Finalizer not run if struct with dtor is not assigned to a variable.

[brson]

This leaks. (and it fails to perform all the other side-effects one might expect from relevant destructors.)

use std::cast::transmute;
use std::libc::c_void;

struct Rattle { croak: ~str }

impl Drop for Rattle {
    fn drop(&mut self) {
        println!("Goodbye {}",self.croak);
    }
}

struct NonCopyable {
    r: Rattle,
    p: *c_void
}

impl Drop for NonCopyable {
    fn drop(&mut self) {
        let p = self.p;
        let _v = unsafe { transmute::<*c_void, ~int>(p) };
    }
}

fn main() {
    let t = ~0;
    let p = unsafe { transmute::<~int, *c_void>(t) };

    let _x = NonCopyable { r: Rattle{ croak: ~"assigned" }, p: p };
    // assigning this to a variable makes it not leak
    NonCopyable { r: Rattle{ croak: ~"unassigned" }, p: p };
}

Namely, running it prints:

% rustc /tmp/issue4734.rs && /tmp/issue4734
warning: no debug symbols in executable (-arch x86_64)
Goodbye assigned

[msullivan]

Confirmed as still a bug. Updated test case in description.

[pnkfelix]

Visiting for bug triage, email from 2013-08-19

Updated test case with a more visible way to see what's going wrong.

It is probably related to #3511

[alexcrichton]

#3511 has landed, but this has not been fixed.

[nikomatsakis]

I expect this is due to that goofy "ignore" mode, which I think ought to be removed from trans. In controlflow.rs, there is this fragment:

        ast::StmtExpr(e, _) | ast::StmtSemi(e, _) => {
            bcx = expr::trans_into(cx, e, expr::Ignore);
        }

I am pretty sure that Ignore mode is not running the relevant destructors. At minimum, we should change this code to check whether type_needs_drop and use trans_to_lvalue if so.