rust-lang/rust · 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

Returned by proc-macro-srv-cli's run() when the binary is compiled WITHOUT the in-rust-tree Cargo feature. The stub run() (gated by #[cfg(not(feature = "in-rust-tree"))]) does nothing and immediately fails with ErrorKind::Unsupported. Only the in-rust-tree build contains the real protocol handler; a standalone crates.io-style build is intentionally non-functional.

Source

Thrown at src/tools/rust-analyzer/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 7088e4b63a)

Solutions

1. Build the CLI from within the rust workspace / rustup toolchain so the in-rust-tree feature is enabled.
2. Add --features in-rust-tree when building standalone: cargo build -p proc-macro-srv-cli --features in-rust-tree.
3. Use the rustup-shipped proc-macro-srv (rustup component) instead of a hand-built binary.
4. Point rust-analyzer's procMacro.server at a known-good binary from the toolchain.

Example fix

# before
# cargo build -p proc-macro-srv-cli   # missing feature

# after
cargo build -p proc-macro-srv-cli --features in-rust-tree

fn ensure_in_tree_feature() {
    // Assert at startup that the binary supports the protocol.
    // Run `proc-macro-srv-cli --help` and check exit code != unsupported.
}

match run(stdin, stdout, fmt) {
    Ok(()) => Ok(()),
    Err(e) if e.kind() == io::ErrorKind::Unsupported
        && e.to_string().contains("in-rust-tree") => {
        // tell user to rebuild with --features in-rust-tree
        Err(e)
    }
    Err(e) => Err(e),
}

When it happens

Trigger: Building proc-macro-srv-cli from source without --features in-rust-tree and then pointing rust-analyzer at the resulting binary. The first request fails instantly with Unsupported because the cfg-gated stub cannot handle stdin/stdout.

Common situations: Building proc-macro-srv-cli standalone (cargo build -p proc-macro-srv-cli) instead of via the rust workspace; vendoring a partial copy of the crate; using a third-party distro package that disabled the feature.

Related errors

• rust-analyzer Language Server is not available. Please, ensu
• Failed to execute ${path} --version.
• Cargo invocation has failed: ${err}
• No compilation artifacts
• Please set `rust-analyzer.profiling.memoryProfile` to the pa

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/59877ecf9115c801. Report an issue: GitHub.