Trait From

1.0.0 · source

pub trait From<T>: Sized {
    // Required method
    fn from(value: T) -> Self;
}

Expand description

Used to do value-to-value conversions while consuming the input value. It is the reciprocal of Into.

One should always prefer implementing From over Into because implementing From automatically provides one with an implementation of Into thanks to the blanket implementation in the standard library.

Only implement Into when targeting a version prior to Rust 1.41 and converting to a type outside the current crate. From was not able to do these types of conversions in earlier versions because of Rust’s orphaning rules. See Into for more details.

Prefer using Into over using From when specifying trait bounds on a generic function. This way, types that directly implement Into can be used as arguments as well.

The From trait is also very useful when performing error handling. When constructing a function that is capable of failing, the return type will generally be of the form Result<T, E>. From simplifies error handling by allowing a function to return a single error type that encapsulates multiple error types. See the “Examples” section and the book for more details.

Note: This trait must not fail. The From trait is intended for perfect conversions. If the conversion can fail or is not perfect, use TryFrom.

§Generic Implementations

From<T> for U implies Into for T
From is reflexive, which means that From<T> for T is implemented

§When to implement `From`

While there’s no technical restrictions on which conversions can be done using a From implementation, the general expectation is that the conversions should typically be restricted as follows:

The conversion is infallible: if the conversion can fail, use TryFrom instead; don’t provide a From impl that panics.
The conversion is lossless: semantically, it should not lose or discard information. For example, i32: From<u16> exists, where the original value can be recovered using u16: TryFrom<i32>. And String: From<&str> exists, where you can get something equivalent to the original value via Deref. But From cannot be used to convert from u32 to u16, since that cannot succeed in a lossless way. (There’s some wiggle room here for information not considered semantically relevant. For example, Box<[T]>: From<Vec<T>> exists even though it might not preserve capacity, like how two vectors can be equal despite differing capacities.)
The conversion is value-preserving: the conceptual kind and meaning of the resulting value is the same, even though the Rust type and technical representation might be different. For example -1_i8 as u8 is lossless, since as casting back can recover the original value, but that conversion is not available via From because -1 and 255 are different conceptual values (despite being identical bit patterns technically). But f32: From<i16> is available because 1_i16 and 1.0_f32 are conceptually the same real number (despite having very different bit patterns technically). String: From<char> is available because they’re both text, but String: From<u32> is not available, since 1 (a number) and "1" (text) are too different. (Converting values to text is instead covered by the Display trait.)
The conversion is obvious: it’s the only reasonable conversion between the two types. Otherwise it’s better to have it be a named method or constructor, like how str::as_bytes is a method and how integers have methods like u32::from_ne_bytes, u32::from_le_bytes, and u32::from_be_bytes, none of which are From implementations. Whereas there’s only one reasonable way to wrap an Ipv6Addr into an IpAddr, thus IpAddr: From<Ipv6Addr> exists.

§Examples

String implements From<&str>:

An explicit conversion from a &str to a String is done as follows:

let string = "hello".to_string();
let other_string = String::from("hello");

assert_eq!(string, other_string);

While performing error handling it is often useful to implement From for your own error type. By converting underlying error types to our own custom error type that encapsulates the underlying error type, we can return a single error type without losing information on the underlying cause. The ‘?’ operator automatically converts the underlying error type to our custom error type with From::from.

use std::fs;
use std::io;
use std::num;

enum CliError {
    IoError(io::Error),
    ParseError(num::ParseIntError),
}

impl From<io::Error> for CliError {
    fn from(error: io::Error) -> Self {
        CliError::IoError(error)
    }
}

impl From<num::ParseIntError> for CliError {
    fn from(error: num::ParseIntError) -> Self {
        CliError::ParseError(error)
    }
}

fn open_and_parse_file(file_name: &str) -> Result<i32, CliError> {
    let mut contents = fs::read_to_string(&file_name)?;
    let num: i32 = contents.trim().parse()?;
    Ok(num)
}

Trait FromCopy item path

§Generic Implementations

§When to implement From

§Examples

Required Methods§

fn from(value: T) -> Self

Object Safety§

Implementors§

impl From<&str> for Box<str>

impl From<&str> for Rc<str>

impl From<&str> for String

impl From<&str> for Arc<str>

impl From<&str> for Vec<u8>

impl From<&CStr> for Box<CStr>

impl From<&CStr> for CString

impl From<&CStr> for Rc<CStr>

impl From<&CStr> for Arc<CStr>

impl From<&OsStr> for Box<OsStr>

impl From<&OsStr> for Rc<OsStr>

impl From<&OsStr> for Arc<OsStr>

impl From<&Path> for Box<Path>

impl From<&Path> for Rc<Path>

impl From<&Path> for Arc<Path>

impl From<&String> for String

impl From<&mut str> for String

impl From<AsciiChar> for char

impl From<AsciiChar> for u8

impl From<AsciiChar> for u16

impl From<AsciiChar> for u32

impl From<AsciiChar> for u64

impl From<AsciiChar> for u128

impl From<Cow<'_, str>> for Box<str>

impl From<Cow<'_, CStr>> for Box<CStr>

impl From<Cow<'_, OsStr>> for Box<OsStr>

impl From<Cow<'_, Path>> for Box<Path>

impl From<TryReserveErrorKind> for TryReserveError

impl From<ErrorKind> for Error

impl From<Infallible> for TryFromSliceError

impl From<Infallible> for TryFromIntError

impl From<bool> for f32

impl From<bool> for f64

impl From<bool> for i8

impl From<bool> for i16

impl From<bool> for i32

impl From<bool> for i64

impl From<bool> for i128

impl From<bool> for isize

impl From<bool> for u8

impl From<bool> for u16

impl From<bool> for u32

impl From<bool> for u64

impl From<bool> for u128

impl From<bool> for usize

impl From<bool> for AtomicBool

impl From<char> for u32

impl From<char> for u64

impl From<char> for u128

impl From<char> for String

impl From<f16> for f64

impl From<f16> for f128

impl From<f32> for f64

impl From<f32> for f128

impl From<f64> for f128

impl From<i8> for f32

impl From<i8> for f64

impl From<i8> for i16

impl From<i8> for i32

impl From<i8> for i64

impl From<i8> for i128

impl From<i8> for isize

impl From<i8> for AtomicI8

impl From<i16> for f32

impl From<i16> for f64

impl From<i16> for i32

impl From<i16> for i64

impl From<i16> for i128

impl From<i16> for isize

impl From<i16> for AtomicI16

impl From<i32> for f64

impl From<i32> for i64

Trait From

§When to implement `From`