Update the existing musl targets to be dynamically linked.

We should change the -musl targets to be consistently dynamically linked to their libc, rather than statically linking the musl libc into them at compile-time. If a MUSL end-user wants the musl-libc to be statically-linked into their program, they will need to specify it explicitly (e.g. via -Ctarget-feature=+crt-static)

Proposed Mitigation Strategy: In order to give the current users a chance to prepare for this change, we will first land a lint on all hosts that target musl. This lint will warn that the defaults here are going to change (namely, that the implicit +crt-static is switching to -crt-static). The lint will be silenced by explicitly providing either +crt-static or -crt-static.

Why we should do this: This would make Rust's -musl targets more consistent on two axes: It would make them more consistent with Rust's non-musl targets (which likewise dynamically link to the target's libc), and it would make them consistent with what musl toolchains expect, especially the toolchains for musl-based Linux distributions. (Today, musl-based Linux distribtuons are locally patching Rust to get the behavior they expect.)

Why we might not do this: It is a breaking change. The current defaults were put in place to support a specific use case of producing shippable binaries, and changing the default will break users who are not explicitly (and, with the current defaults, redundantly) requesting +crt-static. However, the mitigation strategy is intended to address the needs of those users.

Background: Rust currently has Tier 2 targets for a number of platforms using musl for libc (various flavors of ARM, MIPS and x86) as well as some Tier 3 targets (Hexagon, PowerPC, RISC-V, S390x and ARMv7 Thumb). Currently (most of) these targets statically link to musl libc during compilation.
ts could also be added.

Mentors or Reviewers

This MCP is an outcome from the compiler-team design meeting compiler-team#416 (zulip archive)

I expect the stake-holders who participated in that meeting to be involved, though they may not be the actual mentors.

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.