Support -diff=true option in FormatCheck function

[Vince-Chenal]

It would be nice being able to get the diffs output from terraform fmt command.

[radeksimko]

Hi @Vince-Chenal
As far as I can tell the fmt -diff flag is not considered a stable interface from Terraform's perspective, as it doesn't control the output - internally Terraform just calls out to diff and basically passes the output back to the user - i.e. while diff is likely to have stable output itself, Terraform has no control over the stability of it.

https://github.com/hashicorp/terraform/blob/6d03303b7917be206a7fad0c8fe5838af0ac11a9/internal/command/fmt.go#L566-L591

On the positive side you could probably relatively easily implement comparable functionality yourself by copying that linked bytesDiff function from there and pass formatted & unformatted []byte + filename into it, and use that alongside the tfexec.Format(). Would that work for you?

cc @kmoe to confirm the stability expectations on Core side

[Vince-Chenal]

Thanks for you answer @radeksimko 👍
I was going to implement it myself but wanted to make sure it could not be handled from within tfexec.
I'll try implementing your option