Rust Conversion Method Naming

Rule

Pick the prefix that matches the cost and ownership of the conversion:

• as_<type> — free or near-free, returns a borrow (e.g. as_bytes() -> &[u8]).
• to_<type> — non-trivial cost, returns an owned value, leaves self intact (e.g. to_string() -> String).
• into_<type> — consumes self, returns the inner/converted value.
• from_<type> — associated function on the target type that constructs it.
• with_<field> — only for builder-style methods that take and return Self with a field set.

Do not name a non-borrowing method as_*. Do not name a non-builder method with_*. Do not use to_* for a free borrow.

Why

The prefixes come from the Rust API guidelines and are baked into the standard library: readers expect as_ to be cheap, to_ to allocate, into_ to consume, and with_ to return Self. Violating that costs every reader a moment of confusion and erodes trust in the API.

Examples

// Good
impl Header {
    pub fn as_bytes(&self) -> &[u8] { ... }              // cheap borrow
    pub fn to_vec(&self) -> Vec<u8> { ... }              // allocates
    pub fn into_payload(self) -> Payload { self.payload } // consumes
}

impl HeaderBuilder {
    pub fn with_version(mut self, v: u8) -> Self { self.version = v; self }
}

// Bad
impl Header {
    pub fn as_vec(&self) -> Vec<u8> { ... }  // allocates, should be to_vec
    pub fn to_bytes(&self) -> &[u8] { ... }  // borrow, should be as_bytes
}

// Bad: with_* on a non-builder method
fn with_seed(rng: &mut Rng, seed: u64) { ... }   // not returning Self

Source: 0xMiden/protocol — distributed by TomeVault.