Deprecate `-Csoft-float`

[RalfJung]

Proposal

The -Csoft-float flag is unsound and cannot really be salvaged. It should be deprecated and turned into a NOP, or removed.

See https://github.com/rust-lang/rust/issues/129893 for context. The summary is: this flag only has any effect on (32-bit) ARM *eabihf targets. It is equivalent to GCC/clang -mfloat-abi=soft, and as that name makes clear, it changes the ABI. When code built with -Csoft-float calls other code built without that flag (such as the standard library), and there are float types passed in that call, we have an ABI mismatch and hence UB. For that reason, we have the *eabi targets: they ensure that everything is consistently built with the soft-float ABI. This flag might predate those targets; it was added in https://github.com/rust-lang/rust/pull/9617 before Rust 1.0 and has seen very little discussion since then. The ARM folks that spoke up in https://github.com/rust-lang/rust/issues/129893 all agreed it has no use; in the worst case where someone uses a *eabihf target and that flag because there is no corresponding *eabi target, we "just" need to add the corresponding target.

Mentors or Reviewers

It's already implemented at https://github.com/rust-lang/rust/pull/129897, waiting for a review.

Process

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

• [x] 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.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

 @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 @rust-lang/compiler-contributors

[compiler-errors]

👍

@rustbot second

[Urgau]

It's been a over 10 days, there is a second, many thumbs up and no objections. MCP accepted.

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