Rust E0463 "can't find crate for proc_macro" in CI

rustc cannot find the proc_macro crate while building a procedural macro. Proc-macros compile for the host; if the host std/proc-macro component is missing (often after installing only a cross target), the crate is unavailable.

What this error means

The build fails with error[E0463]: can't find crate for proc_macro``. It is deterministic for the installed toolchain/target set.

error[E0463]: can't find crate for `proc_macro`
 --> macros/src/lib.rs:1:1
  |
1 | extern crate proc_macro;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate

Common causes

Host std/proc-macro component missing

Proc-macros build for the host triple. If only a cross-compilation target's std was installed, the host proc_macro crate is absent.

Broken or partial toolchain install

An incomplete rustup install can leave the proc-macro support out of the toolchain.

How to fix it

Install the host toolchain components

Ensure the host std (which provides proc_macro) is present.

rustup toolchain install stable
rustup component add rust-std --toolchain stable
rustc --version --verbose # confirm the host triple

Build proc-macros for the host, not the cross target

Do not pass --target for the proc-macro crate; it must compile for the host.

cargo build --locked   # host build for proc-macro crates

How to prevent it

• Keep the host toolchain fully installed even when adding cross targets.
• Remember proc-macros always build for the host triple.
• Verify rustc --version --verbose shows the expected host.

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