Incorrect detection of proc-macro crate type

[vigoo]

I have a proc-macro crate implementing a deriver logic with #[proc_macro_derive] that compiles correctly.

If I add a harness = false line to the [lib] section though like this:

[package]
name = "golem-wasm-rpc-derive"
version = "0.0.0"
edition = "2021"
license = "Apache-2.0"
homepage = "https://golem.cloud"
repository = "https://github.com/golemcloud/golem"
description = "Golem WASM RPC derivation macros"

[lib]
proc-macro = true
harness = false       # <--- this is the line added

[dependencies]
heck = "0.5.0"
proc-macro2 = "1"
quote = "1"
syn = { version = "2", features = ["full"] }

[dev-dependencies]
test-r = { workspace = true }

then the compilation fails with:

   Compiling golem-wasm-rpc-derive v0.0.0 (.../wasm-rpc-derive)
error: the `#[proc_macro_derive]` attribute is only usable with crates of the `proc-macro` crate type
  --> wasm-rpc-derive/src/lib.rs:25:1
   |
25 | #[proc_macro_derive(IntoValue, attributes(wit_transparent, unit_case, wit_field))]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Meta

rustc --version --verbose:

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: aarch64-apple-darwin
release: 1.86.0
LLVM version: 19.1.7

[ehuss]

Can you show exactly what command you ran? If it is cargo test, then indeed those two options are not going to be compatible with one another.

[vigoo]

@ehuss thanks for the reply, your comment made me realize what is going on more precisely.

So this is in a cargo workspace. After adding the harness = false I can still build the crate with

cargo build -p golem-wasm-rpc-derive

and also

cargo build --workspace

works.

What caused the issue is that our CI build is running

cargo build --workspace --all-targets

and that fails with the provided error.

I still feel like this is a bug (or at least a very strange behavior), as I would not expect disabling the test harness to lead to an error saying the proc-macro crate is not a proc-macro crate.

[ehuss]

Ah. The issue is that --all-targets builds the lib unittest for the proc-macro. With harness=true, this is fine since unittest support is built-in to the compiler so to speak, and it has special support for building that way. Although it has limited use since you still can't use any proc-macros from the unittests. With harness=false, this is essentially building as a binary crate type, and those are incompatible with defining proc-macros, and I doubt we would want to relax that restriction.

I'm not sure what your goal is with setting harness=false, but I would recommend using an integration test for that purpose.

I filed rust-lang/cargo#15519 for providing a better error message.

[vigoo]

Thanks!
Adding the harness = false in this crate of the workspace was just to avoid the need to explicitly exclude this project when running cargo test on the whole workspace, passing test harness parameters not supported by the built-in harness (in every other crate a custom test harness is used). It's definitely not a blocker, just wanted to report the misleading error message.