Trait FromIterator

1.85.0 · Source
pub trait FromIterator<A>: Sized {
    // Required method
    fn from_iter<T>(iter: T) -> Self
       where T: IntoIterator<Item = A>;
}
Expand description

Conversion from an Iterator.

By implementing FromIterator for a type, you define how it will be created from an iterator. This is common for types which describe a collection of some kind.

If you want to create a collection from the contents of an iterator, the Iterator::collect() method is preferred. However, when you need to specify the container type, FromIterator::from_iter() can be more readable than using a turbofish (e.g. ::<Vec<_>>()). See the Iterator::collect() documentation for more examples of its use.

See also: IntoIterator.

§Examples

Basic usage:

let five_fives = std::iter::repeat(5).take(5);

let v = Vec::from_iter(five_fives);

assert_eq!(v, vec![5, 5, 5, 5, 5]);

Using Iterator::collect() to implicitly use FromIterator:

let five_fives = std::iter::repeat(5).take(5);

let v: Vec<i32> = five_fives.collect();

assert_eq!(v, vec![5, 5, 5, 5, 5]);

Using FromIterator::from_iter() as a more readable alternative to Iterator::collect():

use std::collections::VecDeque;
let first = (0..10).collect::<VecDeque<i32>>();
let second = VecDeque::from_iter(0..10);

assert_eq!(first, second);

Implementing FromIterator for your type:

// A sample collection, that's just a wrapper over Vec<T>
#[derive(Debug)]
struct MyCollection(Vec<i32>);

// Let's give it some methods so we can create one and add things
// to it.
impl MyCollection {
    fn new() -> MyCollection {
        MyCollection(Vec::new())
    }

    fn add(&mut self, elem: i32) {
        self.0.push(elem);
    }
}

// and we'll implement FromIterator
impl FromIterator<i32> for MyCollection {
    fn from_iter<I: IntoIterator<Item=i32>>(iter: I) -> Self {
        let mut c = MyCollection::new();

        for i in iter {
            c.add(i);
        }

        c
    }
}

// Now we can make a new iterator...
let iter = (0..5).into_iter();

// ... and make a MyCollection out of it
let c = MyCollection::from_iter(iter);

assert_eq!(c.0, vec![0, 1, 2, 3, 4]);

// collect works too!

let iter = (0..5).into_iter();
let c: MyCollection = iter.collect();

assert_eq!(c.0, vec![0, 1, 2, 3, 4]);

Required Methods§

1.0.0 · Source

fn from_iter<T>(iter: T) -> Self
where T: IntoIterator<Item = A>,

Creates a value from an iterator.

See the module-level documentation for more.

§Examples
let five_fives = std::iter::repeat(5).take(5);

let v = Vec::from_iter(five_fives);

assert_eq!(v, vec![5, 5, 5, 5, 5]);

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors§

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impl FromIterator<(Key, Value)> for Fields

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impl FromIterator<(String, Value)> for rustmax::serde_json::Map<String, Value>

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impl FromIterator<Signal> for nix::sys::signal::SigSet

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impl FromIterator<Signal> for nix::sys::signal::SigSet

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impl FromIterator<TokenTree> for rustmax::proc_macro2::TokenStream

Collects a number of token trees into a single stream.

1.29.0 · Source§

impl FromIterator<TokenTree> for rustmax::proc_macro::TokenStream

Collects a number of token trees into a single stream.

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impl FromIterator<char> for BString

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impl FromIterator<char> for ByteString

1.80.0 · Source§

impl FromIterator<char> for Box<str>

1.0.0 · Source§

impl FromIterator<char> for String

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impl FromIterator<u8> for BString

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impl FromIterator<u8> for ByteString

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impl FromIterator<u8> for Bytes

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impl FromIterator<u8> for BytesMut

1.23.0 · Source§

impl FromIterator<()> for ()

Collapses all unit items from an iterator into one.

This is more useful when combined with higher-level abstractions, like collecting to a Result<(), E> where you only care about errors:

use std::io::*;
let data = vec![1, 2, 3, 4, 5];
let res: Result<()> = data.iter()
    .map(|x| writeln!(stdout(), "{x}"))
    .collect();
assert!(res.is_ok());
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impl FromIterator<usize> for VarBitSet

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impl FromIterator<BString> for BString

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impl FromIterator<FileType> for FileType

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impl FromIterator<AtFlags> for nix::fcntl::AtFlags

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impl FromIterator<AtFlags> for nix::fcntl::AtFlags

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impl FromIterator<FallocateFlags> for nix::fcntl::FallocateFlags

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impl FromIterator<FallocateFlags> for nix::fcntl::FallocateFlags

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impl FromIterator<FdFlag> for nix::fcntl::FdFlag

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impl FromIterator<FdFlag> for nix::fcntl::FdFlag

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impl FromIterator<OFlag> for nix::fcntl::OFlag

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impl FromIterator<OFlag> for nix::fcntl::OFlag

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impl FromIterator<RenameFlags> for nix::fcntl::RenameFlags

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impl FromIterator<RenameFlags> for nix::fcntl::RenameFlags

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impl FromIterator<ResolveFlag> for nix::fcntl::ResolveFlag

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impl FromIterator<ResolveFlag> for nix::fcntl::ResolveFlag

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impl FromIterator<SealFlag> for nix::fcntl::SealFlag

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impl FromIterator<SealFlag> for nix::fcntl::SealFlag

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impl FromIterator<PollFlags> for PollFlags

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impl FromIterator<PosixSpawnFlags> for PosixSpawnFlags

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impl FromIterator<MFdFlags> for MFdFlags

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impl FromIterator<MemFdCreateFlag> for MemFdCreateFlag

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impl FromIterator<SaFlags> for nix::sys::signal::SaFlags

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impl FromIterator<SaFlags> for nix::sys::signal::SaFlags

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impl FromIterator<SfdFlags> for nix::sys::signalfd::SfdFlags

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impl FromIterator<SfdFlags> for nix::sys::signalfd::SfdFlags

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impl FromIterator<Mode> for nix::sys::stat::Mode

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impl FromIterator<Mode> for nix::sys::stat::Mode

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impl FromIterator<SFlag> for nix::sys::stat::SFlag

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impl FromIterator<SFlag> for nix::sys::stat::SFlag

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impl FromIterator<FsFlags> for nix::sys::statvfs::FsFlags

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impl FromIterator<FsFlags> for nix::sys::statvfs::FsFlags

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impl FromIterator<ControlFlags> for ControlFlags

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impl FromIterator<InputFlags> for InputFlags

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impl FromIterator<LocalFlags> for LocalFlags

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impl FromIterator<OutputFlags> for OutputFlags

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impl FromIterator<WaitPidFlag> for nix::sys::wait::WaitPidFlag

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impl FromIterator<WaitPidFlag> for nix::sys::wait::WaitPidFlag

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impl FromIterator<AccessFlags> for nix::unistd::AccessFlags

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impl FromIterator<AccessFlags> for nix::unistd::AccessFlags

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impl FromIterator<CipherCtxFlags> for CipherCtxFlags

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impl FromIterator<CMSOptions> for CMSOptions

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impl FromIterator<OcspFlag> for OcspFlag

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impl FromIterator<Pkcs7Flags> for Pkcs7Flags

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impl FromIterator<ExtensionContext> for ExtensionContext

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impl FromIterator<ShutdownState> for ShutdownState

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impl FromIterator<SslMode> for SslMode

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impl FromIterator<SslOptions> for SslOptions

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impl FromIterator<SslSessionCacheMode> for SslSessionCacheMode

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impl FromIterator<SslVerifyMode> for SslVerifyMode

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impl FromIterator<X509CheckFlags> for X509CheckFlags

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impl FromIterator<X509VerifyFlags> for X509VerifyFlags

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impl FromIterator<Literal> for Seq

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impl FromIterator<CreateFlags> for rustix::backend::fs::inotify::CreateFlags

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impl FromIterator<CreateFlags> for rustix::backend::fs::inotify::CreateFlags

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impl FromIterator<ReadFlags> for rustix::backend::fs::inotify::ReadFlags

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impl FromIterator<ReadFlags> for rustix::backend::fs::inotify::ReadFlags

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impl FromIterator<WatchFlags> for rustix::backend::fs::inotify::WatchFlags

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impl FromIterator<WatchFlags> for rustix::backend::fs::inotify::WatchFlags

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impl FromIterator<Access> for rustix::backend::fs::types::Access

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impl FromIterator<Access> for rustix::backend::fs::types::Access

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impl FromIterator<AtFlags> for rustix::backend::fs::types::AtFlags

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impl FromIterator<AtFlags> for rustix::backend::fs::types::AtFlags

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impl FromIterator<FallocateFlags> for rustix::backend::fs::types::FallocateFlags

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impl FromIterator<FallocateFlags> for rustix::backend::fs::types::FallocateFlags

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impl FromIterator<MemfdFlags> for rustix::backend::fs::types::MemfdFlags

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impl FromIterator<MemfdFlags> for rustix::backend::fs::types::MemfdFlags

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impl FromIterator<Mode> for rustix::backend::fs::types::Mode

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impl FromIterator<Mode> for rustix::backend::fs::types::Mode

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impl FromIterator<OFlags> for rustix::backend::fs::types::OFlags

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impl FromIterator<OFlags> for rustix::backend::fs::types::OFlags

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impl FromIterator<RenameFlags> for rustix::backend::fs::types::RenameFlags

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impl FromIterator<RenameFlags> for rustix::backend::fs::types::RenameFlags

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impl FromIterator<ResolveFlags> for rustix::backend::fs::types::ResolveFlags

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impl FromIterator<ResolveFlags> for rustix::backend::fs::types::ResolveFlags

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impl FromIterator<SealFlags> for rustix::backend::fs::types::SealFlags

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impl FromIterator<SealFlags> for rustix::backend::fs::types::SealFlags

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impl FromIterator<StatVfsMountFlags> for rustix::backend::fs::types::StatVfsMountFlags

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impl FromIterator<StatVfsMountFlags> for rustix::backend::fs::types::StatVfsMountFlags

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impl FromIterator<StatxFlags> for rustix::backend::fs::types::StatxFlags

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impl FromIterator<DupFlags> for rustix::backend::io::types::DupFlags

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impl FromIterator<DupFlags> for rustix::backend::io::types::DupFlags

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impl FromIterator<FdFlags> for rustix::backend::io::types::FdFlags

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impl FromIterator<FdFlags> for rustix::backend::io::types::FdFlags

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impl FromIterator<ReadWriteFlags> for rustix::backend::io::types::ReadWriteFlags

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impl FromIterator<ReadWriteFlags> for rustix::backend::io::types::ReadWriteFlags

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impl FromIterator<MountFlags> for MountFlags

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impl FromIterator<MountPropagationFlags> for MountPropagationFlags

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impl FromIterator<UnmountFlags> for UnmountFlags

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impl FromIterator<IFlags> for rustix::fs::ioctl::IFlags

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impl FromIterator<IFlags> for rustix::fs::ioctl::IFlags

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impl FromIterator<StatxAttributes> for StatxAttributes

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impl FromIterator<StatxFlags> for rustix::fs::statx::StatxFlags

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impl FromIterator<XattrFlags> for rustix::fs::xattr::XattrFlags

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impl FromIterator<XattrFlags> for rustix::fs::xattr::XattrFlags

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impl FromIterator<ByteString> for ByteString

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impl FromIterator<CodegenConfig> for CodegenConfig

1.80.0 · Source§

impl FromIterator<String> for Box<str>

1.4.0 · Source§

impl FromIterator<String> for String

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impl FromIterator<TokenStream> for rustmax::proc_macro2::TokenStream

1.15.0 · Source§

impl FromIterator<TokenStream> for rustmax::proc_macro::TokenStream

A “flattening” operation on token streams, collects token trees from multiple token streams into a single stream.

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impl FromIterator<Modifiers> for Modifiers

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impl FromIterator<Comparator> for VersionReq

1.52.0 · Source§

impl FromIterator<OsString> for OsString

1.80.0 · Source§

impl<'a> FromIterator<&'a char> for Box<str>

1.17.0 · Source§

impl<'a> FromIterator<&'a char> for String

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impl<'a> FromIterator<&'a str> for BString

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impl<'a> FromIterator<&'a str> for ByteString

1.80.0 · Source§

impl<'a> FromIterator<&'a str> for Box<str>

1.0.0 · Source§

impl<'a> FromIterator<&'a str> for String

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impl<'a> FromIterator<&'a u8> for BytesMut

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impl<'a> FromIterator<&'a BStr> for BString

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impl<'a> FromIterator<&'a ByteStr> for ByteString

1.52.0 · Source§

impl<'a> FromIterator<&'a OsStr> for OsString

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impl<'a> FromIterator<&'a [u8]> for BString

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impl<'a> FromIterator<&'a [u8]> for ByteString

1.80.0 · Source§

impl<'a> FromIterator<Cow<'a, str>> for Box<str>

1.19.0 · Source§

impl<'a> FromIterator<Cow<'a, str>> for String

1.52.0 · Source§

impl<'a> FromIterator<Cow<'a, OsStr>> for OsString

1.12.0 · Source§

impl<'a> FromIterator<char> for Cow<'a, str>

1.12.0 · Source§

impl<'a> FromIterator<String> for Cow<'a, str>

1.12.0 · Source§

impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str>

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impl<'a, A, B, C, K0, K1, V> FromIterator<(A, B, C)> for ZeroMap2d<'a, K0, K1, V>
where A: Borrow<K0>, B: Borrow<K1>, C: Borrow<V>, K0: ZeroMapKV<'a> + Ord + ?Sized, K1: ZeroMapKV<'a> + Ord + ?Sized, V: ZeroMapKV<'a> + ?Sized,

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impl<'a, A, B, K, V> FromIterator<(A, B)> for ZeroMap<'a, K, V>
where A: Borrow<K>, B: Borrow<V>, K: ZeroMapKV<'a> + Ord + ?Sized, V: ZeroMapKV<'a> + ?Sized,

1.0.0 · Source§

impl<'a, T> FromIterator<T> for Cow<'a, [T]>
where T: Clone,

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impl<'i> FromIterator<Spanned<DeValue<'i>>> for DeArray<'i>

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impl<A> FromIterator<<A as Array>::Item> for SmallVec<A>
where A: Array,

1.80.0 · Source§

impl<A> FromIterator<Box<str, A>> for Box<str>
where A: Allocator,

1.45.0 · Source§

impl<A> FromIterator<Box<str, A>> for String
where A: Allocator,

1.0.0 · Source§

impl<A, E, V> FromIterator<Result<A, E>> for Result<V, E>
where V: FromIterator<A>,

1.79.0 · Source§

impl<A, EA> FromIterator<(EA₁, EA₂, …, EAₙ)> for (A₁, A₂, …, Aₙ)
where A: Default + Extend<EA>,

This implementation turns an iterator of tuples into a tuple of types which implement Default and Extend.

This is similar to Iterator::unzip, but is also composable with other FromIterator implementations:

let string = "1,2,123,4";

// Example given for a 2-tuple, but 1- through 12-tuples are supported
let (numbers, lengths): (Vec<_>, Vec<_>) = string
    .split(',')
    .map(|s| s.parse().map(|n: u32| (n, s.len())))
    .collect::<Result<_, _>>()?;

assert_eq!(numbers, [1, 2, 123, 4]);
assert_eq!(lengths, [1, 1, 3, 1]);

This trait is implemented for tuples up to twelve items long. The impls for 1- and 3- through 12-ary tuples were stabilized after 2-tuples, in 1.85.0.

1.0.0 · Source§

impl<A, V> FromIterator<Option<A>> for Option<V>
where V: FromIterator<A>,

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impl<B> FromIterator<bool> for BitVec<B>
where B: BitBlock,

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impl<B> FromIterator<usize> for BitSet<B>
where B: BitBlock,

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impl<F> FromIterator<F> for JoinAll<F>
where F: Future,

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impl<F> FromIterator<F> for TryJoinAll<F>
where F: TryFuture,

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impl<Fut> FromIterator<Fut> for rustmax::prelude::future::SelectAll<Fut>
where Fut: Future + Unpin,

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impl<Fut> FromIterator<Fut> for SelectOk<Fut>
where Fut: TryFuture + Unpin,

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impl<Fut> FromIterator<Fut> for FuturesOrdered<Fut>
where Fut: Future,

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impl<Fut> FromIterator<Fut> for FuturesUnordered<Fut>

1.32.0 · Source§

impl<I> FromIterator<I> for Box<[I]>

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impl<K, V> FromIterator<(K, V)> for Value
where K: Into<String>, V: Into<Value>,

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impl<K, V> FromIterator<(K, V)> for Trie<K, V>
where K: TrieKey,

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impl<K, V> FromIterator<(K, V)> for AHashMap<K, V>
where K: Eq + Hash,

1.0.0 · Source§

impl<K, V> FromIterator<(K, V)> for BTreeMap<K, V>
where K: Ord,

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impl<K, V> FromIterator<(K, V)> for rustmax::toml::map::Map<K, V>
where K: Ord + Hash,

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impl<K, V, S> FromIterator<(K, V)> for IndexMap<K, V, S>
where K: Hash + Eq, S: BuildHasher + Default,

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impl<K, V, S> FromIterator<(K, V)> for LiteMap<K, V, S>
where K: Ord, S: StoreFromIterable<K, V>,

1.0.0 · Source§

impl<K, V, S> FromIterator<(K, V)> for rustmax::std::collections::HashMap<K, V, S>
where K: Eq + Hash, S: BuildHasher + Default,

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impl<K, V, S, A> FromIterator<(K, V)> for hashbrown::map::HashMap<K, V, S, A>
where K: Eq + Hash, S: BuildHasher + Default, A: Default + Allocator,

1.0.0 · Source§

impl<P> FromIterator<P> for PathBuf
where P: AsRef<Path>,

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impl<St> FromIterator<St> for rustmax::prelude::stream::SelectAll<St>
where St: Stream + Unpin,

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impl<T> FromIterator<(usize, T)> for Slab<T>

Create a slab from an iterator of key-value pairs.

If the iterator produces duplicate keys, the previous value is replaced with the later one. The keys does not need to be sorted beforehand, and this function always takes O(n) time. Note that the returned slab will use space proportional to the largest key, so don’t use Slab with untrusted keys.

§Examples


let vec = vec![(2,'a'), (6,'b'), (7,'c')];
let slab = vec.into_iter().collect::<Slab<char>>();
assert_eq!(slab.len(), 3);
assert!(slab.capacity() >= 8);
assert_eq!(slab[2], 'a');

With duplicate and unsorted keys:


let vec = vec![(20,'a'), (10,'b'), (11,'c'), (10,'d')];
let slab = vec.into_iter().collect::<Slab<char>>();
assert_eq!(slab.len(), 3);
assert_eq!(slab[10], 'd');
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impl<T> FromIterator<(HeaderName, T)> for HeaderMap<T>

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impl<T> FromIterator<Steal<T>> for Steal<T>

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impl<T> FromIterator<T> for Value
where T: Into<Value>,

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impl<T> FromIterator<T> for ZeroVec<'_, T>
where T: AsULE,

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impl<T> FromIterator<T> for AHashSet<T>
where T: Eq + Hash,

1.0.0 · Source§

impl<T> FromIterator<T> for BTreeSet<T>
where T: Ord,

1.0.0 · Source§

impl<T> FromIterator<T> for BinaryHeap<T>
where T: Ord,

1.0.0 · Source§

impl<T> FromIterator<T> for LinkedList<T>

1.0.0 · Source§

impl<T> FromIterator<T> for VecDeque<T>

1.37.0 · Source§

impl<T> FromIterator<T> for Rc<[T]>

1.37.0 · Source§

impl<T> FromIterator<T> for Arc<[T]>

1.0.0 · Source§

impl<T> FromIterator<T> for Vec<T>

Collects an iterator into a Vec, commonly called via Iterator::collect()

§Allocation behavior

In general Vec does not guarantee any particular growth or allocation strategy. That also applies to this trait impl.

Note: This section covers implementation details and is therefore exempt from stability guarantees.

Vec may use any or none of the following strategies, depending on the supplied iterator:

  • preallocate based on Iterator::size_hint()
    • and panic if the number of items is outside the provided lower/upper bounds
  • use an amortized growth strategy similar to pushing one item at a time
  • perform the iteration in-place on the original allocation backing the iterator

The last case warrants some attention. It is an optimization that in many cases reduces peak memory consumption and improves cache locality. But when big, short-lived allocations are created, only a small fraction of their items get collected, no further use is made of the spare capacity and the resulting Vec is moved into a longer-lived structure, then this can lead to the large allocations having their lifetimes unnecessarily extended which can result in increased memory footprint.

In cases where this is an issue, the excess capacity can be discarded with Vec::shrink_to(), Vec::shrink_to_fit() or by collecting into Box<[T]> instead, which additionally reduces the size of the long-lived struct.

static LONG_LIVED: Mutex<Vec<Vec<u16>>> = Mutex::new(Vec::new());

for i in 0..10 {
    let big_temporary: Vec<u16> = (0..1024).collect();
    // discard most items
    let mut result: Vec<_> = big_temporary.into_iter().filter(|i| i % 100 == 0).collect();
    // without this a lot of unused capacity might be moved into the global
    result.shrink_to_fit();
    LONG_LIVED.lock().unwrap().push(result);
}
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impl<T, F> FromIterator<F> for JoinSet<T>
where F: Future<Output = T> + Send + 'static, T: Send + 'static,

Collect an iterator of futures into a JoinSet.

This is equivalent to calling JoinSet::spawn on each element of the iterator.

§Examples

The main example from JoinSet’s documentation can also be written using collect:

use tokio::task::JoinSet;

#[tokio::main]
async fn main() {
    let mut set: JoinSet<_> = (0..10).map(|i| async move { i }).collect();

    let mut seen = [false; 10];
    while let Some(res) = set.join_next().await {
        let idx = res.unwrap();
        seen[idx] = true;
    }

    for i in 0..10 {
        assert!(seen[i]);
    }
}
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impl<T, N> FromIterator<T> for GenericArray<T, N>
where N: ArrayLength<T>,

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impl<T, P> FromIterator<Pair<T, P>> for Punctuated<T, P>

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impl<T, P> FromIterator<T> for Punctuated<T, P>
where P: Default,

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impl<T, S> FromIterator<T> for IndexSet<T, S>
where T: Hash + Eq, S: BuildHasher + Default,

1.0.0 · Source§

impl<T, S> FromIterator<T> for rustmax::std::collections::HashSet<T, S>
where T: Eq + Hash, S: BuildHasher + Default,

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impl<T, S, A> FromIterator<T> for hashbrown::set::HashSet<T, S, A>
where T: Eq + Hash, S: BuildHasher + Default, A: Default + Allocator,

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impl<T, const CAP: usize> FromIterator<T> for ArrayVec<T, CAP>

Create an ArrayVec from an iterator.

Panics if the number of elements in the iterator exceeds the arrayvec’s capacity.

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impl<T, const N: usize> FromIterator<T> for ArrayFromIter<T, N>