How does implicit language activation work?

[winstliu]

From https://code.visualstudio.com/api/references/activation-events, there is a note that says "Beginning with VS Code 1.74.0, languages contributed by your extension do not require a corresponding onLanguage activation event declaration for your extension to be activated".

I'm dealing with an extension that says it's being activated due to Rust:

But it does not contribute the Rust language.

So what is causing the implicit activation to trigger on Rust?