Skip caching calls (Resolves #667)

[vallentin]

This is just a quick fix to address #667, and open up discussion.

Should we take any assumptions about interior mutability, statics and side-effects? i.e. never cache calls. Someone could potentially even make a "funky" impl Deref, making caching field accesses "wrong".

Alternative suggestion, we could introduce a config to disable caching.

(I'll add tests, update docs, etc. when I know whether we want a config or just take no assumptions and not cache)

---

New behavior:

{{ foo() }} {{ foo() }} {{ bar }}  {{ bar }}

Now generates:

::std::write!(
    writer,
    "{expr0} {expr1} {expr2}  {expr2}",
    expr0 = &::askama::MarkupDisplay::new_unsafe(&(self.foo()), ::askama::Text),
    expr1 = &::askama::MarkupDisplay::new_unsafe(&(self.foo()), ::askama::Text),
    expr2 = &::askama::MarkupDisplay::new_unsafe(&(self.bar), ::askama::Text),
)?;

[Kijewski]

I'm not sure if this doesn't need to be a "deep" check. E.g. foo().0 should still be uncachable, right?

[vallentin]

Oh yeah, you're right. Guess my quick fix, was too quick. I'll fix it later 😅

[djc]

Should I review before you fix it or do you want to fix it before I review? Not sure what's more appropriate here.

[Kijewski]

Rebased on main. I added Expr::is_cachable() to test the recursive case.

[Kijewski]

@vallentin, I hope it's okay that I took this over. :)

[djc]

Thanks!