MIR: broken MIR generated for Box

[arielb1]

#![feature(rustc_attrs, box_syntax)]

#[rustc_mir]
fn xyz(v: Result<[u32; 1024], ()>) -> Result<Box<Box<[u32; 1024]>>,()> {
    Ok(box box match v {
        Ok(x) => x,
        Err(e) => return Err(e)
    })
}

fn main() {
    for i in 1..10000000 { xyz(Err(())); }
}

This generates the following (broken) MIR:

fn(arg0: core::result::Result<[u32; 1024], ()>) -> core::result::Result<Box<Box<
[u32; 1024]>>, ()> {
    let var0: core::result::Result<[u32; 1024], ()>; // v
    let var1: [u32; 1024]; // x
    let var2: (); // e
    let mut tmp0: Box<Box<[u32; 1024]>>;
    let mut tmp1: Box<Box<[u32; 1024]>>;
    let mut tmp2: Box<[u32; 1024]>;
    let mut tmp3: ();
    let mut tmp4: [u32; 1024];
    let mut tmp5: ();

    bb0: {
        var0 = arg0;
        tmp1 = Box(Box<[u32; 1024]>);
        tmp2 = Box([u32; 1024]);
        switch(var0) -> [Ok: bb4, Err: bb5];
    }

    bb1: {
        return;
    }

    bb2: {
        tmp4 = var1;
        (*tmp2) = tmp4;
        goto -> bb6;
    }

    bb3: {
        tmp5 = var2;
        return = core::result::Result::Err(tmp5);
        drop(tmp0) -> bb1;
    }

    bb4: {
        var1 = (var0 as Ok).0;
        goto -> bb2;
    }

    bb5: {
        var2 = (var0 as Err).0;
        goto -> bb3;
    }

    bb6: {
        (*tmp1) = tmp2;
        tmp0 = tmp1;
        return = core::result::Result::Ok(tmp0);
        drop(tmp0) -> bb1;
    }
}

As you can see, tmp0 is dropped but never written to (AFAIK this should be OK because of zeroing) but the boxes are never freed.

[arielb1]

cc @nagisa @nikomatsakis

[nagisa]

Freeing boxes should've been fixed in #31307.

EDIT: I wasn’t looking closely enough, it is indeed broken.

[arielb1]

This is a problem on latest master (f50fb15)