Proposal

Clang supports distributed ThinLTO which is used by bazel and buck2 for C++ to support ThinLTO for extremely large programs. In order to make Rust a full participant of such ecosystems, we need to support emitting the "ThinLTO Index" file during codegen so that the pre-link
indexing step can operate on Rust-originated object files identically to C/C++ ones.

I expect we'd want to come back and try to stabilize this only after doing work to make use of non-rustc linkers supported, which is on our backlog to discuss but not something we can do today.

I have a working proof of concept of this MCP as a patch on my fork of rust-lang/rust. The new format is guarded by requiring users to specify -Zunstable-options.

Mentors or Reviewers

If you have a reviewer or mentor in mind for this work, mention them here. You can put your own name here if you are planning to mentor the work.

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.