Update the existing musl targets to be dynamically linked.

[pnkfelix]

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.

[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.

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

[petrochenkov]

The correct combination of flags to preserve the old behavior is

-C target-feature=+crt-static -C link-self-contained=yes

because the old musl targets not only link libc statically by default, they also link libc and crt objects shipped with rustc by default, which is also undesirable and should be avoided in the new target config.

Besides that, cargo need to be convinced to pass these flags only when compiling for the target, and not for the host (proc macros, build scripts), and we must specify how to do that in the migration diagnostic and documentation.

[petrochenkov]

@rustbot second

[Timmmm]

-C target-feature=+crt-static -C link-self-contained=yes

That's quite a pain when most users using the musl targets are primarily doing it so that they get a statically linked binary and don't have to deal with glibc compatibility issues.

This should only be done once there is a more user friendly way to get the current behaviour.

[Diggsey]

cc @emk who maintains a widely used docker image for building static musl binaries.

[ssokolow]

For context, I use musl targets for using pure Rust to write "shell scripts" that need to meet high standards of reliability, so I consider a Docker-based solution a deal-breaker.

Docker simply adds far too much delay and complexity to the compilation cycle for that kind of use-case... not to mention that I've spent the last few years promoting "For pure-Rust projects, Go-like static builds are as simple as rustup target add and cargo build --target" as a virtue of the language.

[Diggsey]

Docker simply adds far too much delay and complexity to the compilation cycle

Not sure if that was directed at my comment, but I'm not suggesting that everyone should use the docker image, just that the author and users of that docker image would be impacted by this potential change and should know about it.

[ssokolow]

I was unsure what your intent was, so I left whether I was replying to you ambiguous.

EDIT: I don't have a Zulip account (URLO, IRLO, Matrix, Reddit, StackExchange, etc. ...but not Zulip) and I'm not really in the mood to create an account on yet another service right now (assuming I even can, given how many services seem to be restricting themselves to those who have SMS numbers these days), so I'll bow out and hope my interests get adequately represented by someone else.

[wesleywiser]

Hi all, MCP issues aren't meant for discussion, we have a separate Zulip thread for that. Let's please move the discussion to that thread. Thanks!

[androm3da]

I'm happy to review or contribute changes necessary for hexagon to support this change.

[vi]

Will buildability of static musl be still tested automatically on CI even if/when dynamic musl becomes default?

Or will static flavour of musl effectively become Tier 3 instead of Tier 2?

[matthieu-m]

A Zulip thread/stream is very awkward to parse when it comes to searching for whether a question has already been asked, discussed, or answered.

I would recommend aggregating questions as well as their answers (if any) and the salient points of the discussion (if any) together in some sort of FAQ document... for example in the above MCP itself.

That is, I am fine with the discussion occurring on Zulip (or wherever) but I recommend that it be summarized in a more accessible manner (aggregated/summary) and at a more static location (nobody likes to scroll dozens of pages).

[nikomatsakis]

I think adding a FAQ to the template and collecting major points raised in the conversation is a good idea.

[apiraino]

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

[Timmmm]

Uhm... from the thumbs ups here it kinda looks like everyone agreed this was a bad idea (or at least premature without a simple alternative)... and then it was accepted?

You guys should at least post the reasoning behind this decision.

[Timmmm]

I skimmed the Zulip thread and it seems like the conclusion was that initially Rust will look for a system copy of Musl and dynamically link with that, if available, and fall back to static linking. And maybe there will be some kind of --static flag. That seems reasonable.

[apiraino]

@Timmmm a bit more context. Approving and closing the MCP is meant to acknowledge the issue and that actions will be taken ("ok, we gotta do something about it").
As you point out, the general idea is to lay out a clear migration path (deprecation notes, warnings, etc.) where we will fix the default musl target behaviour by providing as much information as possible so to hopefully not disrupt users' workflows (CIs and such).