Better ergonomics when pasting within code blocks in doc comments.

I'm in the situation where I want to paste a multi-line bit of code here:

let (even, odd_map): (Vec<i32>, HashMap<i32, f32>) =
    a.iter()
        .partition_mapped(|&n| n % 2 == 0, |n| n, |&n| (n, (n * 10) as f32));

but when I do, only the first line stays as a doc comment, the rest end up pasted "outside":

What I'm proposing is that I should've ended up with this instead:

/// let (even, odd_map): (Vec<i32>, HashMap<i32, f32>) =
///     a.iter()
///         .partition_mapped(|&n| n % 2 == 0, |n| n, |&n| (n, (n * 10) as f32));

I don't know if this is within the purview of rust-analyzer or not, but I thought I'd bring it up, since it's a very frustrating behavior.

Reproduce the multiline paste behavior in a Rust doc comment using the example from the issue. First determine whether rust-analyzer owns this editor behavior or whether it belongs to the editor, then identify the relevant paste-handling entry point and add coverage for prefixing each pasted line with /// when done.