Confusing error with lib.harness and proc-macro

[ehuss]

The combination of harness=false and proc-macro=true are incompatible since proc-macros cannot be bin crates. I'd recommend making this a warning or error.

The following illustrates the current behavior generating a potentially confusing error:

[package]
name = "foo"
version = "0.1.0"
edition = "2024"

[lib]
proc-macro = true
harness = false

// src/lib.rs
use proc_macro::TokenStream;

#[proc_macro]
pub fn foo(_input: TokenStream) -> TokenStream {
    "".parse().unwrap()
}

#[cfg(test)]
fn main() {}

Running cargo build --all-targets:

   Compiling foo v0.1.0 (/Users/eric/Temp/foo)
error: the `#[proc_macro]` attribute is only usable with crates of the `proc-macro` crate type
 --> src/lib.rs:3:1
  |
3 | #[proc_macro]
  | ^^^^^^^^^^^^^

error: could not compile `foo` (lib test) due to 1 previous error
warning: build failed, waiting for other jobs to finish...

Previously reported at rust-lang/rust#140929.

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cargo 1.89.0-nightly (7918c7eb5 2025-04-27)
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