Add --emit=thin-link-bitcode to enable distributed ThinLTO users

[durin42]

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.

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Process

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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:

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<description of the concern> 

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

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

[tmandry]

@rustbot second

[nnethercote]

Why is the Zulip stream empty?

[bjorn3]

https://rust-lang.zulipchat.com/#narrow/stream/233931-t-compiler.2Fmajor-changes/topic/Add.20.60--emit.3Dthin-link-bitcode.60.20to.20enable.20.E2.80.A6.20compiler-team.23735 is the correct stream. This MCP was renamed, but triagebot doesn't change the link in it's comment.

[apiraino]

@rustbot label -final-comment-period +major-change-accepted