Workaround rust nightly borrowcheck error (#1860)

[mitchmindtree]

Surprisingly, this fixes the error filed at #1860!

This seems suspicious, perhaps indicative of a bug in Rust's non-lexical
lifetime handling on nightly?

The lifetimes in the handlebars::Renderable::render method signature
are quite complicated, and its unclear to me whether or not Rust is
catching some new safety edge-case that wasn't previously handled
correctly...

Possibly related to drop order, which I think is related to the
order of binding statements?

[andrewdavidmackenzie]

Do you plan to release a v0.4.21 on crates.io?

[ehuss]

Thanks so much for the quick fix!

Yea, this fix looks correct. I don't think it is an issue with the borrow checker. What happens is that the handlebars render function has this signature:

fn render<'reg: 'rc, 'rc>(
    &'reg self,
    registry: &'reg Registry<'reg>,
    context: &'rc Context,
    rc: &mut RenderContext<'reg, 'rc>,
    out: &mut dyn Output,
) -> Result<(), RenderError>;

Based on that signature, RenderContext is mutable and has a 'rc lifetime, which means in theory it could receive references from &'rc Context. If local_ctx is dropped before local_rc, then in theory local_rc could contain a dangling reference into something from local_ctx.

I don't think that happens. The RenderContext is mutable for other reasons.

I don't have time right now to do a release, but I will make one later today.