[MIR] double drop with slice patterns

[arielb1]

Version

$ rustc -V
rustc 1.11.0-dev (42b7c32ac 2016-07-02)

STR

#![feature(slice_patterns, rustc_attrs)]

struct NoisyDrop;
impl Drop for NoisyDrop {
    fn drop(&mut self) { println!("splat!"); }
}

#[rustc_mir]
fn main() {
    let [x] = [NoisyDrop];
}

Expected Result

NoisyDrop gets dropped once

Actual Result

NoisyDrop is dropped twice

[brson]

Should this go on the mir milestone?

[arielb1]

@brson

slice_patterns is feature-gated and quite buggy.

[arielb1]

Fixing the issue for arrays would be quite easy. The primary problem is that you can move out of slices

#![feature(slice_patterns, advanced_slice_patterns, box_syntax, box_patterns)]

fn cond() -> bool { false }

fn make() -> Box<[Box<[Box<[String]>]>]> {
    box [box [box ["hello".to_string(), "world".to_string()]]]
}

fn main() {
    let b3 = make();

    match (b3, cond()) {
        (box [box [box [s, ..], ..], ..], true) => {
            println!("{}", s);
        }
        (box [.., box [.., box [.., s]]], _) => {
            println!("{}", s);
        }
        _ => {}
    }
}

Here we must drop only "hello" in the first branch, and only "world" in the second branch, but I do not see any reliable way of doing it.

[aturon]

This is not blocking the move to MIR, as @arielb1 notes.

[bluss]

Still an issue in rustc 1.15.0-nightly (0bd2ce62b 2016-11-19)