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

Memory profiling is not enabled for this build of rust-analy

Error message

Memory profiling is not enabled for this build of rust-analyzer.

To build rust-analyzer with profiling support, pass `--features dhat --profile dev-rel` to `cargo build` when building from source, or pass `--enable-profiling` to `cargo xtask`.

What it means

The `rust-analyzer/memoryUsage` LSP request (`handle_memory_usage`) requires the `dhat` cargo feature. When the running binary was built without it, the handler returns this error explaining exactly how to rebuild with profiling support. It is a deliberate, documented build-configuration guard, not a bug.

Source

Thrown at crates/rust-analyzer/src/handlers/request.rs:134

            .status(file_id)
            .unwrap_or_else(|_| "Analysis retrieval was cancelled".to_owned()),
    );

    buf.push_str("\nVersion: \n");
    format_to!(buf, "{}", crate::version());

    buf.push_str("\nConfiguration: \n");
    format_to!(buf, "{:#?}", snap.config);

    Ok(buf)
}

pub(crate) fn handle_memory_usage(_state: &mut GlobalState, _: ()) -> anyhow::Result<String> {
    let _p = tracing::info_span!("handle_memory_usage").entered();

    #[cfg(not(feature = "dhat"))]
    {
        Err(anyhow::anyhow!(
            "Memory profiling is not enabled for this build of rust-analyzer.\n\n\
            To build rust-analyzer with profiling support, pass `--features dhat --profile dev-rel` to `cargo build`
            when building from source, or pass `--enable-profiling` to `cargo xtask`."
        ))
    }
    #[cfg(feature = "dhat")]
    {
        if let Some(dhat_output_file) = _state.config.dhat_output_file() {
            let mut profiler = crate::DHAT_PROFILER.lock().unwrap();
            let old_profiler = profiler.take();
            // Need to drop the old profiler before creating a new one.
            drop(old_profiler);
            *profiler = Some(dhat::Profiler::builder().file_name(&dhat_output_file).build());
            Ok(format!(
                "Memory profile was saved successfully to {dhat_output_file}.\n\n\
                See https://docs.rs/dhat/latest/dhat/#viewing for how to inspect the profile."
            ))
        } else {

View on GitHub (pinned to e8f7e90aa3)

Solutions

Rebuild from source with `cargo build --features dhat --profile dev-rel` (inside rust-analyzer's repo).
Or build via `cargo xtask --enable-profiling install`.
Point the editor at the newly built binary and retry the memoryUsage request.
If profiling isn't actually needed, use an OS-level tool (heaptrack, valgrind/massif) instead of this request.

Example fix

// before
cargo build

// after
cargo build --features dhat --profile dev-rel

Defensive patterns

Strategy: fallback

Validate before calling

// detect a profiling-capable build before calling the request
fn supports_memory_usage(build_features: &[&str]) -> bool {
    build_features.contains(&"dhat")
}

Try / catch

match request_memory_usage() {
    Err(e) if e.to_string().contains("Memory profiling is not enabled") => {
        eprintln!("falling back to OS profiler; rebuild with --features dhat for dhat profiles");
        spawn_external_profiler() // heaptrack / massif
    }
    other => other,
}

Prevention

For profiling sessions, build rust-analyzer with `cargo xtask --enable-profiling install`.
Check the server's feature set (or docs) before exposing profiling UI commands.
Keep an OS-level heap profiler available as a fallback.

When it happens

Trigger: Invoking `rust-analyzer/memoryUsage` (e.g. via `Editor: RA Memory Usage` command in VS Code) against a rust-analyzer binary compiled without `--features dhat`.

Common situations: Developers profiling rust-analyzer's heap usage from a distro- or marketplace-supplied binary that was not built with profiling; CI binaries and default builds also lack the feature.

Related errors

AI-assisted analysis of rust-lang/rust-analyzer@e8f7e90aa3 (2026-09-03). Data as JSON: /api/errors/96894a2e93c73a28. Report an issue: GitHub.