arrayTypeSentinel

fn arrayTypeSentinel(gz: *GenZir, scope: *Scope, ri: ResultInfo, node: Ast.Node.Index) !Zir.Inst.Ref {
    const astgen = gz.astgen;
    const tree = astgen.tree;

    const len_node, const extra_index = tree.nodeData(node).node_and_extra;
    const extra = tree.extraData(extra_index, Ast.Node.ArrayTypeSentinel);

    if (tree.nodeTag(len_node) == .identifier and
        mem.eql(u8, tree.tokenSlice(tree.nodeMainToken(len_node)), "_"))
    {
        return astgen.failNode(len_node, "unable to infer array size", .{});
    }
    const len = try reachableExprComptime(gz, scope, .{ .rl = .{ .coerced_ty = .usize_type } }, len_node, node, .array_length);
    const elem_type = try typeExpr(gz, scope, extra.elem_type);
    const sentinel = try reachableExprComptime(gz, scope, .{ .rl = .{ .coerced_ty = elem_type } }, extra.sentinel, node, .array_sentinel);

    const result = try gz.addPlNode(.array_type_sentinel, node, Zir.Inst.ArrayTypeSentinel{
        .len = len,
        .elem_type = elem_type,
        .sentinel = sentinel,
    });
    return rvalue(gz, ri, result, node);
}