Allow Trailing Commas in Arrays Inside Template Expressions

[Aequitosh]

Basically what the title says.

To give an example of what I mean, this here compiles just fine:

<nav class="flex justify-between items-center max-w-7xl gap-x-12">
    {% for (route, name) in [
        ("/about", "About"),
        ("/contact", "Contact"),
        ("/events", "Events"),
        ("/history", "History")
    ] %}
    <div class="gap-x-6 leading-6 hover:bg-gray-200">
        <a href="{{ route }}"><span>{{ name }}</span></a>
    </div>
    {% endfor %}
</nav>

But this here doesn't:

<nav class="flex justify-between items-center max-w-7xl gap-x-12">
    {% for (route, name) in [
        ("/about", "About"),
        ("/contact", "Contact"),
        ("/events", "Events"),
        ("/history", "History"),
    ] %}
    <div class="gap-x-6 leading-6 hover:bg-gray-200">
        <a href="{{ route }}"><span>{{ name }}</span></a>
    </div>
    {% endfor %}
</nav>

Note the little comma after the ("/history", "History") tuple in the second example.

I'm not sure if this is trivial to implement or not, but I thought I might bring it to y'alls' attention.

It wasn't that of a big deal in the end though, so I wouldn't call this a "major issue" or whatever, not by a long shot. Though, since Rust allows such trailing commas, I think askama should as well.

For reference, this is the error I got:

error: problems parsing template source at row 30, column 31 near:
       ",\n    ] %}\n    <div class=\"gap-x-6 leadi"...
  --> src/routes/mod.rs:96:10
   |
96 | #[derive(Template)]
   |          ^^^^^^^^
   |
   = note: this error originates in the derive macro `Template` (in Nightly builds, run with -Z macro-backtrace for more info)

Running with RUSTFLAGS="-Zmacro-backtrace" cargo +nightly build gives the following:

error: problems parsing template source at row 30, column 31 near:
       ",\n    ] %}\n    <div class=\"gap-x-6 leadi"...
  --> src/routes/mod.rs:96:10
   |
96 | #[derive(Template)]
   |          ^^^^^^^^ in this derive macro expansion
   |
  ::: /home/max/.cargo/registry/src/index.crates.io-6f17d22bba15001f/askama_derive-0.12.5/src/lib.rs:22:1
   |
22 | pub fn derive_template(input: TokenStream) -> TokenStream {
   | --------------------------------------------------------- in this expansion of `#[derive(Template)]`

Neither error was too helpful, but it does point to line 30, column 31, which in my code is precisely the closing parenthesis before the trailing comma; careful reading helps at least.

[djc]

Should be easy enough to implement, do you want to try submitting a PR? You'll want to start here:

https://github.com/djc/askama/blob/main/askama_parser/src/expr.rs#L263

[Aequitosh]

Oh yeah sure! I'll see to it once I have the time. Doesn't look too complicated at first glance.