Vec is missing INDEX_GET protocol and support for range indexing

[jasal82]

Vec indexing is inconsistent. For example, indexing a single item in a vector works fine:

let v = [0, 1, 2];
let item = v[1];

Obtaining slices into the vector fails, however:

let slice = v[1..];
// fails with 'the index get operation `Vec[Range]` is not supported'

For String types, on the other hand, this works as expected. I have a patch for Rune ready that I can contribute.

[jasal82]

There's one thing that confuses me and needs to be clarified for the patch: Even after removing both get and Protocol::INDEX_GET instance functions from Vec's native module declaration I can still do let item = v[1] in the Rune script, which is completely unexpected.

[udoprog]

Vector indexing by a number is an internal Vm operation. Range indexing could probably also go there.

[jasal82]

Ok I understand. Intuitively I would think that neither the integer nor the range indexing should be an internal Vm operation because if there are instance functions in the native modules (like get()) which are just another frontend to the indexing you have to implement the operations twice.

[udoprog]

Hm, Vm operation exists because it accelerates the operation. We avoid additional dispatch to the external function which involves a bit of stack mangling.

I'd be open to being able to disable internal acceleration maybe it could be done through a feature flag, but then you'd run the risk of feature parity. All though disabling it completely should probably be supported at some point for folks that really wants a completely bare virtual machine.