I was reading through the Material Design guidelines and noticed this restriction on the snackbar usage: "Don’t place icons in snackbars."
Why not? What would be the rationale for that?
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You can see that at multiple parts:
Component: Snackbar Priority: Low priority
Component: Banner Priority: Prominent, medium priority
Component: Dialog Priority: Highest priority
As it is writte, a snack bar has low priority. MD also tells you to use Alerts if you need to use a icon. This means that Icons are prioritized higher than low on the Material Design Guidelines.
An Alert is a sub display of Dialog which has a priority of Highest priority compared to the Low priority of the snackbar.
This is what MD says about Alerts: "Alert dialogs interrupt users with urgent information, details, or actions."
Snackbars are not supposed to "interrupt" users but just quickly inform them. Any visual stimulant, such as Icons, could interrupt the user. So the most logical explanation is, that Icons are having a priority of atleast medium, as snackbars have a low priority. Here is a full part about icons on material design you should dive into for more information.
I think the answer is about user interaction with Snackbars. When a Snackbar (or toast!) appears on screen, I just scan it quickly to check everything went well. Not reading the content of the Snackbar. Using icons needs more attention to understand the message. As you can see "Alert" is recommended on Google Material guideline.