Add diff in output of --check (for CI use cases)

Edit 2025-03-06 by @lydell: This is a very long thread. It starts out with me asking for use cases for this feature. People then provide use cases over and over. It is clear that this is a wanted feature. There is no need to add more comments about that, but do react with 👍 on this post. There is a pull request https://github.com/prettier/prettier/pull/12598 for this feature, but it is (in my opinion) a bit lacking: https://github.com/prettier/prettier/pull/12598#issuecomment-2338648470. I’m not a maintainer anymore, but it seems to me like a good PR for this would be merged.

More people submit PR for documents in Markdown files or other things via Github web editor. If they make a mistake then the CI fails but there is no chance to inform the user whats wrong.

I see some infos should be with the --debug-check but currently there is no output with this option.

Environments:

• Prettier Version: 1.18.2
• Running Prettier via: cli
• Runtime: node?
• Operating System: linux

Steps to reproduce:

prettier "**/*.{md,css,scss,yaml,yml,ts}" "--check"

Expected behavior:

Show each error and the changes like with --debug-check here in #4415

Actual behavior:

Show nothing about the errors:

> @ prettier /home/circleci/project
> prettier "**/*.{md,css,scss,yaml,yml,ts}" "--check"

Checking formatting...
docs/contributing/environment.md
Code style issues found in the above file(s). Forgot to run Prettier?

Also there are no more infos available with this commands:

prettier "**/*.{md,css,scss,yaml,yml,ts}" "--debug-check"
prettier "**/*.{md,css,scss,yaml,yml,ts}" "--debug-check" "--check"

Start with the Prettier CLI behavior for --check and --debug-check, then read pull request #12598 and its linked review comment to understand the existing approach and remaining concerns. Reproduce the command shown in the issue and compare the desired diagnostic diff with the current output; the work is done when --check reports the relevant changes for CI users.