[clang-format] Support .clang-format-ignore file

[mkurdej]

An idea split from #27467.
The gist is to be able to specify files that should not be formatted.
It could work in a similar way as .gitignore files (possibly the same syntax).

[mydeveloperday]

I like this idea, I often have generated files that I don't want to clang-format. (resource.h).

I notice clang-power tools has this capability built in. PowerTools

[thoni56]

I too, like this. I was googling for a way to do just that, ignore generated files although they match the pattern clang-format usually would format. One example is C source generated from Yacc.

[Badokas]

I was looking online how to ignore formatting for 3rd party code in specific folders and bumped into this issue. +1 for this feature!

[hrandib]

Another case is to exclude 3rd party submodule without polluting it with another .clang-format file just to ignore, +1 from me

[benner]

Another case is to exclude 3rd party submodule without polluting it with another .clang-format file just to ignore, +1 from me

this is exactly why I want this too

[Hadatko]

+1

[owenca]

I've come up with the following spec for the .clang-format-ignore file:

• A blank line is skipped.
• Leading and trailing spaces of a line are trimmed.
• A line starting with a hash (#) is a comment.
• A non-comment line is a single pattern.
• The slash (/) is used as the directory separator.
• A pattern is relative to the directory of the .clang-format-ignore file (or the root directory if the pattern starts with a slash).
• If a pattern ends with a slash, all files in a directory are ignored.
• If a pattern is a single dot (.), all files in the directory of the .clang-format-ignore file are ignored.
• Patterns follow the glob rules POSIX 2.13.1, 2.13.2, and Rule 1 of 2.13.3.
• A pattern is negated if it starts with a bang (!).

Multiple .clang-format-ignore files are supported similar to the .clang-format files, with a lower directory level file overriding voiding the higher level ones.

[owenca]

cc @mydeveloperday @HazardyKnusperkeks @rymiel

[bersbersbers]

A pattern is relative to ... the root directory if the pattern starts with a slash

Is that intentionally different from how .gitignore files work? https://git-scm.com/docs/gitignore

with a lower directory level file overriding the higher level ones

This appears a little unclear. Does the file override the other file, meaning, all of that other file's patterns are ignored, or are the patterns merged (that would be .gitignore behavior).

[HazardyKnusperkeks]

* A pattern is relative to the directory of the `.clang-format-ignore` file (or the root directory if the pattern starts with a slash).

Do we need support for absolute paths? That should be an anti-pattern for code repositories.
Also, then you would need a way to define absolute paths on windows.

I'm not against this feature, but I really don't think its that useful resp. one should spent time on implementing it.

You want to disable formatting of a single source file? // clang-format off in that file.
You want to disable formatting for third party code? .clang-format with DisableFormat: true in that directory resp. your directory for all 3rd party code (that's what I'm doing)
You want to disable formatting of generated code? Same as above, eventually you need to move such files in a separate directory. (Which is in my opinion also a good idea.)