Introduce mixin declaration to interface

[taichi-ishitani]

Close #1212

Adds interface mixin via a new mixin declaration. Inside an interface, mixin <interface>; mixes in the members (variables, functions, and modports) of another interface; the mixed-in members are expanded into the interface as if they were declared directly.

Syntax

mixin is an interface declaration item:

interface InterfaceC {
    mixin InterfaceA;
    mixin InterfaceB;

    // additional members of the interface
}

Generic arguments can be passed to a mixed-in interface:

interface InterfaceC::<W: u32> {
    mixin InterfaceA::<W>;

    // ...
}

Restrictions on a mixed-in interface

A mixed-in interface must satisfy all of the following; otherwise an InvalidMixin error is reported:

• It must be an interface, not a proto interface.
• It must not have parameters (generic parameters are allowed).
• It must not itself mix in other interfaces (nested mixin is not supported yet).
• Its member names must not conflict with the interface's own members or with the other mixed-in interfaces' members.

Generated SystemVerilog

The mixed-in members are expanded inline into the interface. Given:

package DataPkg::<W: u32> {
    type Data = logic<W>;
}

interface Req::<W: u32> {
    import DataPkg::<W>::*;
    var command: Data;
    modport mp_req {
        command: output,
    }
}

interface Rsp {
    var status: logic;
    modport mp_rsp {
        status: input,
    }
}

interface Bus::<W: u32> {
    mixin Req::<W>;
    mixin Rsp;

    modport mp {
        ..same(mp_req, mp_rsp)
    }
}

Bus is emitted with the mixed-in members expanded (names simplified for readability):

interface Bus__W;
    // mixed in from Req
    DataPkg__W::Data command;
    modport mp_req (output command);

    // mixed in from Rsp
    logic status;
    modport mp_rsp (input status);

    // the interface's own member
    modport mp (
        output command,
        input  status
    );
endinterface

Notes:

• A member referenced through a mixed-in interface's import is emitted in package-qualified form (e.g. DataPkg__W::Data) instead of merging the import statements. This avoids conflicts when mixed-in interfaces import same-named symbols from different packages.
• Generic arguments passed to a mixed-in interface are propagated into the expanded members.

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks

---

_{Comparing taichi-ishitani:mixin_interface (75500ce) with master (48df938)¹}

Footnotes

1. No successful run was found on master (75500ce) during the generation of this report, so 48df938 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[dalance]

The whole changes look good.
Could you add a snapshot test for InvalidMixin error?

[dalance]

Could you use crate::HashSet instead of std?

[taichi-ishitani]

I changed it to crate::HashSet.

[taichi-ishitani]

The whole changes look good. Could you add a snapshot test for InvalidMixin error?

I added snapshot tests for InvalidMixin error.