Trait signature_core::lib::From
1.0.0 · source · [−]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
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,
Converts an usize into an AtomicUsize.
Converts a NonZeroU32 into an u32
Converts a NonZeroU16 into an u16
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 an isize into an AtomicIsize.
Converts a NonZeroUsize into an usize
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 NonZeroI16 into an i16
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<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 NonZeroIsize into an isize
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,
Converts a NonZeroI128 into an i128
Converts a NonZeroU128 into an u128
Converts a NonZeroI32 into an i32
impl<'a, T, N> From<&'a mut [T]> for &'a mut GenericArray<T, N> where
N: ArrayLength<T>,
impl<'a, T, N> From<&'a mut [T]> for &'a mut GenericArray<T, N> where
N: ArrayLength<T>,
Convert the Choice wrapper into a bool, depending on whether
the underlying u8 was a 0 or a 1.
Note
This function exists to avoid having higher-level cryptographic protocol implementations duplicating this pattern.
The intended use case for this conversion is at the end of a
higher-level primitive implementation: for example, in checking a keyed
MAC, where the verification should happen in constant-time (and thus use
a Choice) but it is safe to return a bool at the end of the
verification.
Converts a &str into a Box<str>
This conversion allocates on the heap
and performs a copy of s.
Examples
let boxed: Box<str> = Box::from("hello");
println!("{}", boxed);Convert a clone-on-write slice into a vector.
If s already owns a Vec<T>, it will be returned directly.
If s is borrowing a slice, a new Vec<T> will be allocated and
filled by cloning s’s items into it.
Examples
let o: Cow<[i32]> = Cow::Owned(vec![1, 2, 3]);
let b: Cow<[i32]> = Cow::Borrowed(&[1, 2, 3]);
assert_eq!(Vec::from(o), Vec::from(b));Convert a vector into a boxed slice.
If v has excess capacity, its items will be moved into a
newly-allocated buffer with exactly the right capacity.
Examples
assert_eq!(Box::from(vec![1, 2, 3]), vec![1, 2, 3].into_boxed_slice());Converts a Cow<'_, str> into a Box<str>
When cow is the Cow::Borrowed variant, this
conversion allocates on the heap and copies the
underlying str. Otherwise, it will try to reuse the owned
String’s allocation.
Examples
use std::borrow::Cow;
let unboxed = Cow::Borrowed("hello");
let boxed: Box<str> = Box::from(unboxed);
println!("{}", boxed);let unboxed = Cow::Owned("hello".to_string());
let boxed: Box<str> = Box::from(unboxed);
println!("{}", boxed);Converts a BinaryHeap<T> into a Vec<T>.
This conversion requires no data movement or allocation, and has constant time complexity.
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 &[T] into a Box<[T]>
This conversion allocates on the heap
and performs a copy of slice.
Examples
// create a &[u8] which will be used to create a Box<[u8]>
let slice: &[u8] = &[104, 101, 108, 108, 111];
let boxed_slice: Box<[u8]> = Box::from(slice);
println!("{:?}", boxed_slice);Turn a VecDeque<T> into a Vec<T>.
This never needs to re-allocate, but does need to do O(n) data movement if the circular buffer doesn’t happen to be at the beginning of the allocation.
Examples
use std::collections::VecDeque;
// This one is *O*(1).
let deque: VecDeque<_> = (1..5).collect();
let ptr = deque.as_slices().0.as_ptr();
let vec = Vec::from(deque);
assert_eq!(vec, [1, 2, 3, 4]);
assert_eq!(vec.as_ptr(), ptr);
// This one needs data rearranging.
let mut deque: VecDeque<_> = (1..5).collect();
deque.push_front(9);
deque.push_front(8);
let ptr = deque.as_slices().1.as_ptr();
let vec = Vec::from(deque);
assert_eq!(vec, [8, 9, 1, 2, 3, 4]);
assert_eq!(vec.as_ptr(), ptr);Converts a Cow<'_, [T]> into a Box<[T]>
When cow is the Cow::Borrowed variant, this
conversion allocates on the heap and copies the
underlying slice. Otherwise, it will try to reuse the owned
Vec’s allocation.
Converts a T into a Box<T>
The conversion allocates on the heap and moves t
from the stack into it.
Examples
let x = 5;
let boxed = Box::new(5);
assert_eq!(Box::from(x), boxed);Converts a Box<str> into a Box<[u8]>
This conversion does not allocate on the heap and happens in place.
Examples
// create a Box<str> which will be used to create a Box<[u8]>
let boxed: Box<str> = Box::from("hello");
let boxed_str: Box<[u8]> = Box::from(boxed);
// create a &[u8] which will be used to create a Box<[u8]>
let slice: &[u8] = &[104, 101, 108, 108, 111];
let boxed_slice = Box::from(slice);
assert_eq!(boxed_slice, boxed_str);Converts a Vec<T> into a BinaryHeap<T>.
This conversion happens in-place, and has O(n) time complexity.
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");Always evaluates to TryReserveErrorKind::CapacityOverflow.
Converts a [T; N] into a Box<[T]>
This conversion moves the array to newly heap-allocated memory.
Examples
let boxed: Box<[u8]> = Box::from([4, 2]);
println!("{:?}", boxed);use std::collections::LinkedList;
let list1 = LinkedList::from([1, 2, 3, 4]);
let list2: LinkedList<_> = [1, 2, 3, 4].into();
assert_eq!(list1, list2);impl<R, G, T> From<T> for ReentrantMutex<R, G, T> where
R: RawMutex,
G: GetThreadId,
impl<R, G, T> From<T> for ReentrantMutex<R, G, T> where
R: RawMutex,
G: GetThreadId,
pub fn from(t: T) -> ReentrantMutex<R, G, T>
Implementors
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.