gpui: Add Global marker trait

[osiewicz]

This should prevent a class of bugs where one queries the wrong type of global, which results in oddities at runtime.

Release Notes:

• N/A

[ConradIrwin]

I'm curious why the existing type stuff didn't work?

[osiewicz]

It is error prone when you're e.g. refactoring globals (or querying globals in general) - if you use wrong type as the argument of cx.global, you're only gonna notice that at runtime.
This whole thing started in #7023

[maxdeviant]

It is error prone when you're e.g. refactoring globals (or querying globals in general) - if you use wrong type as the argument of cx.global, you're only gonna notice that at runtime. This whole thing started in #7023

Yea, what the Global trait gives us is finer-grained control over what can be a global (and accessed via the global methods on the context).

So a common pattern we've started doing is having private types implement Global and then expose getters/setters on the inner type. This forces callers to go through a predefined interface instead of having carte blanche access to globals from anywhere.

It should make dealing with globals a lot more manageable (and also more type-safe).

[osiewicz]

It is also easier to tell what is global in the codebase.

Call us to get a quote on better type safety. :P

[ConradIrwin]

Makes sense! I was imagining "oddities" to include type confusion and other odd things, but this makes sense to me

[maxdeviant]

Makes sense! I was imagining "oddities" to include type confusion and other odd things, but this makes sense to me

Ah yea, the "oddities" is probably better read as "panics" 🤣