Terraform - passing for_each output in the same module

I am writing a terraform script with modules. I have a module that creates azure application insights using for_each as below:

resource "azurerm_application_insights" "appinsights-apps" {
    for_each            = var.app_insights_names  
    name                = each.key
    location            = var.location
    resource_group_name = var.app_insights_rg_name
    application_type    = "web"
    disable_ip_masking  = true
    workspace_id        = var.sf_la_workspace_resource_id
}

Now in want the same module to also create a diagnostic setting for every app insights created:

resource "azurerm_monitor_diagnostic_setting" "appinsights-diag-settings" {
    depends_on         = [azurerm_application_insights.appinsights-apps]

    for_each           = azurerm_application_insights.appinsights-apps
    name               = "diag"
    target_resource_id = azurerm_application_insights.appinsights-apps[*].id
    storage_account_id = data.azurerm_storage_account.logs-storage.id

    enabled_log {
        category_group  = "allLogs"
    }
}

The above does not work for me. I need to somehow pass the id's of application insights created earlier to target_resource_id variable. Should I use terraform output for this? If yes how do I reference the output within the same module?

Without getting into the details of how things work in Azure, the easiest solution would be the following:

resource "azurerm_application_insights" "appinsights-apps" {
  for_each            = var.app_insights_names  
  name                = each.key
  location            = var.location
  resource_group_name = var.app_insights_rg_name
  application_type    = "web"
  disable_ip_masking  = true
  workspace_id        = var.sf_la_workspace_resource_id
}

resource "azurerm_monitor_diagnostic_setting" "appinsights-diag-settings" {
    for_each           = azurerm_application_insights.appinsights-apps
    name               = "diag"
    target_resource_id = each.value.id
    storage_account_id = data.azurerm_storage_account.logs-storage.id

    enabled_log {
        category_group  = "allLogs"
    }
}

Since you are using for_each chaining between resources, the keys are the same as for the azurerm_application_insights resource, but you also get the attributes from that resource. This is why you can use each.value.id to access the id attribute of the resource.