Lightning Components: can aura:id be set to an component attribute?

The alternative to this is to dynamically create the component using $A.createComponent and since attribute is accessible via the JavaScript one can assign dynamic values to aura:I'd

Currently the expression syntax is not supported in the aura:id attribute

No, it's not possible to dynamically set aura:id based on aura:attribute for any component, it should always point to literal/hardcoded string value.

Below statement is mentioned in the docs.

aura:id doesn't support expressions. You can only assign literal string values to aura:id.

Having aura:id point to Aura Expression, will not throw error when saving the markup.

<div aura:id="{!v.whatever}" /> //No error

But when trying find the div using component.find() , you won't be able to locate the div(i.e aura:html) it will return undefined.

component.find(cmp.get("v.whatever")) // returns undefined

It seems to be possible to save an component having markup like

<div aura:id="v.whatever" />

however an usage like this in the controller

cmp.find( cmp.get("v.whatever") )

will come back empty. So my assumption is, that it is not possible by design - even if the markup parser does not complain about it.