Emit an error if a syntax config already exists

[GuillaumeGomez]

Fixes https://github.com/djc/askama/issues/674.

I realized that warnings were easy to ignore so I decided to turn it into an error. What do you think?

[GuillaumeGomez]

The checks I'm running are far from enough. I'll update it.

[djc]

Out of curiosity, is the custom syntax you're working with intended for rustdoc or some other project? What's the use case?

[GuillaumeGomez]

No, it's just that I don't like having settings being silently ignored. That's pretty much it. ^^'

EDIT: However my previous PRs were definitely for rustdoc. So when the new version is released, I'll update a few things in rustdoc.

[GuillaumeGomez]

It should be ready now.

[djc]

So can you explain again what problem this is trying to solve? This seems like a bunch of complicated code trying to improve diagnostics for what's like an edge case of an edge case -- in other words, extremely niche...

[GuillaumeGomez]

When your config adds a syntax which already exists. A problem I actually encountered in a test of this crate. :laughing:

[djc]

Okay, I'm inclined not to take this.

[GuillaumeGomez]

Understandeable!