fastframe-log

Log to stderr and a file for bug reports, keep private data out, and record panics without their payload.

A desktop app launched from a menu has no terminal: the log file is what people attach to bug reports, so it must never hold message contents, phone numbers, keys, tokens, or pairing payloads. This is the setup ZapFast and Spotifast share.

Usage

fastframe_log::Logging::new("zapfast", env!("CARGO_PKG_VERSION"))
    // Used when RUST_LOG is unset.
    .filter(if verbose { "info,zapfast=debug" } else { "warn,zapfast=info" })
    // Created fresh each run. Leave it out for demo runs (stderr only).
    .file(dirs.log_file())
    // One line per panic, appended.
    .panic_log(dirs.panic_log())
    // Optional: rewrite the lines of chatty or private targets.
    .redact(|record, _message| {
        record
            .target()
            .starts_with("whatsapp_rust")
            .then_some("protocol diagnostic (private details omitted)".into())
    })
    .init()?;

init logs Starting zapfast 0.16.3 on linux (x86_64) once the logger is up, at info under the app’s name as its target, so a filter such as warn,zapfast=info keeps it; it also warns (on stderr) if the log file could not be created. The app’s name and version are passed in; this crate’s version is not the app’s.

Lines look like [2026-09-25T10:00:00Z WARN zapfast::backend] message.

Panics

log_panics (called by init when .panic_log is set) replaces the default panic hook. Each panic appends one line to the panic log and prints it to stderr:

2026-09-25T10:00:00Z zapfast 0.16.3 on thread "main", panic at src/app.rs:10:5 (payload omitted)

The payload is never written anywhere: an expect or a formatted panic message can quote whatever the code was handling. The source location is enough to find the panic.

Apps on tracing

Logging is a log logger, behind the default logger feature. The panic hook and the redaction helpers use only the standard library and work with any facade, so an app on tracing takes them without the logger:

fastframe-log = { git = "https://github.com/crmne/fastframe", rev = "<commit>", default-features = false }

fastframe_log::log_panics(state_dir.join("panic.log"), "solco", env!("CARGO_PKG_VERSION"));
tracing::warn!(error = %fastframe_log::redact::links(&error.to_string()), "download failed");

log_panics replaces the panic hook without chaining the previous one (the default hook would print the payload). Install it first: a hook set later that chains std::panic::take_hook(), such as Solco’s session recorder, then runs before it and keeps working.

There is no tracing subscriber in this crate: only one app logs through tracing, and it writes no log file. One can follow when a second app needs it.

Redaction

For single messages, before they are logged:

use fastframe_log::redact;

// Error strings from HTTP clients quote URLs with their tokens.
log::warn!("sticker download failed: {}", redact::links(&error.to_string()));

// Add the shapes an app knows are private.
let cdn = |word: &str| redact::is_link(word) || word.contains("oh=");
log::warn!("{}", redact::words(&error, cdn));

For a whole dependency whose messages can carry private data, pass a Redactor to .redact and summarise its lines into fixed categories.

Redaction is a net, not the rule: the rule is not to format private values into log lines at levels that ship.