Didn't expect rust to support dropping outer variables in a loop

1

This actually compiles: (note the drop(writer) inside a for loop)

let mut batches = Vec::new();
let mut ty = hecs::ColumnBatchType::new();
ty.add::<i32>();

let batch_size = 32;
let mut builder = ty.clone().into_batch(batch_size);

let mut writer = builder.writer::<i32>().unwrap();

for i in 0..10 {
    if let Err(v) = writer.push(i) {
        drop(writer);

        let old_builder = std::mem::replace(&mut builder, ty.clone().into_batch(batch_size));
        batches.push(old_builder.build().unwrap());

        writer = builder.writer::<i32>().unwrap();

        writer.push(v).unwrap();
    }
}

drop(writer);
batches.push(builder.build().unwrap());

New knowledge learned:
I can drop a Non-Copy value inside a loop as long as I re-assign it later.

2

There’s quite a lot of things you can do with local variables, because the compiler can see the control flow in the whole function and determine that there is no case where it would be read while empty.

3 
fn test() {
    let mut s = String::new();
    drop(s);
    s = String::new();
}

The above code can be compiled too, not just in a loop.