Trait core::convert::From1.0.0[][src]

pub trait From<T>: Sized {
#[lang = "from"]    fn from(_: T) -> Self;

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 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>. The From trait simplifies error handling by allowing a function to return a single error type that encapsulate multiple error types. See the “Examples” section and the book for more details.

Note: This trait must not fail. If the conversion can fail, use TryFrom.

Generic Implementations


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 by calling Into<CliError>::into which is automatically provided when implementing From. The compiler then infers which implementation of Into should be used.

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

enum CliError {

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

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

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()?;

Required methods

#[lang = "from"]fn from(_: T) -> Self[src]

Performs the conversion.

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impl From<!> for Infallible1.34.0[src]

impl From<!> for TryFromIntError[src]

impl From<Infallible> for TryFromSliceError1.36.0[src]

impl From<Infallible> for TryFromIntError1.34.0[src]

impl From<NonZeroI8> for NonZeroI161.41.0[src]

Converts NonZeroI8 to NonZeroI16 losslessly.

impl From<NonZeroI8> for NonZeroI321.41.0[src]

Converts NonZeroI8 to NonZeroI32 losslessly.

impl From<NonZeroI8> for NonZeroI641.41.0[src]

Converts NonZeroI8 to NonZeroI64 losslessly.

impl From<NonZeroI8> for NonZeroI1281.41.0[src]

Converts NonZeroI8 to NonZeroI128 losslessly.

impl From<NonZeroI8> for NonZeroIsize1.41.0[src]

Converts NonZeroI8 to NonZeroIsize losslessly.

impl From<NonZeroI8> for i81.31.0[src]

fn from(nonzero: NonZeroI8) -> Self[src]

Converts a NonZeroI8 into an i8

impl From<NonZeroI16> for NonZeroI321.41.0[src]

Converts NonZeroI16 to NonZeroI32 losslessly.

impl From<NonZeroI16> for NonZeroI641.41.0[src]

Converts NonZeroI16 to NonZeroI64 losslessly.

impl From<NonZeroI16> for NonZeroI1281.41.0[src]

Converts NonZeroI16 to NonZeroI128 losslessly.

impl From<NonZeroI16> for NonZeroIsize1.41.0[src]

Converts NonZeroI16 to NonZeroIsize losslessly.

impl From<NonZeroI16> for i161.31.0[src]

fn from(nonzero: NonZeroI16) -> Self[src]

Converts a NonZeroI16 into an i16

impl From<NonZeroI32> for NonZeroI641.41.0[src]

Converts NonZeroI32 to NonZeroI64 losslessly.

impl From<NonZeroI32> for NonZeroI1281.41.0[src]

Converts NonZeroI32 to NonZeroI128 losslessly.

impl From<NonZeroI32> for i321.31.0[src]

fn from(nonzero: NonZeroI32) -> Self[src]

Converts a NonZeroI32 into an i32

impl From<NonZeroI64> for NonZeroI1281.41.0[src]

Converts NonZeroI64 to NonZeroI128 losslessly.

impl From<NonZeroI64> for i641.31.0[src]

fn from(nonzero: NonZeroI64) -> Self[src]

Converts a NonZeroI64 into an i64

impl From<NonZeroI128> for i1281.31.0[src]

fn from(nonzero: NonZeroI128) -> Self[src]

Converts a NonZeroI128 into an i128

impl From<NonZeroIsize> for isize1.31.0[src]

fn from(nonzero: NonZeroIsize) -> Self[src]

Converts a NonZeroIsize into an isize

impl From<NonZeroU8> for NonZeroI161.41.0[src]

Converts NonZeroU8 to NonZeroI16 losslessly.

impl From<NonZeroU8> for NonZeroI321.41.0[src]

Converts NonZeroU8 to NonZeroI32 losslessly.

impl From<NonZeroU8> for NonZeroI641.41.0[src]

Converts NonZeroU8 to NonZeroI64 losslessly.

impl From<NonZeroU8> for NonZeroI1281.41.0[src]

Converts NonZeroU8 to NonZeroI128 losslessly.

impl From<NonZeroU8> for NonZeroIsize1.41.0[src]

Converts NonZeroU8 to NonZeroIsize losslessly.

impl From<NonZeroU8> for NonZeroU161.41.0[src]

Converts NonZeroU8 to NonZeroU16 losslessly.

impl From<NonZeroU8> for NonZeroU321.41.0[src]

Converts NonZeroU8 to NonZeroU32 losslessly.

impl From<NonZeroU8> for NonZeroU641.41.0[src]

Converts NonZeroU8 to NonZeroU64 losslessly.

impl From<NonZeroU8> for NonZeroU1281.41.0[src]

Converts NonZeroU8 to NonZeroU128 losslessly.

impl From<NonZeroU8> for NonZeroUsize1.41.0[src]

Converts NonZeroU8 to NonZeroUsize losslessly.

impl From<NonZeroU8> for u81.31.0[src]

fn from(nonzero: NonZeroU8) -> Self[src]

Converts a NonZeroU8 into an u8

impl From<NonZeroU16> for NonZeroI321.41.0[src]

Converts NonZeroU16 to NonZeroI32 losslessly.

impl From<NonZeroU16> for NonZeroI641.41.0[src]

Converts NonZeroU16 to NonZeroI64 losslessly.

impl From<NonZeroU16> for NonZeroI1281.41.0[src]

Converts NonZeroU16 to NonZeroI128 losslessly.

impl From<NonZeroU16> for NonZeroU321.41.0[src]

Converts NonZeroU16 to NonZeroU32 losslessly.

impl From<NonZeroU16> for NonZeroU641.41.0[src]

Converts NonZeroU16 to NonZeroU64 losslessly.

impl From<NonZeroU16> for NonZeroU1281.41.0[src]

Converts NonZeroU16 to NonZeroU128 losslessly.

impl From<NonZeroU16> for NonZeroUsize1.41.0[src]

Converts NonZeroU16 to NonZeroUsize losslessly.

impl From<NonZeroU16> for u161.31.0[src]

fn from(nonzero: NonZeroU16) -> Self[src]

Converts a NonZeroU16 into an u16

impl From<NonZeroU32> for NonZeroI641.41.0[src]

Converts NonZeroU32 to NonZeroI64 losslessly.

impl From<NonZeroU32> for NonZeroI1281.41.0[src]

Converts NonZeroU32 to NonZeroI128 losslessly.

impl From<NonZeroU32> for NonZeroU641.41.0[src]

Converts NonZeroU32 to NonZeroU64 losslessly.

impl From<NonZeroU32> for NonZeroU1281.41.0[src]

Converts NonZeroU32 to NonZeroU128 losslessly.

impl From<NonZeroU32> for u321.31.0[src]

fn from(nonzero: NonZeroU32) -> Self[src]

Converts a NonZeroU32 into an u32

impl From<NonZeroU64> for NonZeroI1281.41.0[src]

Converts NonZeroU64 to NonZeroI128 losslessly.

impl From<NonZeroU64> for NonZeroU1281.41.0[src]

Converts NonZeroU64 to NonZeroU128 losslessly.

impl From<NonZeroU64> for u641.31.0[src]

fn from(nonzero: NonZeroU64) -> Self[src]

Converts a NonZeroU64 into an u64

impl From<NonZeroU128> for u1281.31.0[src]

fn from(nonzero: NonZeroU128) -> Self[src]

Converts a NonZeroU128 into an u128

impl From<NonZeroUsize> for usize1.31.0[src]

fn from(nonzero: NonZeroUsize) -> Self[src]

Converts a NonZeroUsize into an usize

impl From<bool> for AtomicBool1.24.0[src]

fn from(b: bool) -> Self[src]

Converts a bool into an AtomicBool.


use std::sync::atomic::AtomicBool;
let atomic_bool = AtomicBool::from(true);
assert_eq!(format!("{:?}", atomic_bool), "true")

impl From<bool> for i81.28.0[src]

Converts a bool to a i8. The resulting value is 0 for false and 1 for true values.


assert_eq!(i8::from(true), 1);
assert_eq!(i8::from(false), 0);

impl From<bool> for i161.28.0[src]

Converts a bool to a i16. The resulting value is 0 for false and 1 for true values.


assert_eq!(i16::from(true), 1);
assert_eq!(i16::from(false), 0);

impl From<bool> for i321.28.0[src]

Converts a bool to a i32. The resulting value is 0 for false and 1 for true values.


assert_eq!(i32::from(true), 1);
assert_eq!(i32::from(false), 0);

impl From<bool> for i641.28.0[src]

Converts a bool to a i64. The resulting value is 0 for false and 1 for true values.


assert_eq!(i64::from(true), 1);
assert_eq!(i64::from(false), 0);

impl From<bool> for i1281.28.0[src]

Converts a bool to a i128. The resulting value is 0 for false and 1 for true values.


assert_eq!(i128::from(true), 1);
assert_eq!(i128::from(false), 0);

impl From<bool> for isize1.28.0[src]

Converts a bool to a isize. The resulting value is 0 for false and 1 for true values.


assert_eq!(isize::from(true), 1);
assert_eq!(isize::from(false), 0);

impl From<bool> for u81.28.0[src]

Converts a bool to a u8. The resulting value is 0 for false and 1 for true values.


assert_eq!(u8::from(true), 1);
assert_eq!(u8::from(false), 0);

impl From<bool> for u161.28.0[src]

Converts a bool to a u16. The resulting value is 0 for false and 1 for true values.


assert_eq!(u16::from(true), 1);
assert_eq!(u16::from(false), 0);

impl From<bool> for u321.28.0[src]

Converts a bool to a u32. The resulting value is 0 for false and 1 for true values.


assert_eq!(u32::from(true), 1);
assert_eq!(u32::from(false), 0);

impl From<bool> for u641.28.0[src]

Converts a bool to a u64. The resulting value is 0 for false and 1 for true values.


assert_eq!(u64::from(true), 1);
assert_eq!(u64::from(false), 0);

impl From<bool> for u1281.28.0[src]

Converts a bool to a u128. The resulting value is 0 for false and 1 for true values.


assert_eq!(u128::from(true), 1);
assert_eq!(u128::from(false), 0);

impl From<bool> for usize1.28.0[src]

Converts a bool to a usize. The resulting value is 0 for false and 1 for true values.


assert_eq!(usize::from(true), 1);
assert_eq!(usize::from(false), 0);

impl From<char> for u321.13.0[src]

fn from(c: char) -> Self[src]

Converts a char into a u32.


use std::mem;

let c = 'c';
let u = u32::from(c);
assert!(4 == mem::size_of_val(&u))

impl From<char> for u641.51.0[src]

fn from(c: char) -> Self[src]

Converts a char into a u64.


use std::mem;

let c = '👤';
let u = u64::from(c);
assert!(8 == mem::size_of_val(&u))

impl From<char> for u1281.51.0[src]

fn from(c: char) -> Self[src]

Converts a char into a u128.


use std::mem;

let c = '⚙';
let u = u128::from(c);
assert!(16 == mem::size_of_val(&u))

impl From<f32> for f641.6.0[src]

Converts f32 to f64 losslessly.

impl From<i8> for AtomicI81.34.0[src]

fn from(v: i8) -> Self[src]

Converts an i8 into an AtomicI8.

impl From<i8> for f321.6.0[src]

Converts i8 to f32 losslessly.

impl From<i8> for f641.6.0[src]

Converts i8 to f64 losslessly.

impl From<i8> for i161.5.0[src]

Converts i8 to i16 losslessly.

impl From<i8> for i321.5.0[src]

Converts i8 to i32 losslessly.

impl From<i8> for i641.5.0[src]

Converts i8 to i64 losslessly.

impl From<i8> for i1281.26.0[src]

Converts i8 to i128 losslessly.

impl From<i8> for isize1.5.0[src]

Converts i8 to isize losslessly.

impl From<i16> for AtomicI161.34.0[src]

fn from(v: i16) -> Self[src]

Converts an i16 into an AtomicI16.

impl From<i16> for f321.6.0[src]

Converts i16 to f32 losslessly.

impl From<i16> for f641.6.0[src]

Converts i16 to f64 losslessly.

impl From<i16> for i321.5.0[src]

Converts i16 to i32 losslessly.

impl From<i16> for i641.5.0[src]

Converts i16 to i64 losslessly.

impl From<i16> for i1281.26.0[src]

Converts i16 to i128 losslessly.

impl From<i16> for isize1.26.0[src]

Converts i16 to isize losslessly.

impl From<i32> for AtomicI321.34.0[src]

fn from(v: i32) -> Self[src]

Converts an i32 into an AtomicI32.

impl From<i32> for f641.6.0[src]

Converts i32 to f64 losslessly.

impl From<i32> for i641.5.0[src]

Converts i32 to i64 losslessly.

impl From<i32> for i1281.26.0[src]

Converts i32 to i128 losslessly.

impl From<i64> for AtomicI641.34.0[src]

fn from(v: i64) -> Self[src]

Converts an i64 into an AtomicI64.

impl From<i64> for i1281.26.0[src]

Converts i64 to i128 losslessly.

impl From<isize> for AtomicIsize1.23.0[src]

fn from(v: isize) -> Self[src]

Converts an isize into an AtomicIsize.

impl From<u8> for AtomicU81.34.0[src]

fn from(v: u8) -> Self[src]

Converts an u8 into an AtomicU8.

impl From<u8> for char1.13.0[src]

Maps a byte in 0x00..=0xFF to a char whose code point has the same value, in U+0000..=U+00FF.

Unicode is designed such that this effectively decodes bytes with the character encoding that IANA calls ISO-8859-1. This encoding is compatible with ASCII.

Note that this is different from ISO/IEC 8859-1 a.k.a. ISO 8859-1 (with one less hyphen), which leaves some “blanks”, byte values that are not assigned to any character. ISO-8859-1 (the IANA one) assigns them to the C0 and C1 control codes.

Note that this is also different from Windows-1252 a.k.a. code page 1252, which is a superset ISO/IEC 8859-1 that assigns some (not all!) blanks to punctuation and various Latin characters.

To confuse things further, on the Web ascii, iso-8859-1, and windows-1252 are all aliases for a superset of Windows-1252 that fills the remaining blanks with corresponding C0 and C1 control codes.

fn from(i: u8) -> Self[src]

Converts a u8 into a char.


use std::mem;

let u = 32 as u8;
let c = char::from(u);
assert!(4 == mem::size_of_val(&c))

impl From<u8> for f321.6.0[src]

Converts u8 to f32 losslessly.

impl From<u8> for f641.6.0[src]

Converts u8 to f64 losslessly.

impl From<u8> for i161.5.0[src]

Converts u8 to i16 losslessly.

impl From<u8> for i321.5.0[src]

Converts u8 to i32 losslessly.

impl From<u8> for i641.5.0[src]

Converts u8 to i64 losslessly.

impl From<u8> for i1281.26.0[src]

Converts u8 to i128 losslessly.

impl From<u8> for isize1.26.0[src]

Converts u8 to isize losslessly.

impl From<u8> for u161.5.0[src]

Converts u8 to u16 losslessly.

impl From<u8> for u321.5.0[src]

Converts u8 to u32 losslessly.

impl From<u8> for u641.5.0[src]

Converts u8 to u64 losslessly.

impl From<u8> for u1281.26.0[src]

Converts u8 to u128 losslessly.

impl From<u8> for usize1.5.0[src]

Converts u8 to usize losslessly.

impl From<u16> for AtomicU161.34.0[src]

fn from(v: u16) -> Self[src]

Converts an u16 into an AtomicU16.

impl From<u16> for f321.6.0[src]

Converts u16 to f32 losslessly.

impl From<u16> for f641.6.0[src]

Converts u16 to f64 losslessly.

impl From<u16> for i321.5.0[src]

Converts u16 to i32 losslessly.

impl From<u16> for i641.5.0[src]

Converts u16 to i64 losslessly.

impl From<u16> for i1281.26.0[src]

Converts u16 to i128 losslessly.

impl From<u16> for u321.5.0[src]

Converts u16 to u32 losslessly.

impl From<u16> for u641.5.0[src]

Converts u16 to u64 losslessly.

impl From<u16> for u1281.26.0[src]

Converts u16 to u128 losslessly.

impl From<u16> for usize1.26.0[src]

Converts u16 to usize losslessly.

impl From<u32> for AtomicU321.34.0[src]

fn from(v: u32) -> Self[src]

Converts an u32 into an AtomicU32.

impl From<u32> for f641.6.0[src]

Converts u32 to f64 losslessly.

impl From<u32> for i641.5.0[src]

Converts u32 to i64 losslessly.

impl From<u32> for i1281.26.0[src]

Converts u32 to i128 losslessly.

impl From<u32> for u641.5.0[src]

Converts u32 to u64 losslessly.

impl From<u32> for u1281.26.0[src]

Converts u32 to u128 losslessly.

impl From<u64> for AtomicU641.34.0[src]

fn from(v: u64) -> Self[src]

Converts an u64 into an AtomicU64.

impl From<u64> for i1281.26.0[src]

Converts u64 to i128 losslessly.

impl From<u64> for u1281.26.0[src]

Converts u64 to u128 losslessly.

impl From<usize> for AtomicUsize1.23.0[src]

fn from(v: usize) -> Self[src]

Converts an usize into an AtomicUsize.

impl<'a, T> From<&'a Option<T>> for Option<&'a T>1.30.0[src]

fn from(o: &'a Option<T>) -> Option<&'a T>[src]

Converts from &Option<T> to Option<&T>.


Converts an Option<String> into an Option<usize>, preserving the original. The map method takes the self argument by value, consuming the original, so this technique uses as_ref to first take an Option to a reference to the value inside the original.

let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());

println!("Can still print s: {:?}", s);

assert_eq!(o, Some(18));

impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T>1.30.0[src]

fn from(o: &'a mut Option<T>) -> Option<&'a mut T>[src]

Converts from &mut Option<T> to Option<&mut T>


let mut s = Some(String::from("Hello"));
let o: Option<&mut String> = Option::from(&mut s);

match o {
    Some(t) => *t = String::from("Hello, Rustaceans!"),
    None => (),

assert_eq!(s, Some(String::from("Hello, Rustaceans!")));

impl<T> From<!> for T1.34.0[src]

Stability note: This impl does not yet exist, but we are “reserving space” to add it in the future. See rust-lang/rust#64715 for details.

impl<T> From<*mut T> for AtomicPtr<T>1.23.0[src]

impl<T> From<T> for Option<T>1.12.0[src]

fn from(val: T) -> Option<T>[src]

Copies val into a new Some.


let o: Option<u8> = Option::from(67);

assert_eq!(Some(67), o);

impl<T> From<T> for Poll<T>1.36.0[src]

fn from(t: T) -> Poll<T>[src]

Convert to a Ready variant.


assert_eq!(Poll::from(true), Poll::Ready(true));

impl<T> From<T> for Cell<T>1.12.0[src]

impl<T> From<T> for RefCell<T>1.12.0[src]

impl<T> From<T> for UnsafeCell<T>1.12.0[src]

impl<T> From<T> for OnceCell<T>[src]

impl<T> From<T> for T[src]

impl<T: ?Sized> From<&'_ T> for NonNull<T>1.25.0[src]

impl<T: ?Sized> From<&'_ mut T> for NonNull<T>1.25.0[src]

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