fn f>(t: T) accepts many sources.

TryFrom common; owned TryFrom avoids extra alloc when already owned.

From/Into & TryFrom

Q: From vs custom new?

new for domain semantics; TryFrom when converting from external representation.

Q: serde with TryFrom?

Deserialize to DTO then TryInto domain type in handler.

Q: TryFrom error type?

Use domain error enum, not String.

Q: clippy cast?

Prefer TryFrom over as for user input.

Q: test roundtrip?

assert_eq!(x, T::from(U::from(x))) when bijection holds.

Q: document conversions?

Rustdoc examples showing from, into, try_from.

Implement From and TryFrom for explicit, discoverable conversions between types. Prefer these over ad-hoc as casts and constructor naming soup.

Recipe

impl From<u64> for UserId {
    fn from(id: u64) -> Self { UserId(id) }
}
let id: UserId = 42.into();
 
impl TryFrom<String> for Email {
    type Error = ParseError;
    fn try_from(s: String) -> Result<Self, Self::Error> { Email::parse(s) }
}

When to reach for this: Boundary conversions, newtypes, parsing external input.

Working Example

struct Meters(f64);
struct Feet(f64);
 
impl From<Feet> for Meters {
    fn from(f: Feet) -> Self { Meters(f.0 * 0.3048) }
}
 
impl TryFrom<&str> for UserId {
    type Error = std::num::ParseIntError;
    fn try_from(s: &str) -> Result<Self, Self::Error> {
        Ok(UserId(s.parse()?))
    }
}

What this demonstrates:

Into auto-implemented when From exists
Fallible parsing uses TryFrom + ?
Conversions are symmetric and grep-friendly
API guidelines prefer From over into_foo methods

Deep Dive

Implement From on destination type. Chain with ? in handlers: let id = UserId::try_from(path_segment)?;. Avoid From that can fail; use TryFrom instead.

Gotchas

Lossy From - silent truncation. Fix: only infallible conversions in From; use TryFrom for validation.
From recursion - infinite loop. Fix: one direction only or distinct types.
Orphan rule - cannot impl From<Foreign> for Foreign. Fix: newtype wrapper.
Ambiguous Into - multiple targets. Fix: use turbofish UserId::from(x).
as casts for enums - unsafe for values. Fix: TryFrom<u8> for wire formats.

Alternatives

Alternative	Use When	Don't Use When
`parse()` inherent	single type string parse	cross-type standard trait
`as` primitive cast	numeric widening rules clear	domain validation
`map` on Option	light transform	public API conversion

FAQs

From vs custom new?

new for domain semantics; TryFrom when converting from external representation.

serde with TryFrom?

Deserialize to DTO then TryInto domain type in handler.

blanket From?

Avoid conflicting impls; follow orphan rules.

Into in generics?

fn f<T: Into<UserId>>(t: T) accepts many sources.

TryFrom error type?

Use domain error enum, not String.

ref conversions?

TryFrom<&str> common; owned TryFrom<String> avoids extra alloc when already owned.

clippy cast?

Prefer TryFrom over as for user input.

test roundtrip?

assert_eq!(x, T::from(U::from(x))) when bijection holds.

foreign crate type?

Newtype then From/TryFrom on newtype.

document conversions?

Rustdoc examples showing from, into, try_from.

Standard Library (std) Docs - Complete API reference for the standard library.
The Rust Programming Language (The Book) - Canonical beginner-to-intermediate guide.
Rust By Example - Runnable examples covering language features and standard library.
Programming Rust: Fast, Safe Systems Development (Amazon) - Blandy, Orendorff, Tindall.
Rust for Rustaceans (No Starch) - No Starch Press.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

Recipe

impl From<u64> for UserId {
    fn from(id: u64) -> Self { UserId(id) }
}
let id: UserId = 42.into();
 
impl TryFrom<String> for Email {
    type Error = ParseError;
    fn try_from(s: String) -> Result<Self, Self::Error> { Email::parse(s) }
}

When to reach for this: Boundary conversions, newtypes, parsing external input.

Working Example

struct Meters(f64);
struct Feet(f64);
 
impl From<Feet> for Meters {
    fn from(f: Feet) -> Self { Meters(f.0 * 0.3048) }
}
 
impl TryFrom<&str> for UserId {
    type Error = std::num::ParseIntError;
    fn try_from(s: &str) -> Result<Self, Self::Error> {
        Ok(UserId(s.parse()?))
    }
}

What this demonstrates:

Into auto-implemented when From exists
Fallible parsing uses TryFrom + ?
Conversions are symmetric and grep-friendly
API guidelines prefer From over into_foo methods

Deep Dive

Implement From on destination type. Chain with ? in handlers: let id = UserId::try_from(path_segment)?;. Avoid From that can fail; use TryFrom instead.

Gotchas

Lossy From - silent truncation. Fix: only infallible conversions in From; use TryFrom for validation.
From recursion - infinite loop. Fix: one direction only or distinct types.
Orphan rule - cannot impl From<Foreign> for Foreign. Fix: newtype wrapper.
Ambiguous Into - multiple targets. Fix: use turbofish UserId::from(x).
as casts for enums - unsafe for values. Fix: TryFrom<u8> for wire formats.

Alternatives

Alternative	Use When	Don't Use When
`parse()` inherent	single type string parse	cross-type standard trait
`as` primitive cast	numeric widening rules clear	domain validation
`map` on Option	light transform	public API conversion

FAQs

From vs custom new?

new for domain semantics; TryFrom when converting from external representation.

serde with TryFrom?

Deserialize to DTO then TryInto domain type in handler.

blanket From?

Avoid conflicting impls; follow orphan rules.

Into in generics?

fn f<T: Into<UserId>>(t: T) accepts many sources.

TryFrom error type?

Use domain error enum, not String.

ref conversions?

TryFrom<&str> common; owned TryFrom<String> avoids extra alloc when already owned.

clippy cast?

Prefer TryFrom over as for user input.

test roundtrip?

assert_eq!(x, T::from(U::from(x))) when bijection holds.

foreign crate type?

Newtype then From/TryFrom on newtype.

document conversions?

Rustdoc examples showing from, into, try_from.

Standard Library (std) Docs - Complete API reference for the standard library.
The Rust Programming Language (The Book) - Canonical beginner-to-intermediate guide.
Rust By Example - Runnable examples covering language features and standard library.
Programming Rust: Fast, Safe Systems Development (Amazon) - Blandy, Orendorff, Tindall.
Rust for Rustaceans (No Starch) - No Starch Press.

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

From/Into & TryFrom

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From/Into & TryFrom