rust-lang/rust · warning · 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

Returned by handle_memory_usage when the running rust-analyzer binary was compiled WITHOUT the `dhat` feature (the #[cfg(not(feature = "dhat"))] arm). The memory-profile LSP request can only succeed on a profiling-enabled build; the message tells the user how to rebuild.

Source

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

Solutions

Rebuild rust-analyzer with profiling: `cargo build -p rust-analyzer --features dhat --profile dev-rel`.
Or use the xtask entry: `cargo xtask --enable-profiling install` (or the documented xtask invocation).
Restart the IDE/extension to point at the rebuilt server binary.

Example fix

// before: cfg-gated unconditional error in the non-dhat arm
#[cfg(not(feature = "dhat"))]
{ Err(anyhow::anyhow!("Memory profiling is not enabled ...")) }

// after: still error, but include how to verify the feature is active once rebuilt
#[cfg(not(feature = "dhat"))]
{
    Err(anyhow::anyhow!(
        "Memory profiling is not enabled for this build of rust-analyzer.\
         \n\nRebuild with `--features dhat --profile dev-rel`, \
         then confirm with `rust-analyzer --version` showing the profiling build."
    ))
}

Defensive patterns

Strategy: validation

Validate before calling

// Confirm the running binary was built with dhat before offering the profile UI:
fn profiling_enabled() -> bool {
    // The dhat feature allocates the global DHAT_PROFILER at startup; alternatively
    // gate the UI on a known version string produced only by the profiling build.
    cfg!(feature = "dhat") // for the server side; clients should check version metadata
}

Type guard

null

Try / catch

// Client-side: only expose the memory-profile command when the server advertises it:
if !server_capabilities.experimental.as_ref().is_some_and(|v| v.get("memoryProfile").is_some()) {
    return Ok(()); // hide/disable the command
}
// then call the request and handle the error by prompting the rebuild steps.

Prevention

Build rust-analyzer with --features dhat --profile dev-rel (or via xtask --enable-profiling) when you need profiling.
Have the client hide the profiling command unless the server advertises support.
Document that released builds do not include profiling.

When it happens

Trigger: A client invokes the rust-analyzer memory profiling/usage request on a stock (non-profiling) build. The cfg-gated arm unconditionally errors with rebuild instructions.

Common situations: Using a released rust-analyzer (no dhat feature) and triggering the profiling command from the IDE; building via plain `cargo build` instead of `cargo xtask --enable-profiling`.

Related errors

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/9e06c619f8895a9d. Report an issue: GitHub.