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
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. 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
impliesInto
<U> for T
From
is reflexive, which means thatFrom<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 aFrom
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 usingu16: TryFrom<i32>
. AndString: From<&str>
exists, where you can get something equivalent to the original value viaDeref
. ButFrom
cannot be used to convert fromu32
tou16
, 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, sinceas
casting back can recover the original value, but that conversion is not available viaFrom
because-1
and255
are different conceptual values (despite being identical bit patterns technically). Butf32: From<i16>
is available because1_i16
and1.0_f32
are conceptually the same real number (despite having very different bit patterns technically).String: From<char>
is available because they’re both text, butString: From<u32>
is not available, since1
(a number) and"1"
(text) are too different. (Converting values to text is instead covered by theDisplay
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 likeu32::from_ne_bytes
,u32::from_le_bytes
, andu32::from_be_bytes
, none of which areFrom
implementations. Whereas there’s only one reasonable way to wrap anIpv6Addr
into anIpAddr
, thusIpAddr: 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);
RunWhile 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)
}
RunRequired Methods§
Object Safety§
Implementors§
impl From<&str> for Box<str>
impl From<&str> for Box<dyn Error>
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
Intended for use for errors not exposed to the user, where allocating onto the heap (for normal construction via Error::new) is too costly.
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<f32> for f64
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
impl From<i32> for i128
impl From<i32> for AtomicI32
impl From<i64> for i128
impl From<i64> for AtomicI64
impl From<isize> for AtomicIsize
impl From<!> for Infallible
impl From<!> for TryFromIntError
impl From<u8> for char
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.
impl From<u8> for f32
impl From<u8> for f64
impl From<u8> for i16
impl From<u8> for i32
impl From<u8> for i64
impl From<u8> for i128
impl From<u8> for isize
impl From<u8> for u16
impl From<u8> for u32
impl From<u8> for u64
impl From<u8> for u128
impl From<u8> for usize
impl From<u8> for ExitCode
impl From<u8> for AtomicU8
impl From<u16> for f32
impl From<u16> for f64
impl From<u16> for i32
impl From<u16> for i64
impl From<u16> for i128
impl From<u16> for u32
impl From<u16> for u64
impl From<u16> for u128
impl From<u16> for usize
impl From<u16> for AtomicU16
impl From<u32> for f64
impl From<u32> for i64
impl From<u32> for i128
impl From<u32> for u64
impl From<u32> for u128
impl From<u32> for Ipv4Addr
impl From<u32> for AtomicU32
impl From<u64> for i128
impl From<u64> for u128
impl From<u64> for AtomicU64
impl From<u128> for Ipv6Addr
impl From<usize> for AtomicUsize
impl From<__m128> for Simd<f32, 4>
impl From<__m128d> for Simd<f64, 2>
impl From<__m128i> for Simd<i8, 16>
impl From<__m128i> for Simd<i16, 8>
impl From<__m128i> for Simd<i32, 4>
impl From<__m128i> for Simd<i64, 2>
impl From<__m128i> for Simd<isize, 2>
impl From<__m128i> for Simd<u8, 16>
impl From<__m128i> for Simd<u16, 8>
impl From<__m128i> for Simd<u32, 4>
impl From<__m128i> for Simd<u64, 2>
impl From<__m128i> for Simd<usize, 2>
impl From<__m256> for Simd<f32, 8>
impl From<__m256d> for Simd<f64, 4>
impl From<__m256i> for Simd<i8, 32>
impl From<__m256i> for Simd<i16, 16>
impl From<__m256i> for Simd<i32, 8>
impl From<__m256i> for Simd<i64, 4>
impl From<__m256i> for Simd<isize, 4>
impl From<__m256i> for Simd<u8, 32>
impl From<__m256i> for Simd<u16, 16>
impl From<__m256i> for Simd<u32, 8>
impl From<__m256i> for Simd<u64, 4>
impl From<__m256i> for Simd<usize, 4>
impl From<__m512> for Simd<f32, 16>
impl From<__m512d> for Simd<f64, 8>
impl From<__m512i> for Simd<i8, 64>
impl From<__m512i> for Simd<i16, 32>
impl From<__m512i> for Simd<i32, 16>
impl From<__m512i> for Simd<i64, 8>
impl From<__m512i> for Simd<isize, 8>
impl From<__m512i> for Simd<u8, 64>
impl From<__m512i> for Simd<u16, 32>
impl From<__m512i> for Simd<u32, 16>
impl From<__m512i> for Simd<u64, 8>
impl From<__m512i> for Simd<usize, 8>
impl From<LayoutError> for TryReserveErrorKind
impl From<Box<str>> for String
impl From<Box<CStr>> for CString
impl From<Box<OsStr>> for OsString
impl From<Box<Path>> for PathBuf
impl From<CString> for Box<CStr>
impl From<CString> for Rc<CStr>
impl From<CString> for Arc<CStr>
impl From<CString> for Vec<u8>
impl From<NulError> for Error
impl From<OsString> for Box<OsStr>
impl From<OsString> for PathBuf
impl From<OsString> for Rc<OsStr>
impl From<OsString> for Arc<OsStr>
impl From<File> for OwnedFd
impl From<File> for OwnedHandle
impl From<File> for Stdio
impl From<Stderr> for Stdio
impl From<Stdout> for Stdio
impl From<Ipv4Addr> for IpAddr
impl From<Ipv4Addr> for u32
impl From<Ipv6Addr> for IpAddr
impl From<Ipv6Addr> for u128
impl From<SocketAddrV4> for SocketAddr
impl From<SocketAddrV6> for SocketAddr
impl From<TcpListener> for OwnedFd
impl From<TcpListener> for OwnedSocket
impl From<TcpStream> for OwnedFd
impl From<TcpStream> for OwnedSocket
impl From<UdpSocket> for OwnedFd
impl From<UdpSocket> for OwnedSocket
impl From<NonZero<i8>> for i8
impl From<NonZero<i8>> for NonZero<i16>
impl From<NonZero<i8>> for NonZero<i32>
impl From<NonZero<i8>> for NonZero<i64>
impl From<NonZero<i8>> for NonZero<i128>
impl From<NonZero<i8>> for NonZero<isize>
impl From<NonZero<i16>> for i16
impl From<NonZero<i16>> for NonZero<i32>
impl From<NonZero<i16>> for NonZero<i64>
impl From<NonZero<i16>> for NonZero<i128>
impl From<NonZero<i16>> for NonZero<isize>
impl From<NonZero<i32>> for i32
impl From<NonZero<i32>> for NonZero<i64>
impl From<NonZero<i32>> for NonZero<i128>
impl From<NonZero<i64>> for i64
impl From<NonZero<i64>> for NonZero<i128>
impl From<NonZero<i128>> for i128
impl From<NonZero<isize>> for isize
impl From<NonZero<u8>> for u8
impl From<NonZero<u8>> for NonZero<i16>
impl From<NonZero<u8>> for NonZero<i32>
impl From<NonZero<u8>> for NonZero<i64>
impl From<NonZero<u8>> for NonZero<i128>
impl From<NonZero<u8>> for NonZero<isize>
impl From<NonZero<u8>> for NonZero<u16>
impl From<NonZero<u8>> for NonZero<u32>
impl From<NonZero<u8>> for NonZero<u64>
impl From<NonZero<u8>> for NonZero<u128>
impl From<NonZero<u8>> for NonZero<usize>
impl From<NonZero<u16>> for u16
impl From<NonZero<u16>> for NonZero<i32>
impl From<NonZero<u16>> for NonZero<i64>
impl From<NonZero<u16>> for NonZero<i128>
impl From<NonZero<u16>> for NonZero<u32>
impl From<NonZero<u16>> for NonZero<u64>
impl From<NonZero<u16>> for NonZero<u128>
impl From<NonZero<u16>> for NonZero<usize>
impl From<NonZero<u32>> for u32
impl From<NonZero<u32>> for NonZero<i64>
impl From<NonZero<u32>> for NonZero<i128>
impl From<NonZero<u32>> for NonZero<u64>
impl From<NonZero<u32>> for NonZero<u128>
impl From<NonZero<u64>> for u64
impl From<NonZero<u64>> for NonZero<i128>
impl From<NonZero<u64>> for NonZero<u128>
impl From<NonZero<u128>> for u128
impl From<NonZero<usize>> for usize
impl From<OwnedFd> for File
impl From<OwnedFd> for TcpListener
impl From<OwnedFd> for TcpStream
impl From<OwnedFd> for UdpSocket
impl From<OwnedFd> for PidFd
impl From<OwnedFd> for UnixDatagram
impl From<OwnedFd> for UnixListener
impl From<OwnedFd> for UnixStream
impl From<OwnedFd> for ChildStderr
Create a ChildStderr
from the provided OwnedFd
.
The provided file descriptor must point to a pipe
with the CLOEXEC
flag set.
impl From<OwnedFd> for ChildStdin
Create a ChildStdin
from the provided OwnedFd
.
The provided file descriptor must point to a pipe
with the CLOEXEC
flag set.
impl From<OwnedFd> for ChildStdout
Create a ChildStdout
from the provided OwnedFd
.
The provided file descriptor must point to a pipe
with the CLOEXEC
flag set.
impl From<OwnedFd> for Stdio
impl From<PidFd> for OwnedFd
impl From<UnixDatagram> for OwnedFd
impl From<UnixListener> for OwnedFd
impl From<UnixStream> for OwnedFd
impl From<OwnedHandle> for File
impl From<OwnedHandle> for ChildStderr
Create a ChildStderr
from the provided OwnedHandle
.
The provided handle must be asynchronous, as reading and writing from and to it is implemented using asynchronous APIs.
impl From<OwnedHandle> for ChildStdin
Create a ChildStdin
from the provided OwnedHandle
.
The provided handle must be asynchronous, as reading and writing from and to it is implemented using asynchronous APIs.
impl From<OwnedHandle> for ChildStdout
Create a ChildStdout
from the provided OwnedHandle
.
The provided handle must be asynchronous, as reading and writing from and to it is implemented using asynchronous APIs.
impl From<OwnedHandle> for Stdio
impl From<OwnedSocket> for TcpListener
impl From<OwnedSocket> for TcpStream
impl From<OwnedSocket> for UdpSocket
impl From<PathBuf> for Box<Path>
impl From<PathBuf> for OsString
impl From<PathBuf> for Rc<Path>
impl From<PathBuf> for Arc<Path>
impl From<Child> for OwnedHandle
impl From<ChildStderr> for OwnedFd
impl From<ChildStderr> for OwnedHandle
impl From<ChildStderr> for Stdio
impl From<ChildStdin> for OwnedFd
impl From<ChildStdin> for OwnedHandle
impl From<ChildStdin> for Stdio
impl From<ChildStdout> for OwnedFd
impl From<ChildStdout> for OwnedHandle
impl From<ChildStdout> for Stdio
impl From<ExitStatusError> for ExitStatus
impl From<Alignment> for usize
impl From<Alignment> for NonZero<usize>
impl From<Rc<str>> for Rc<[u8]>
impl From<Simd<f32, 4>> for __m128
impl From<Simd<f32, 8>> for __m256
impl From<Simd<f32, 16>> for __m512
impl From<Simd<f64, 2>> for __m128d
impl From<Simd<f64, 4>> for __m256d
impl From<Simd<f64, 8>> for __m512d
impl From<Simd<i8, 16>> for __m128i
impl From<Simd<i8, 32>> for __m256i
impl From<Simd<i8, 64>> for __m512i
impl From<Simd<i16, 8>> for __m128i
impl From<Simd<i16, 16>> for __m256i
impl From<Simd<i16, 32>> for __m512i
impl From<Simd<i32, 4>> for __m128i
impl From<Simd<i32, 8>> for __m256i
impl From<Simd<i32, 16>> for __m512i
impl From<Simd<i64, 2>> for __m128i
impl From<Simd<i64, 4>> for __m256i
impl From<Simd<i64, 8>> for __m512i
impl From<Simd<isize, 2>> for __m128i
impl From<Simd<isize, 4>> for __m256i
impl From<Simd<isize, 8>> for __m512i
impl From<Simd<u8, 16>> for __m128i
impl From<Simd<u8, 32>> for __m256i
impl From<Simd<u8, 64>> for __m512i
impl From<Simd<u16, 8>> for __m128i
impl From<Simd<u16, 16>> for __m256i
impl From<Simd<u16, 32>> for __m512i
impl From<Simd<u32, 4>> for __m128i
impl From<Simd<u32, 8>> for __m256i
impl From<Simd<u32, 16>> for __m512i
impl From<Simd<u64, 2>> for __m128i
impl From<Simd<u64, 4>> for __m256i
impl From<Simd<u64, 8>> for __m512i
impl From<Simd<usize, 2>> for __m128i
impl From<Simd<usize, 4>> for __m256i
impl From<Simd<usize, 8>> for __m512i
impl From<String> for Box<str>
impl From<String> for Box<dyn Error + Send + Sync>
impl From<String> for Box<dyn Error>
impl From<String> for OsString
impl From<String> for PathBuf
impl From<String> for Rc<str>
impl From<String> for Arc<str>
impl From<String> for Vec<u8>
impl From<RecvError> for RecvTimeoutError
impl From<RecvError> for TryRecvError
impl From<Arc<str>> for Arc<[u8]>
impl From<Vec<NonZero<u8>>> for CString
impl From<[u8; 4]> for IpAddr
impl From<[u8; 4]> for Ipv4Addr
impl From<[u8; 16]> for IpAddr
impl From<[u8; 16]> for Ipv6Addr
impl From<[u16; 8]> for IpAddr
impl From<[u16; 8]> for Ipv6Addr
impl<'a> From<&'a str> for Cow<'a, str>
impl<'a> From<&'a CStr> for Cow<'a, CStr>
impl<'a> From<&'a CString> for Cow<'a, CStr>
impl<'a> From<&'a OsStr> for Cow<'a, OsStr>
impl<'a> From<&'a OsString> for Cow<'a, OsStr>
impl<'a> From<&'a Path> for Cow<'a, Path>
impl<'a> From<&'a PathBuf> for Cow<'a, Path>
impl<'a> From<&'a String> for Cow<'a, str>
impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a>
impl<'a> From<Cow<'a, str>> for Box<dyn Error>
impl<'a> From<Cow<'a, str>> for String
impl<'a> From<Cow<'a, CStr>> for CString
impl<'a> From<Cow<'a, OsStr>> for OsString
impl<'a> From<Cow<'a, Path>> for PathBuf
impl<'a> From<CString> for Cow<'a, CStr>
impl<'a> From<OsString> for Cow<'a, OsStr>
impl<'a> From<PathBuf> for Cow<'a, Path>
impl<'a> From<String> for Cow<'a, str>
impl<'a, 'b> From<Cow<'b, str>> for Box<dyn Error + Send + Sync + 'a>
impl<'a, B> From<Cow<'a, B>> for Rc<B>
impl<'a, B> From<Cow<'a, B>> for Arc<B>
impl<'a, E> From<E> for Box<dyn Error + 'a>where
E: Error + 'a,
impl<'a, E> From<E> for Box<dyn Error + Send + Sync + 'a>
impl<'a, T> From<&'a Option<T>> for Option<&'a T>
impl<'a, T> From<&'a [T]> for Cow<'a, [T]>where
T: Clone,
impl<'a, T> From<&'a Vec<T>> for Cow<'a, [T]>where
T: Clone,
impl<'a, T> From<&'a mut Option<T>> for Option<&'a mut T>
impl<'a, T> From<Cow<'a, [T]>> for Vec<T>
impl<'a, T> From<Vec<T>> for Cow<'a, [T]>where
T: Clone,
impl<'a, T, const N: usize> From<&'a [T; N]> for Cow<'a, [T]>where
T: Clone,
impl<'data> From<&'data mut [u8]> for BorrowedBuf<'data>
Create a new BorrowedBuf
from a fully initialized slice.
impl<'data> From<&'data mut [MaybeUninit<u8>]> for BorrowedBuf<'data>
Create a new BorrowedBuf
from an uninitialized buffer.
Use set_init
if part of the buffer is known to be already initialized.
impl<A> From<Box<str, A>> for Box<[u8], A>where
A: Allocator,
impl<E> From<E> for Report<E>where
E: Error,
impl<I> From<(I, u16)> for SocketAddr
impl<K, V, const N: usize> From<[(K, V); N]> for HashMap<K, V, RandomState>
impl<K, V, const N: usize> From<[(K, V); N]> for BTreeMap<K, V>where
K: Ord,
impl<T> From<&[T]> for Box<[T]>where
T: Clone,
impl<T> From<&[T]> for Rc<[T]>where
T: Clone,
impl<T> From<&[T]> for Arc<[T]>where
T: Clone,
impl<T> From<&[T]> for Vec<T>where
T: Clone,
impl<T> From<&mut [T]> for Vec<T>where
T: Clone,
impl<T> From<Cow<'_, [T]>> for Box<[T]>where
T: Clone,
impl<T> From<[T; 1]> for (T,)
impl<T> From<[T; 2]> for (T, T)
impl<T> From<[T; 3]> for (T, T, T)
impl<T> From<[T; 4]> for (T, T, T, T)
impl<T> From<[T; 5]> for (T, T, T, T, T)
impl<T> From<[T; 6]> for (T, T, T, T, T, T)
impl<T> From<[T; 7]> for (T, T, T, T, T, T, T)
impl<T> From<[T; 8]> for (T, T, T, T, T, T, T, T)
impl<T> From<[T; 9]> for (T, T, T, T, T, T, T, T, T)
impl<T> From<[T; 10]> for (T, T, T, T, T, T, T, T, T, T)
impl<T> From<[T; 11]> for (T, T, T, T, T, T, T, T, T, T, T)
impl<T> From<[T; 12]> for (T, T, T, T, T, T, T, T, T, T, T, T)
impl<T> From<!> for T
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.