Adding module hierarchy for library creation and explicit imports

[rbkettlewell]

This is a language feature proposal.

Incorporating the ability to publish projects and add dependencies similar in spirit to Rust is a great addition to Veryl. However, one current limitation, it seems to me, is that there isn't a way to organize modules in a hierarchical fashion. I realize that this is likely to align with System Verilog's flat global namespace for modules and what is expected from EDA tools. However, I believe the pros outweigh the cons of organizing modules hierarchically into sharable libraries and that this promotes code reuse.

Lets imagine we are building a core library which contains useful building blocks for digital design:

core
    src
        register
            reg_a.vl
            reg_b.vl
        fifo
            fifo_d.vl
            fifo_s.vl
        ...
    Veryl.toml
    Veryl.pub

If one then adds this core library as a dependency for a project it would be nice to be able to use its modules explicitly as such:

module foo
use core::register::reg_a;
(...){
    // some user logic
    inst a_foo : reg_a(...) 

Or alternatively:

module foo
use core;
(...){
    // some user logic
    inst a_foo : core::register::reg_a(...) 

This would be preferable over implicit importing once the dependency is defined, where something like this would be allowed:

module foo(...){
    // some user logic
    inst a_foo : core::register::reg_a(...) 

Perhaps a mod.vl could be added to the core library taking Rust's older approach to organizing modules in a crate. This file mod.vl for the register subdirectory could have the following contents:

mod reg_a;
mod reg_b;

A similar mod.vl would thus be added for each logical grouping of Veryl modules like fifo and so on.

Finally the core library would be updated to look like the following:

core
    src
        register
            reg_a.vl
            reg_b.vl
            mod.vl
        fifo
            fifo_d.vl
            fifo_s.vl
            mod.vl
        ...
    Veryl.toml
    Veryl.pub