It depends. I wrote a pre-commit hook (in shell, not precommit the tool) at a previous job that ran terraform fmt on any staged files (and add the changes to the commit) because I was really tired of having people push commits that would then fail for trivial things. It was overrideable with an env var.
IMO if there’s a formatting issue, and the tool knows how it should look, it should fix it for you.
The standard way for this with current tools is to have the formatter/linter make the changes but exit with a non-zero status, failing the hook. Then the person reviews the changes, stages, and commits. (That's what our setup currently has `tofu fmt` do.)
But if you don't want to have hooks modify code, in a case like this you can also just use `tofu validate`. Our setup does `tflint` and `tofu validate` for this purpose, neither of which modifies the code.
This is also, of course, a reasonable place to have people use `tofu plan`. It you want bad code to fail as quickly as possible, you can do:
tflint -> tfsec -> tofu validate -> tofu plan
That'll catch everything Terraform will let you catch before deploy time— most of it very quickly— without modifying any code.
> make the changes but exit with a non-zero status
That's reasonable. My personal (and that of my team at the time) take was that I was willing to let formatting - and only formatting - be auto-merged into the commit, since that isn't going to impact logic. For anything else, though, I would definitely want to let submitter review the changes.
IMO if there’s a formatting issue, and the tool knows how it should look, it should fix it for you.