rust-style

name	rust-style
description	General Rust conventions for Pluto. Use it once for understanding the codebase better.

Quality Gate

Run from pluto/ before declaring any work done:

cargo fmt --all --check
cargo clippy --workspace --all-targets --all-features -- -D warnings
cargo test --workspace --all-features
cargo deny check

All must pass clean.

---

Error Handling

Define module-local error enums with thiserror:

#[derive(Debug, thiserror::Error)]
pub enum Error {
    #[error("message: {0}")]
    Variant(String),

    #[error(transparent)]
    Underlying(#[from] OtherError),
}

pub type Result<T> = std::result::Result<T, Error>;

Rules:

• errors.New("msg") → enum variant with #[error("msg")] (match strings exactly)
• errors.Wrap(err, "...") → #[from] / #[source] where appropriate
• Every field/payload in an error variant must appear in its #[error("...")] format string. If a field is not surfaced in the error message, either include it or remove it from the variant. Dead payload (captured but never displayed) is not allowed.
• Always propagate with ?; never swallow errors with .ok() or filter_map in production code
• No unwrap(), expect(), panic!() outside of test code
• No anyhow in library crates; use typed errors everywhere

---

Arithmetic

All arithmetic must be checked — arithmetic_side_effects = "deny" is enforced:

// Bad
let x = a + b;

// Good
let x = a.checked_add(b).ok_or(Error::Overflow)?;

---

Casts

Never use as for numeric type conversions — use fallible conversions with try_from:

// Bad - will cause clippy errors
let x = value as u32;
let y = some_usize as u64;

// Good - use try_from with proper error handling
let x = u32::try_from(value)?;
let y = u64::try_from(some_usize).expect("message explaining why this is safe");

Rules:

• Always use TryFrom/try_from for numeric conversions between different types
• Handle conversion failures explicitly (either with ? or expect with justification)
• The only acceptable use of expect is when the conversion is guaranteed to succeed (e.g., usize to u64 on 64-bit platforms)
• Clippy will error on unchecked as casts: cast_possible_truncation, cast_possible_wrap, cast_sign_loss

---

Conversion Traits

When implementing a conversion, implement TryFrom (not TryInto) and From (not Into):

// Bad - implementing TryInto directly
impl TryInto<Foo> for Bar {
    type Error = Error;
    fn try_into(self) -> Result<Foo, Error> { /* ... */ }
}

// Good - implement TryFrom; the matching TryInto comes for free
impl TryFrom<Bar> for Foo {
    type Error = Error;
    fn try_from(value: Bar) -> Result<Foo, Error> { /* ... */ }
}

Rules:

• Implement TryFrom<A> for B / From<A> for B. The standard library's blanket impls then provide TryInto/Into automatically; the reverse is not true.
• When specifying trait bounds on a generic function, the preference flips: bound on TryInto/Into so callers that implement only those traits still work.

---

Async / Tokio

• Prefer async/await for I/O and network-bound code; use Tokio as the runtime.
• In async contexts, use tokio::fs / tokio::io instead of blocking std::fs / std::io.
• If you must call blocking or CPU-heavy code from async (crypto, large serialization, filesystem walks), isolate it with tokio::task::spawn_blocking.
• Prefer Tokio sync primitives (tokio::sync::*) over std::sync::* when tasks may .await.
• Use tokio::time for timeouts, sleeps, and intervals (avoid std::thread::sleep).

---

Code Style

• Naming: modules/functions snake_case, types PascalCase, constants SCREAMING_SNAKE_CASE.
• Formatting: prefer named arguments in formatting macros:
  • ✅ format!("hello {name}")
  • ❌ format!("hello {}", name)
• Documentation:
  • Prefer copying doc comments from Go and adapting to Rust conventions (avoid “Type is a …”).
  • Avoid leaving TODOs in merged code. If a short-lived internal note is necessary, use // TODO: and remove before PR merge.

Generalized Parameter Types

Prefer generic parameters over concrete types when a function only needs the behavior of a trait. This mirrors the standard library's own conventions and makes functions callable with a wider range of inputs without extra allocations.

Instead of	Prefer	Accepts
&str	impl AsRef<str>	&str, String, &String, …
&Path	impl AsRef<Path>	&str, String, PathBuf, &Path, …
&[u8]	impl AsRef<[u8]>	&[u8], Vec<u8>, arrays, …
&Vec<T>	impl AsRef<[T]>	Vec<T>, slices, arrays, …
String (owned, read-only)	impl Into<String>	&str, String, …

Examples:

// accepts &str, String, PathBuf, &Path, …
fn read_file(path: impl AsRef<std::path::Path>) -> std::io::Result<String> {
    std::fs::read_to_string(path.as_ref())
}

// accepts &str, String, &String, …
fn print_message(msg: impl AsRef<str>) {
    println!(“{}”, msg.as_ref());
}

// accepts &[u8], Vec<u8>, arrays, …
fn hash_bytes(data: impl AsRef<[u8]>) -> [u8; 32] {
    sha256(data.as_ref())
}

Rules:

• Call .as_ref() once at the top of the function and bind it to a local variable when the value is used in multiple places.
• Do not use impl AsRef<T> if the function immediately converts to an owned type anyway — use impl Into<T> (or just accept the owned type) in that case.
• Applies to public and private functions alike; the gain is ergonomics, not just API surface.

Testing

• Translate Go tests to Rust where applicable; keep similar test names for cross-reference.
• Reuse the same fixtures/golden files when possible.
• Use #[tokio::test] for async tests.
• Use test-case for repeated/parameterized tests (including async):

#[cfg(test)]
mod tests {
    use test_case::test_case;

    #[test_case(1, 2 ; "small")]
    #[test_case(10, 20 ; "large")]
    fn adds(a: u64, b: u64) {
        let _ = (a, b);
    }

    #[test_case("a" ; "case_a")]
    #[test_case("b" ; "case_b")]
    #[tokio::test]
    async fn async_cases(input: &str) {
        let _ = input;
    }
}

• For hashing/serialization parity, generate Go-derived test vectors and hardcode them as Rust fixtures.

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Pluto-Specific Checklist

Apply when reviewing or porting code:

• Ordering::SeqCst is justified; prefer Relaxed/AcqRel for standalone flags.
• Error::Io wraps std::io::Error (not String) to preserve ErrorKind.
• New public functions accept impl AsRef<[u8]> / impl AsRef<str> rather than concrete slice refs where appropriate.
• No unwrap() / expect() / panic!() outside test code.
• All arithmetic uses checked ops (checked_add, checked_mul, …).
• Tests mirror the Go test names and shapes where applicable.
• use declarations appear before all other items in each file.
• No dead payload in error variants (every captured field appears in the #[error("...")] string).
• Conversions implement TryFrom/From rather than TryInto/Into (not caught by Clippy for TryInto).