pub trait From<T> {
fn from(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 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
From<T> for UimpliesInto<U> for TFromis reflexive, which means thatFrom<T> for Tis implemented
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 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 {
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)
}Required methods
Implementations on Foreign Types
Creates an IpAddr::V6 from an eight element 16-bit array.
Examples
use std::net::{IpAddr, Ipv6Addr};
let addr = IpAddr::from([
525u16, 524u16, 523u16, 522u16,
521u16, 520u16, 519u16, 518u16,
]);
assert_eq!(
IpAddr::V6(Ipv6Addr::new(
0x20d, 0x20c,
0x20b, 0x20a,
0x209, 0x208,
0x207, 0x206
)),
addr
);Create a new cell with its contents set to value.
Example
#![feature(once_cell)]
use std::lazy::SyncOnceCell;
let a = SyncOnceCell::from(3);
let b = SyncOnceCell::new();
b.set(3)?;
assert_eq!(a, b);
Ok(())Converts a File into a Stdio
Examples
File will be converted to Stdio using Stdio::from under the hood.
use std::fs::File;
use std::process::Command;
// With the `foo.txt` file containing `Hello, world!"
let file = File::open("foo.txt").unwrap();
let reverse = Command::new("rev")
.stdin(file) // Implicit File conversion into a Stdio
.output()
.expect("failed reverse command");
assert_eq!(reverse.stdout, b"!dlrow ,olleH");Intended for use for errors not exposed to the user, where allocating onto the heap (for normal construction via Error::new) is too costly.
Converts a ChildStderr into a Stdio
Examples
use std::process::{Command, Stdio};
let reverse = Command::new("rev")
.arg("non_existing_file.txt")
.stderr(Stdio::piped())
.spawn()
.expect("failed reverse command");
let cat = Command::new("cat")
.arg("-")
.stdin(reverse.stderr.unwrap()) // Converted into a Stdio here
.output()
.expect("failed echo command");
assert_eq!(
String::from_utf8_lossy(&cat.stdout),
"rev: cannot open non_existing_file.txt: No such file or directory\n"
);Converts a SocketAddrV4 into a SocketAddr::V4.
Converts a SocketAddrV6 into a SocketAddr::V6.
Converts a tuple struct (Into<IpAddr>, u16) into a SocketAddr.
This conversion creates a SocketAddr::V4 for an IpAddr::V4
and creates a SocketAddr::V6 for an IpAddr::V6.
u16 is treated as port of the newly created SocketAddr.
Converts a ChildStdout into a Stdio
Examples
ChildStdout will be converted to Stdio using Stdio::from under the hood.
use std::process::{Command, Stdio};
let hello = Command::new("echo")
.arg("Hello, world!")
.stdout(Stdio::piped())
.spawn()
.expect("failed echo command");
let reverse = Command::new("rev")
.stdin(hello.stdout.unwrap()) // Converted into a Stdio here
.output()
.expect("failed reverse command");
assert_eq!(reverse.stdout, b"!dlrow ,olleH\n");Creates an IpAddr::V6 from a sixteen element byte array.
Examples
use std::net::{IpAddr, Ipv6Addr};
let addr = IpAddr::from([
25u8, 24u8, 23u8, 22u8, 21u8, 20u8, 19u8, 18u8,
17u8, 16u8, 15u8, 14u8, 13u8, 12u8, 11u8, 10u8,
]);
assert_eq!(
IpAddr::V6(Ipv6Addr::new(
0x1918, 0x1716,
0x1514, 0x1312,
0x1110, 0x0f0e,
0x0d0c, 0x0b0a
)),
addr
);Creates an Ipv6Addr from an eight element 16-bit array.
Examples
use std::net::Ipv6Addr;
let addr = Ipv6Addr::from([
525u16, 524u16, 523u16, 522u16,
521u16, 520u16, 519u16, 518u16,
]);
assert_eq!(
Ipv6Addr::new(
0x20d, 0x20c,
0x20b, 0x20a,
0x209, 0x208,
0x207, 0x206
),
addr
);Converts a ChildStdin into a Stdio
Examples
ChildStdin will be converted to Stdio using Stdio::from under the hood.
use std::process::{Command, Stdio};
let reverse = Command::new("rev")
.stdin(Stdio::piped())
.spawn()
.expect("failed reverse command");
let _echo = Command::new("echo")
.arg("Hello, world!")
.stdout(reverse.stdin.unwrap()) // Converted into a Stdio here
.output()
.expect("failed echo command");
// "!dlrow ,olleH" echoed to consoleCreates an Ipv6Addr from a sixteen element byte array.
Examples
use std::net::Ipv6Addr;
let addr = Ipv6Addr::from([
25u8, 24u8, 23u8, 22u8, 21u8, 20u8, 19u8, 18u8,
17u8, 16u8, 15u8, 14u8, 13u8, 12u8, 11u8, 10u8,
]);
assert_eq!(
Ipv6Addr::new(
0x1918, 0x1716,
0x1514, 0x1312,
0x1110, 0x0f0e,
0x0d0c, 0x0b0a
),
addr
);Converts a NonZeroIsize into an isize
impl<T, const LANES: usize> From<Mask<T, LANES>> for [bool; LANES] where
T: MaskElement,
LaneCount<LANES>: SupportedLaneCount,
impl<T, const LANES: usize> From<Mask<T, LANES>> for [bool; LANES] where
T: MaskElement,
LaneCount<LANES>: SupportedLaneCount,
Converts from &mut Option<T> to Option<&mut T>
Examples
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!")));Converts a NonZeroI64 into an i64
Converts a NonZeroU64 into an u64
impl<T, const LANES: usize> From<[T; LANES]> for Simd<T, LANES> where
T: SimdElement,
LaneCount<LANES>: SupportedLaneCount,
impl<T, const LANES: usize> From<[T; LANES]> for Simd<T, LANES> where
T: SimdElement,
LaneCount<LANES>: SupportedLaneCount,
impl<T, const LANES: usize> From<Mask<T, LANES>> for Simd<T, LANES> where
T: MaskElement,
LaneCount<LANES>: SupportedLaneCount,
impl<T, const LANES: usize> From<Mask<T, LANES>> for Simd<T, LANES> where
T: MaskElement,
LaneCount<LANES>: SupportedLaneCount,
Converts a NonZeroI128 into an i128
Converts a NonZeroU32 into an u32
Converts a NonZeroU16 into an u16
impl<T, const LANES: usize> From<[bool; LANES]> for Mask<T, LANES> where
T: MaskElement,
LaneCount<LANES>: SupportedLaneCount,
impl<T, const LANES: usize> From<[bool; LANES]> for Mask<T, LANES> where
T: MaskElement,
LaneCount<LANES>: SupportedLaneCount,
impl<T, const LANES: usize> From<Simd<T, LANES>> for [T; LANES] where
T: SimdElement,
LaneCount<LANES>: SupportedLaneCount,
impl<T, const LANES: usize> From<Simd<T, LANES>> for [T; LANES] where
T: SimdElement,
LaneCount<LANES>: SupportedLaneCount,
Converts a NonZeroUsize into an usize
Converts a NonZeroU128 into an u128
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.
Converts from &Option<T> to Option<&T>.
Examples
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 from 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));Converts a NonZeroI16 into an i16
Converts a NonZeroI32 into an i32
use std::collections::BinaryHeap;
let mut h1 = BinaryHeap::from([1, 4, 2, 3]);
let mut h2: BinaryHeap<_> = [1, 4, 2, 3].into();
while let Some((a, b)) = h1.pop().zip(h2.pop()) {
assert_eq!(a, b);
}Converts a Vec<T> into a BinaryHeap<T>.
This conversion happens in-place, and has O(n) time complexity.
use std::collections::LinkedList;
let list1 = LinkedList::from([1, 2, 3, 4]);
let list2: LinkedList<_> = [1, 2, 3, 4].into();
assert_eq!(list1, list2);Always evaluates to TryReserveErrorKind::CapacityOverflow.
Converts a clone-on-write string to an owned
instance of String.
This extracts the owned string, clones the string if it is not already owned.
Example
// If the string is not owned...
let cow: Cow<str> = Cow::Borrowed("eggplant");
// It will allocate on the heap and copy the string.
let owned: String = String::from(cow);
assert_eq!(&owned[..], "eggplant");Implementors
1.45.0 · source
impl<'a, B> From<Cow<'a, B>> for Rc<B> where
B: ToOwned + ?Sized,
Rc<B>: From<&'a B>,
Rc<B>: From<<B as ToOwned>::Owned>,
1.45.0 · source
impl<'a, B> From<Cow<'a, B>> for Arc<B> where
B: ToOwned + ?Sized,
Arc<B>: From<&'a B>,
Arc<B>: From<<B as ToOwned>::Owned>,
impl<'a, E> From<E> for Box<dyn Error + Sync + Send + 'a, Global> where
E: 'a + Error + Send + Sync,
1.14.0 · source
impl<'a, T> From<Cow<'a, [T]>> for Vec<T, Global> where
[T]: ToOwned,
<[T] as ToOwned>::Owned == Vec<T, Global>,
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.