Compiler thinks variable needs to be mutable when getting an owned slice

[grantslatton]

https://play.rust-lang.org/?gist=be40620b431766ae921d89b3e2c0b9e5&version=nightly

fn main() {
    let vec_a: Vec<u32> = vec![];
    let mut vec_b: Vec<u32> = vec![];
    vec_b.append(&mut vec_a[..].to_owned());
}

error[E0596]: cannot borrow immutable local variable `vec_a` as mutable
 --> src/main.rs:4:23
  |
2 |     let vec_a: Vec<u32> = vec![];
  |         ----- consider changing this to `mut vec_a`
3 |     let mut vec_b: Vec<u32> = vec![];
4 |     vec_b.append(&mut vec_a[..].to_owned());
  |                       ^^^^^ cannot borrow mutably

[jonas-schievink]

This is correct - you aren't getting an owned slice, you're getting a &mut [u32], a mutable slice. Since the data the slice points to is owned by vec_a, you can only mutably refer to it if you have a mutable reference to vec_a itself.

[jonas-schievink]

You can also use &vec_a instead of &mut vec_a, since the mutability isn't needed here (only vec_b needs to be mutable, since you append to it).

[grantslatton]

@jonas-schievink but as far as I know, I getting an immutable slice. &mut should apply to the Vec<u32> returned by to_owned. Additional parentheses to clarify this don't help:

https://play.rust-lang.org/?gist=266b22ca7d45c52200c068fa34bb5b61&version=nightly

fn main() {
    let vec_a: Vec<u32> = vec![];
    let mut vec_b: Vec<u32> = vec![];
    vec_b.append(&mut (vec_a[..].to_owned()));
}

In the above code, vec_a[..] should be a &[u32], right? And (vec_a[..].to_owned()) should be a Vec<u32>, right? Why can't I get a &mut of the owned Vec?

And why does splitting it into 2 lines work?

https://play.rust-lang.org/?gist=1600e7488b3de8e7ed489828af1c6727&version=nightly

fn main() {
    let vec_a: Vec<u32> = vec![];
    let mut vec_b: Vec<u32> = vec![];
    let mut owned = vec_a[..].to_owned();
    vec_b.append(&mut owned);
}

The above code compiles, but all I've done is introduce an intermediate variable owned.

[grantslatton]

@jonas-schievink

You can also use &vec_a instead of &mut vec_a, since the mutability isn't needed here (only vec_b needs to be mutable, since you append to it).

appends type signature is fn append(&mut self, other: &mut Vec<T>)

See https://doc.rust-lang.org/std/vec/struct.Vec.html#method.append

[bluss]

The compiler is calling IndexMut here, and that's wrong. vec_a[..].to_vec() only needs Index

[jonas-schievink]

Ah, right, that's an actual bug, of course: #28935

[bluss]

Thanks both! This is a duplicate of #28935 and it seems like an exact match.