Use v0 mangling by default on nightly

[davidtwco]

Proposal

Following-on from discussions prompted by compiler-team#934, @wesleywiser and I were discussing how we could move the v0 mangling forward in general. v0 is already being used successfully for everything at Microsoft and we've waited quite a long time for support to exist in downstream tools that interact with Rust symbols.

Using v0 mangling by default, but only on nightly builds, is a useful way to identify potential issues while ensuring users can opt-out with -Csymbol-mangling-version=legacy (which isn't possible on stable). After some indeterminate period of time, we could enable v0 on stable too, with more confidence that it won't cause major issues.

If we decide to do this, we should write a blog post announcing that this change will happen, laying out the advantages and (few and minor!) disadvantages of the v0 mangling scheme, so that if users notice a difference then they can find a blog post explaining the change.

Mentors or Reviewers

@wesleywiser and @davidtwco

Process

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

• File an issue describing the proposal.
• A compiler team member who is knowledgeable in the area can second by writing @rustbot second or kickoff a team FCP with @rfcbot fcp $RESOLUTION.
  • Refer to Proposals, Approvals and Stabilization docs for when a second is sufficient, or when a full team FCP is required.
• Once an MCP is seconded, the Final Comment Period begins.
  • Final Comment Period lasts for 10 days after all outstanding concerns are solved.
  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

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 can formally be registered here by adding a comment.

@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

[oli-obk]

@rustbot second

[mati865]

@rustbot second

[ojeda]

In case it is helpful, as another data point, Rust for Linux has also been using v0 since its early days (~ end of 2020).

[davidtwco]

Draft blog post for this at rust-lang/blog.rust-lang.org#1730