Proposal: Add variant of CustomizeDiffFunc that returns diag.Diagnostics

[gdavison]

Use-cases

There are times when a CustomizeDiffFunc would ideally return a warning message. The current option is to add a log message, but that is only visible to the practitioner if logging is enabled.

Proposal

Create a variant of CustomizeDiffFunc, e.g., CustomizeDiffFuncDiags, that returns a diag.Diagnostics instead of error.

[arxeiss]

Yes, please. I would like to see this improvement as well.

Our use case

User can create config in special syntax via Terraform attribute. Or in our Web Console, where they can use Drag 'n' Drop UI Tool.
But, if they do via Terraform, we cannot render diagram in UI tool.

So what can happen is, they use our UI tool to design it and then export that syntax to Terraform.
But after terraform apply the diagram data are lost.

I would like to show them Warning, that by applying changes, diagram will be discarded. But I cannot do that now because.
I have the code below, which shows that warning. But only after applying. So it is a bit late

// "was_configured_via_ui" is set as Computed

// ReadContext function
wasConfiguredViaUI := false
if resp.ConfigType == "ui_rich_json" {
    wasConfiguredViaUI = true
}
data.Set("was_configured_via_ui", wasConfiguredViaUI)

// UpdateContext 
if data.Get("was_configured_via_ui").(bool) {
	*d = append(*d, diag.Diagnostic{
		Severity:      diag.Warning,
		Summary:       "Config was managed by the Console UI. This change will discard UI Diagram.",
	})
}