Enable clang-format for rustc

[dianqk]

Proposal

I plan to use clang-format to format the C++ code in compiler/rustc_llvm/llvm-wrapper. The issue originates from: rust-lang/rust#123510.

To achieve this, I intend to integrate clang-format into our LLVM build process. Although adding clang-format will increase maintenance costs, even minor version differences (e.g., between clang-format 18.1.7 and 18.1.6) can lead to formatting discrepancies. Using a consistent build version ensures uniform formatting results, preventing inconsistencies with the CI environment and reducing the difficulties developers face when configuring their environments.

I plan to add new fields in config.toml to control clang-format behavior:

[llvm]
# Use a specified version of clang-format, the most common scenario.
clang-format = '5a5152f653959d14d68613a3a8a033fb65eec021'
# Use a clang-format from a specified path.
clang-format = '/path/to/clang-format'
# Use the same version as the llvm-project submodule; if not available it will build from local.
clang-format = 'submodule'
# For specific reasons, do not use clang-format.
clang-format = false

Mentors or Reviewers

Myself or @workingjubilee.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

@rustbot concern reason-for-concern 
<description of the concern> 

Concerns can be lifted with:

@rustbot resolve reason-for-concern 

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler @rust-lang/compiler-contributors

[dianqk]

We can use clang-format provided by pip: https://pypi.org/project/clang-format/.