The citation from https://dlang.org/spec/hash-map.html
Static Initialization of AAs
----------------------------
immutable long[string] aa = [
"foo": 5,
"bar": 10,
"baz": 2000
];
unittest
{
assert(aa["foo"] == 5);
assert(aa["bar"] == 10);
assert(aa["baz"] == 2000);
}
Judging by the text, it is static initialization, during
compilation.
