Remove -Csoft-float

[RalfJung]

Proposal

The -Csoft-float flag has been added in pre-1.0 times (rust-lang/rust#9617) without much discussion, probably with the intent to mirror -msoft-float in C compilers. It never properly mirrored clang though because it only affected the LLVM float ABI setting, not the "soft-float" target feature (the clang flag sets both). It is also blatantly unsound because it affects how float arguments are passed (rust-lang/rust#129893), making it UB to invoke parts of the standard library.

The flag got deprecated with Rust 1.83 (released November 2024), and in the year since then, nobody spoke up in rust-lang/rust#129893 to have it preserved. I think it is time to remove it. I can totally imagine us bringing back a similar flag, properly registered as a target modifier to preserve soundness, but that should come with a coherent design that works for more than one architecture (the flag only does anything on ARM).

I assume this will need an FCP since it is a breaking change.

So far we apparently often turned removed flags into such warnings, OTOH doing this for an ABI-affecting flag seems like a potentially bad idea. Therefore, we'll turn usage of this flag into a hard error.

Mentors or Reviewers

Implementation already available at rust-lang/rust#152973

Process

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Note

Concerns (0 active)

• error-or-ignore resolved in this comment

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Important

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cc @rust-lang/compiler

[Noratrieb]

@rustbot second

[Noratrieb]

@rustbot concern error-or-ignore

[Noratrieb]

We have decided to make it an error
@rustbot resolve error-or-ignore

[rustbot]

The final comment period is now complete, this major change is now accepted. 🎉

As the automated representative, I want to express gratitude to the author for their work and everyone else who contributed to this major change proposal.

If you think this major change shouldn't have been accepted, feel free to remove the major-change-accepted label and reopen this issue.

Note

Further progress is now tracked over at rust-lang/rust#154106.