VecDeque should support splice / extend_front

VecDeque should support splice for convenience.
(SliceDeque supports it, but it doesn't build on wasm and seems kind of abandoned (several unmerged PRs with no updates).)

VecDeque should also support extend_front (like repeated push_front from an iterator. Compared to calling q.splice(..0, iter), the elements added with q.extend_front(iter) would be in reverse order).

I had to switch over to using a Vec because I couldn't efficiently insert multiple items into a VecDeque. Having a splice method would've been really useful. If I can find some time these days, I might be able to work on a PR.

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@tomkarw

@tomkarw Thanks for working on this. Btw, what's the current status? :)

Sorry for the late response, apparently I don't get GitHub notification via email.

TBH, I thought it will be easier. The unsafe code needed kinda scared me, and I got a job 4 days after claiming this.
I'll try to revisit it now, but will probably look for a mentor/guidance.

And what about a .prepend() to mirror .append()?

Hi, I just found out I'm encountering a case where prepend() and/or extend_front() methods on VecDeque could be really useful.
I'm doing the work by iterating over other vector and push_front repeatedly, but that feels not idiomatic and I guess not optimal at all.

If no one is available for this I might take a look at some point.

+1 for implementing this. Repeatedly calling push_front has a significant performance impact for my use-case, so I ended-up hacking something together on top of a Vec.

I wonder what the blockers are here? I might give implementing prepend a shot this weekend.

No blockers, just lack of time. Feel free to pick it up, I'll be happy to see it implemented.

For the simple case of trying to avoid repeated shifts on insert into the middle, the following should add only O(2n+m) to the complexity of extend(), where n is length pre-insert and m is length of iter.

use std::collections::VecDeque;

trait VecDequeExt<T> {
    fn insert_iter<I>(&mut self, index: usize, iter: I) where 
        I: IntoIterator<Item = T>;
}

impl<T> VecDequeExt<T> for VecDeque<T> {
    fn insert_iter<I>(&mut self, index: usize, iter: I) where 
            I: IntoIterator<Item = T> {
        let n = self.len() - index;
        self.rotate_right(n);
        self.extend(iter);
        self.rotate_left(n);
    }
}

fn main() {
    let mut deque = VecDeque::new();
    deque.push_back(0);
    deque.push_back(1);
    deque.push_back(4);
    deque.push_back(5);
    
    deque.insert_iter(2, vec![2, 3]);
    assert_eq!(deque.into_iter().collect::<Vec<_>>(), vec![0, 1, 2, 3, 4, 5]);
}

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tracking issue: #146975, related PRs are linked there if people here want to check them out

Rollup merge of rust-lang#146861 - antonilol:vec_deque_extend_front, …

Unrolled build for #146861

Rollup merge of rust-lang#146861 - antonilol:vec_deque_extend_front, …

Closing this in favour of the tracking issue #146975

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VecDeque should support splice / extend_front #69939

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VecDeque should support splice / extend_front #69939

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cynecx commented on Dec 23, 2020

tomkarw commented on Mar 28, 2022

Boscop commented on May 11, 2022

tomkarw commented on Jul 16, 2022

drwilco commented on Apr 25, 2023

qvignaud commented on Jun 10, 2024

jnb commented on Aug 13, 2024

nhtyy commented on Dec 12, 2024

tomkarw commented on Dec 13, 2024

jlnstrk commented on Mar 26, 2025

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antonilol commented on Oct 30, 2025

Dylan-DPC commented on Nov 7, 2026

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