LLVM assertion failed in code with closures

[netvl]

This code (as reported in this SO question):

fn foo(t: &mut int){
    println!("{}", t);
}

fn main() {
    let test = 10;

    let h = move || {
        let mut r = &mut test.clone();
        foo(r);
    };

    h();
}

causes an assertion failure in LLVM code:

Assertion failed: (S->getType()->isPtrOrPtrVectorTy() && "Invalid cast"), function CreatePointerCast, file /Users/rustbuild/src/rust-buildbot/slave/nightly-mac/build/src/llvm/lib/IR/Instructions.cpp, line 2522.

[japaric]

I think this is a duplicate of #19141. Right now, when you assign a closure to a variable you always end with a boxed closure, and using move with a boxed closure shouldn't be allowed (the bug is that the compiler lets you do that) because that kind of closure doesn't capture by value. Once you've created this invalid closure, depending on how you created it or what you do with it, you'll get different LLVM assertions.

The workaround here is to annotate the closure (like move |:|) to make the compiler do the Right Thing and give you back an "unboxed" closure.

cc @nikomatsakis