How to solve 'cannot move out of a shared reference'?

help
1

The following code:

#![allow(dead_code)]

#[derive(Debug, Clone)]
pub struct SmallFiles {
    pub entries: Vec<u8>,
}

#[derive(Debug, Clone)]
pub struct Block {
    pub pos: u64,
    pub size: usize,
}

#[derive(Debug, Clone)]
pub struct BigFile {
    pub blocks: Vec<Block>,
}

#[derive(Debug, Clone)]
pub enum TocEntry {
    SmallFiles(SmallFiles),
    BigFile(BigFile),
}

#[derive(Debug, Clone)]
pub struct Toc {
    pub entries: Vec<TocEntry>,
}

fn main() {
    let mut toc = Toc { entries: Vec::new() };
    let big_file = BigFile { blocks: Vec::new() };
    toc.entries.push(TocEntry::BigFile(big_file));
    let block = Block { pos: 0, size: 0 };
    match &toc.entries[0] {
        TocEntry::BigFile(mut e) => e.blocks.push(block),
        _ => {}
    }
}

fails to compile with error:

Compiling playground v0.0.1 (/playground)
error[E0507]: cannot move out of a shared reference
  --> src/main.rs:35:11
   |
35 |     match &toc.entries[0] {
   |           ^^^^^^^^^^^^^^^
36 |         TocEntry::BigFile(mut e) => e.blocks.push(block),
   |                           -----
   |                           |
   |                           data moved here
   |                           move occurs because `e` has type `BigFile`, which does not implement the `Copy` trait

error: aborting due to previous error

I've tried it with "ref mut" in the catch clause and also tried RefCell. But I haven't found a solution which compiles. How can I solve the above issue?

2

Taking a mutable reference and removing the mut works:

    match &mut toc.entries[0] {
        TocEntry::BigFile(e) => e.blocks.push(block),
        _ => {}
    }

To elaborate on why this works, in your above example mut e causes Rust to attempt to move the data in the BigFile into a mutable variable called e. Just writing e alone tells Rust to borrow it instead. You also need to take a mutable reference to the content at the index instead of a shared one so that you can mutate it.

3

Thank you very much for the quick fix! It solved my issue.

4

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