rust-lang/cargo · error · AlreadyPrintedError

no sessions found{context}

Error message

no sessions found{context}

What it means

`cargo report rebuilds` (an experimental -Z build-analysis feature) reads JSON build logs from the cargo home / target dir. If find_log_file returns None (no log file for the workspace or the given --id), it prints a 'no sessions found' report and bails at rebuilds.rs:63. The note tells you to enable -Z build-analysis to start producing logs.

Source

Thrown at src/ops/cargo_report/rebuilds.rs:63

        } else {
            String::new()
        };
        let (title, note) = if let Some(id) = &opts.id {
            (
                format!("session `{id}` not found{context}"),
                "run `cargo report sessions` to list available sessions",
            )
        } else {
            (
                format!("no sessions found{context}"),
                "run command with `-Z build-analysis` to generate log files",
            )
        };
        let report = [Level::ERROR
            .primary_title(title)
            .element(Level::NOTE.message(note))];
        gctx.shell().print_report(&report, false)?;
        return Err(AlreadyPrintedError::new(anyhow::anyhow!("")).into());
    };

    let ctx = prepare_context(&log)
        .with_context(|| format!("failed to analyze log at `{}`", log.display()))?;
    let ws_root = ws.map(|ws| ws.root()).unwrap_or(gctx.cwd());

    display_report(gctx, ctx, &run_id, ws_root)?;

    Ok(())
}

struct Context {
    root_rebuilds: Vec<RootRebuild>,
    units: HashMap<UnitIndex, UnitInfo>,
    total_cached: usize,
    total_new: usize,
    total_rebuilt: usize,
}

View on GitHub (pinned to 0e07a15537)

Solutions

1. Run a build with the feature enabled, e.g. `cargo build -Z build-analysis`, then re-run the report.
2. If you passed --id, list valid sessions first with `cargo report sessions`.
3. Confirm you are on a nightly toolchain that supports -Z build-analysis.

Example fix

# before
$ cargo report rebuilds
error: no sessions found

# after
$ cargo +nightly build -Z build-analysis
$ cargo +nightly report rebuilds

# Only report if at least one build-analysis log exists:
log_dir="${CARGO_TARGET_DIR:-target}/build-analysis"
if [ ! -d "$log_dir" ] || [ -z "$(ls -A "$log_dir" 2>/dev/null)" ]; then
  echo "no sessions; run: cargo +nightly build -Z build-analysis" >&2
  exit 1
fi
cargo report rebuilds

When it happens

Trigger: Running `cargo report rebuilds` (or with `--id X`) without ever having built with `-Z build-analysis`, or after logs were garbage-collected.

Common situations: Exploring the build-analysis reporting before enabling the feature; pointing --id at a non-existent session; logs pruned by an older cargo.

Related errors

• no sessions found{context}
• no sessions found{context}
• no timing data found in log
• not implemented
• running the file `{cmd}` requires `-Zscript`

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/da95c2fa7b9ed63f.json. Report an issue: GitHub.