Map values cannot be reassigned once they are set

Reproduction:

let map: hmap[] = []
let map[key] = 1
let map[key] = 2
echo @map

Prints key 1 instead of key 2. Both HashMaps and BTreeMaps are affected.

Expected behaviour:

I'd expect it to work the same way as integer variables work:

let m: int = 0
let m = 1
let m = 2
echo $m  # Prints 2

Build information:

rustc -V: rustc 1.33.0-nightly (4c2be9c97 2019-01-22)

git rev-parse HEAD: 7e45ae99