Allow string interpolation in property names

[alex-frankel]

Those strings should support string interpolation so I can use expressions:

var uamiId = resourceId(subId, rgName, 'Microsoft.ManagedIdentity/userAssignedIdentities', uamiName)

resource dScript '...' = {
  ...
  properties: {
    managedIdentities: {
      '${uamiId}': {}
    }
  }
}

[majastrz]

ship it!

[anthony-c-martin]

A drawback of allowing unrestricted interpolation that we should probably discuss: all our type safety goes out of the window as soon as anyone uses it.

For example:

{
  '${myKey}': true // if myKey evaluates to 'abc' at runtime, we've got a duplicate key
  abc: false
}

Or:

...inside resource declaration...
properties: {
  '${myKey}': true // myKey evaluates to something that the resource schema doesn't allow
}

[majastrz]

We could probably restrict it to places that are using additionalProperties in the swagger.

An even more restrictive option could be to only allow its use at identity.userAssignedIdentities.* property names and use feedback to discover other places where it's needed. (Or do we need to scan quickstart ourselves?)

[majastrz]

What if we required the property name (not property value) to be of type resourceId on properties inside identity.userAssignedIdentities? This would be the same type that we're planning to use for setting deployment scopes, which is a subtype of string.

I think this would give us decent type safety and would prevent arbitrary strings from being used that way. Since we're planning functions to construct resourceId values anyway, the users could construct them out of component parts pretty easily. Could also consider exposing resourceId as a parameter or output type, which would benefit module validation as well.