I'm trying to use one instance's privateIp in the user data of another instance. I tried doing something like

function getIp (server: aws.ec2.Instance) {
  return pulumi.all([server.privateIp]).apply(([privateIp]) => {
                      return pulumi.interpolate`${privateIp}`;
                  });
}

to get the IP, but it still gives me the calling toString on an output error instead of the value. Thoughts on how to do this?

You can use

pulumi.interpolate

directly in that case.

const userData = pulumi.interpolate`
IP Address: ${instance.publicIp}
Subnet ID: ${vpc.publicSubnetIds[0]}
`;

👏 👏 👏 👏

For two strings you can use .all and then .apply