rust-lang/rust-analyzer · error · io::Error

proc-macro-srv-cli needs to be compiled with the `in-rust-tr

Error message

proc-macro-srv-cli needs to be compiled with the `in-rust-tree` feature to function

What it means

This error is returned by the stub `run` function that is compiled when the `in-rust-tree` cargo feature is disabled. The binary ships two implementations of `run`: a real one that reads requests from stdin and dispatches proc-macro expansions, and this no-op guard that simply errors out. It fires whenever the user invokes proc-macro-srv-cli built without the feature, because only `in-rust-tree` builds link against the rustc driver and the proc-macro bridge needed to expand macros; a standalone build has no expansion backend, so any attempt to run it aborts immediately with this sentinel error.

Source

Thrown at crates/proc-macro-srv-cli/src/main.rs:100

    fn from_str(input: &str, _ignore_case: bool) -> Result<Self, String> {
        match input {
            "json-legacy" => Ok(ProtocolFormatArg(ProtocolFormat::JsonLegacy)),
            "bidirectional-postcard-prototype" => {
                Ok(ProtocolFormatArg(ProtocolFormat::BidirectionalPostcardPrototype))
            }
            _ => Err(format!("unknown protocol format: {input}")),
        }
    }
}

#[cfg(not(feature = "in-rust-tree"))]
fn run(
    _: &mut std::io::BufReader<std::io::Stdin>,
    _: &mut std::io::Stdout,
    _: ProtocolFormat,
) -> std::io::Result<()> {
    Err(std::io::Error::new(
        std::io::ErrorKind::Unsupported,
        "proc-macro-srv-cli needs to be compiled with the `in-rust-tree` feature to function"
            .to_owned(),
    ))
}

View on GitHub (pinned to e8f7e90aa3)

Solutions

1. Rebuild or reinstall the binary with the `in-rust-tree` feature enabled (e.g. `cargo build --features in-rust-tree`)
2. Use the rustup-distributed rust-analyzer, whose proc-macro-srv component is built in-tree with the required feature
3. Prefer invoking proc-macro expansion through rust-analyzer, which spawns a compatible proc-macro-srv automatically

When it happens

Trigger: Thrown at crates/proc-macro-srv-cli/src/main.rs:100 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of rust-lang/rust-analyzer@e8f7e90aa3 (2026-09-03). Data as JSON: /api/errors/b9dd6855d8220d25. Report an issue: GitHub.