Rollup merge of #76310 - scottmcm:array-try_from-vec, r=dtolnay
Add `[T; N]: TryFrom<Vec<T>>` (insta-stable) This is very similar to the [existing](https://doc.rust-lang.org/nightly/std/convert/trait.TryFrom.html#impl-TryFrom%3CBox%3C%5BT%5D%3E%3E) `Box<[T; N]>: TryFrom<Box<[T]>>`, but allows avoiding the `shrink_to_fit` if you have a vector and not a boxed slice. Like the slice equivalents of this, it fails if the length of the vector is not exactly `N`. This uses `Vec<T>` as the `Error` type to return the input, like how the `Rc<[T]> -> Rc<[T; N]>` (and Arc) ones also reflect the input directly in the error type. ```rust #[stable(feature = "array_try_from_vec", since = "1.47.0")] impl<T, const N: usize> TryFrom<Vec<T>> for [T; N] { type Error = Vec<T>; fn try_from(mut vec: Vec<T>) -> Result<[T; N], Vec<T>>; } ``` Inspired by this zulip thread: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/APIs.20for.20getting.20stuff.20from.20a.20Vec.20by.20owned/near/209048103
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@ -55,6 +55,7 @@
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#![stable(feature = "rust1", since = "1.0.0")]
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use core::cmp::{self, Ordering};
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use core::convert::TryFrom;
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use core::fmt;
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use core::hash::{Hash, Hasher};
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use core::intrinsics::{arith_offset, assume};
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@ -2754,6 +2755,57 @@ impl From<&str> for Vec<u8> {
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}
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}
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#[stable(feature = "array_try_from_vec", since = "1.48.0")]
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impl<T, const N: usize> TryFrom<Vec<T>> for [T; N] {
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type Error = Vec<T>;
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/// Gets the entire contents of the `Vec<T>` as an array,
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/// if its size exactly matches that of the requested array.
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///
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/// # Examples
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///
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/// ```
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/// use std::convert::TryInto;
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/// assert_eq!(vec![1, 2, 3].try_into(), Ok([1, 2, 3]));
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/// assert_eq!(<Vec<i32>>::new().try_into(), Ok([]));
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/// ```
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///
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/// If the length doesn't match, the input comes back in `Err`:
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/// ```
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/// use std::convert::TryInto;
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/// let r: Result<[i32; 4], _> = (0..10).collect::<Vec<_>>().try_into();
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/// assert_eq!(r, Err(vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9]));
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/// ```
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///
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/// If you're fine with just getting a prefix of the `Vec<T>`,
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/// you can call [`.truncate(N)`](Vec::truncate) first.
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/// ```
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/// use std::convert::TryInto;
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/// let mut v = String::from("hello world").into_bytes();
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/// v.sort();
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/// v.truncate(2);
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/// let [a, b]: [_; 2] = v.try_into().unwrap();
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/// assert_eq!(a, b' ');
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/// assert_eq!(b, b'd');
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/// ```
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fn try_from(mut vec: Vec<T>) -> Result<[T; N], Vec<T>> {
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if vec.len() != N {
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return Err(vec);
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}
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// SAFETY: `.set_len(0)` is always sound.
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unsafe { vec.set_len(0) };
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// SAFETY: A `Vec`'s pointer is always aligned properly, and
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// the alignment the array needs is the same as the items.
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// We checked earlier that we have sufficient items.
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// The items will not double-drop as the `set_len`
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// tells the `Vec` not to also drop them.
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let array = unsafe { ptr::read(vec.as_ptr() as *const [T; N]) };
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Ok(array)
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}
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}
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////////////////////////////////////////////////////////////////////////////////
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// Clone-on-write
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////////////////////////////////////////////////////////////////////////////////
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