glib-macros: Mark property getters as #[must_use]

[AaronErhardt]

This PR should fix #1186

I'm still not convinced that quote_spanned is correctly used (because there are obviously a lot of tokens which have nothing to do with the span of the attribute), but I don't know the reasoning behind the code, so adding #[must_use] to the codegen is the easiest fix and actually a useful addition even if you don't use clippy::must_use_candidate.

[ranfdev]

quote_spanned is required mostly when dealing with user input.

In this piece of code, the cast <#ty as #crate_ident::Property> could fail, and without a span the user is left with a seemingly random error without any direction on what's causing it.

I added the span to the entire generated code without knowing about the possible consequences with clippy... I agree that quote_spanned should be used more granularly.

I don't agree on adding #[must_use] on a getter though, because the returned value doesn't have to be used at all.

[AaronErhardt]

I don't agree on adding #[must_use] on a getter though, because the returned value doesn't have to be used at all.

What's the point of using a getter without using the value?

[ranfdev]

A custom getter could trigger a side effect and you may not need the actual value.
It's an edge case but it's technically true.

Apart from that edge case, in the general case rust getters don't have a #[must_use] attribute.

#[must_use] should only be used if discarding the value triggers a different behavior than using the value, as with a Future. If you use a Future, you get a side effect, if you don't use it, the Future is simply discarded.

In our case whether we use the returned value or not, nothing changes, so adding #[must_use] is incorrect and confusing.

[AaronErhardt]

Alright, that makes sense. I just assumed that #[must_use] was supposed to be used more often, likely because I though the clippy lint should suggest idiomatic code, but it's under "pedantic" after all.

I guess a proper fix would then be about keeping the function declaration largely mixed site and only use the attribute span for the type cast.

[ranfdev]

Ok, I've read more about #[must_use].
Apparently it is adopted in more cases than I had anticipated, even in the stdlib (https://blog.rust-lang.org/2018/06/21/Rust-1.27.html#must_use-on-functions and https://std-dev-guide.rust-lang.org/policy/must-use.html).

Considering that, most of the times, the user calls a getter to actually get and use the returned value and not to produce a side effect, I assume adding #[must_use] on a getter is correct.