Optional macro arguments without default value

[seijikun]

Soo... maybe I'm just too dumb, but I often run into a problem like this:

{% macro my_product(name, features = []) %}
    {% if features.is_empty() %}
        ...
    {% endif %}
{% endmacro %}

{{ my_product("Awesome TV", features = ["Full-HD", "no Full-HD"]) }}
{{ my_product("Less Awesome TV") }}

This obviously doesn't compile, because the default value for features ([]) is no fully specified type. The compiler knows it's an array, but not what its item is. Now, some cases of this could probably be "fixed" by #514, but another Idea I got is this:

{% macro my_product(name, features?) %}
    {% if is features defined %}
        ..
    {% endif %}
{% endmacro %}

{{ my_product("Awesome TV", features = ["Full-HD", "no Full-HD"]) }}
{{ my_product("Less Awesome TV") }}

This would allow specifying optional arguments without default value. (If not specified - they are simply not defined).
The (IMHO massive) advantage of this is, that each if is (not) defined is resolved at compile-time - and the code for the other branch is never generated. This reduces the code generated - and resolves the problem of unspecified types in the default case.

(The shown example is much simpler than what I typically have - where the array items are complex objects)

[GuillaumeGomez]

I'm a bit reluctant for adding a ? in this position. If we want to add new syntax on top of the official jinja one, I'd prefer if it was as close to rust's as possible. So I think allowing to add types with : ty is the best approach here.

[seijikun]

Just one addition (first had to verify it):

The type annotation syntax can only fix some cases of this, because it's very restrictive for variable bindings in Rust.
Let's say you have these types:

struct TV {}
struct Radio {}

trait MediaDevice {}

impl MediaDevice for TV {}
impl MediaDevice for Radio {}

Writing a macro with an optional argument that can accept an array of either of the two types TV and Radio will not be possible, because the impl Trait syntax is not allowed for variable bindings:

let md: &[impl MediaDevice] = &[TV {}];

[GuillaumeGomez]

But

let x: &[Box<dyn MediaDevice>] = &[];

is allowed

[seijikun]

True. Also:

let x: &[&dyn MediaDevice] = &[&TV{}, &Radio{}];