[clang-format] Allow for Short namespaces to be on the same line

[tiagomacarios]

clang-format: Add "AllowShortNamespacesOnASingleLine" option
Proposed adding a AllowShortNamespacesOnASingleLine option, but the patch did not make it in. Would it be possible for us to consider applying that patch or looking at other ways to accomplish the same functionality?

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Check that no other contributor has already been assigned to this issue. If you believe that no one is actually working on it despite an assignment, ping the person. After one week without a response, the assignee may be changed.
2. In the comments of this issue, request for it to be assigned to you, or just create a pull request after following the steps below. Mention this issue in the description of the pull request.
3. Fix the issue locally.
4. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
5. Create a Git commit.
6. Run git clang-format HEAD~1 to format your changes.
7. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation. Mention this issue in the description of the pull request.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Tiago (tiagomacarios)

[clang-format: Add "AllowShortNamespacesOnASingleLine" option](https://reviews.llvm.org/D11851) Proposed adding a AllowShortNamespacesOnASingleLine option, but the patch did not make it in. Would it be possible for us to consider applying that patch or looking at other ways to accomplish the same functionality?

[mydeveloperday]

I would recommend supporting

namespace A { namespace B { class C; }}

as well as

namespace A { class C; }}

[galenelias]

I am able to pick this up, however there is an edge case I'm not sure how to solve:

// What happens if we format with '{BasedOnStyle: llvm, ColumnLimit: 30}'?
namespace A { namespace B { class C; }}

// Option 1
namespace A {
namespace B {
class C;
}
}

//Option 2 - I assume this one
namespace A {
namespace B { class C; }
}

[JessehMSFT]

Our main desire for AllowShortNamespacesOnASingleLine is to reduce the number of lines these declarations take up. I agree that Option 2 most closely aligns with that goal.

We're also conscious that clang-tidy's modernize-concat-nested-namespaces could be a useful tool to prevent hitting column limits in this scenario.

[galenelias]

Yah, also just to point it out, since it might not be super obvious, but the nested namespaces also really doesn't play nice with the FixNamespaceComments option. To demonstrate:

namespace A { namespace B { namespace C { class D; } } }

// Becomes this, instead of a no-op.  Even worse with ShortNamespaceLines: 0
namespace A {
namespace B { namespace C { class D; } } // namespace B
} // namespace A

My current plan is that this will be the output, but really it seems very awkward to reconcile AllowShortNamespacesOnASingleLine with FixNamespaceComments and nested namespaces.