Crate secret_fmt 

secret-fmt

Accidental logging of PII (emails, IP addresses) or secrets (API keys, passwords) is a massive security liability.

secret-fmt provides a tiny, zero-dependency #![no_std] wrapper type called Secret<T> that prevents accidental logging by overriding Debug and Display to emit "[REDACTED]".

It has zero trait bounds, meaning you can wrap any third-party type instantly without having to write boilerplate.

Usage

use secret_fmt::{Secret, redact};

// 1. Wrap your sensitive data
let my_secret: Secret<String> = "super_secret_api_token".to_string().into();

// 2. Accidental logging is safe!
println!("User token is: {}", my_secret);     // Prints: User token is: [REDACTED]
println!("Debug token is: {:?}", my_secret);  // Prints: Debug token is: [REDACTED]

// 3. Log inline references safely without ownership
let token = "sensitive_data";
println!("Inline redact: {}", redact!(&token)); // Prints: [REDACTED]

// 4. Access the data only when you actually need it
let actual_token: &String = my_secret.as_inner();
assert_eq!(actual_token, "super_secret_api_token");

Serde Safety (Optional Feature)

Unlike other crates, Secret<T> intentionally does not implement Serialize by default. If a user serializes a database struct and the password field writes "[REDACTED]", it causes data destruction. Instead, secret-fmt forces you to be explicit about how the data leaves your system using #[serde(serialize_with)].

In Cargo.toml:

[dependencies]
secret-fmt = { version = "0.1", features = ["serde"] }

use secret_fmt::{Secret, serialize_redacted, serialize_actual};
use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize)]
struct UserResponse {
    id: u32,
    
    // Explicitly choose to redact this field when serialized to JSON
    #[serde(serialize_with = "serialize_redacted::serialize")]
    api_key: Secret<String>,
    
    // Explicitly choose to serialize the real underlying value for an upstream API
    #[serde(serialize_with = "serialize_actual::serialize")]
    pass_through: Secret<String>,
}

let incoming = r#"{"id": 123, "api_key": "real_token", "pass_through": "stripe_token"}"#;
let user: UserResponse = serde_json::from_str(incoming).unwrap();

// Outbound JSON explicitly enforces your chosen policies
let outbound = serde_json::to_string(&user).unwrap();
assert_eq!(outbound, r#"{"id":123,"api_key":"[REDACTED]","pass_through":"stripe_token"}"#);

Why not secrecy?

The secrecy crate is fantastic, but it enforces the Zeroize trait on everything it wraps to clear memory on drop. If you are dealing with types from external crates (like reqwest::HeaderValue), you have to write boilerplate wrappers to use secrecy.

Secret makes no attempt to clear memory—it focuses exclusively on stopping string/log/JSON leakage. Because it drops the Zeroize requirement, it works instantly on any type out of the box with zero dependencies. A zero-dependency wrapper type that prevents accidental logging of sensitive data.

Secret<T> wraps any value and explicitly overrides its Debug and Display (and optionally Serialize) implementations to emit "[REDACTED]".

Unlike other secrecy crates, Secret has zero trait bounds. You can wrap any type instantly without implementing boilerplate traits like Zeroize.

Note: Secret intentionally does not implement Deref to prevent implicit coercions that could bypass the redaction formatting.

Macros

redact

Wraps a reference or value in a Secret on the fly.

Structs

Secret

A wrapper type that redacts its contents when logged or formatted.