addParam

    fn addParam(
        gz: *GenZir,
        param_gz: *GenZir,
        /// Previous parameters, which might be referenced in `param_gz` (the new parameter type).
        /// `ref`s of these instructions will be put into this param's type body, and removed from `AstGen.ref_table`.
        prev_param_insts: []const Zir.Inst.Index,
        ty_is_generic: bool,
        tag: Zir.Inst.Tag,
        /// Absolute token index. This function does the conversion to Decl offset.
        abs_tok_index: Ast.TokenIndex,
        name: Zir.NullTerminatedString,
    ) !Zir.Inst.Index {
        const gpa = gz.astgen.gpa;
        const param_body = param_gz.instructionsSlice();
        const body_len = gz.astgen.countBodyLenAfterFixupsExtraRefs(param_body, prev_param_insts);
        try gz.astgen.instructions.ensureUnusedCapacity(gpa, 1);
        try gz.astgen.extra.ensureUnusedCapacity(gpa, @typeInfo(Zir.Inst.Param).@"struct".field_names.len + body_len);

        const payload_index = gz.astgen.addExtraAssumeCapacity(Zir.Inst.Param{
            .name = name,
            .type = .{
                .body_len = @intCast(body_len),
                .is_generic = ty_is_generic,
            },
        });
        gz.astgen.appendBodyWithFixupsExtraRefsArrayList(&gz.astgen.extra, param_body, prev_param_insts);
        param_gz.unstack();

        const new_index: Zir.Inst.Index = @fromBackingInt(@intCast(gz.astgen.instructions.len));
        gz.astgen.instructions.appendAssumeCapacity(.{
            .tag = tag,
            .data = .{ .pl_tok = .{
                .src_tok = gz.tokenIndexToRelative(abs_tok_index),
                .payload_index = payload_index,
            } },
        });
        gz.instructions.appendAssumeCapacity(new_index);
        return new_index;
    }