.
Could suggestions be added to error messages?

[jyn514]

Here are the commands cargo generates for a crate-type = ["cdylib", "staticlib"] crate:

   Compiling example v0.1.0 (/home/jyn/src/example)
     Running `/home/jyn/.local/lib/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name example --edition=2021 src/main.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=113 --crate-type bin --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 -C metadata=70605fc8f37b61e6 -C extra-filename=-70605fc8f37b61e6 --out-dir /home/jyn/.local/lib/cargo/target/debug/deps -C incremental=/home/jyn/.local/lib/cargo/target/debug/incremental -L dependency=/home/jyn/.local/lib/cargo/target/debug/deps`
     Running `/home/jyn/.local/lib/rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --crate-name example --edition=2021 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --diagnostic-width=113 --crate-type cdylib --crate-type staticlib --emit=dep-info,link -C embed-bitcode=no -C debuginfo=2 -C metadata=45e74d774f239f04 --out-dir /home/jyn/.local/lib/cargo/target/debug/deps -C incremental=/home/jyn/.local/lib/cargo/target/debug/incremental -L dependency=/home/jyn/.local/lib/cargo/target/debug/deps`

There are two things going on:

1. Cargo doesn't pass --emit=metadata to rustc when building the library.
2. Cargo doesn't wait until the library finishes building to start building the binary.

Also, there is a bug in rustc: passing --emit=metadata generates an empty file:

; rustc --crate-name example --crate-type cdylib --crate-type staticlib --emit=dep-info,link,metadata  src/lib.rs 
; file libexample.rmeta 
libexample.rmeta: empty

I guess cargo's behavior could make sense if you were only building the library (e.g. to link into a C program), but I think cargo should not allow having any target in the package other than the library, since it knows just by crate-type = lib being missing that they'll fail to compile.

@rust-lang/cargo could you transfer this to the cargo repository, please? I don't have permissions.

[jyn514]

Also, there is a bug in rustc: passing --emit=metadata generates an empty file:

ah, this is #67293.

[ehuss]

I'm not entirely clear exactly what this issue is asking for. @dannywillems, can you include the exact Cargo.toml that you had, the file structure of your project, and what the error said?

Perhaps you can also include what you were trying to accomplish? I don't know what exactly was meant by "use a binary in a library". If you mean access code from a binary in a library, that isn't how Cargo works. Intra-package dependencies go the other way, where binaries can only access code from a library.

[jyn514]

@ehuss https://gitlab.com/dannywillems/rust-playbook/-/tree/main/0002-cargo-lib-bin has an MCVE

[ehuss]

Oh, thanks! Sorry I missed that, that's exactly what I need.

So I'm not sure there is a lot cargo can do in this case. It doesn't know anything about the code, or whether or not you intend to access a library from the binary. crate-type = ["cdylib", "staticlib"] is a legitimate configuration, and it is fine to write binaries that don't use the library as a Rust library. The only option I can think of is that rustc could try to detect if there is a static lib, dylib, or zero-length rmeta in the search path, and let you know that it found the non-rust-library types and provide some kind of hint like what is recommended here.