`soft-float` feature on mips-unknown-linux-musl is listed as regular feature by rustc, but emits an unstability warning during compilation

Location (URL)

Within CLI output of rustc +stable --target mips-unknown-linux-musl --print target-features

Summary

When listing the target features for mips-unknown-linux-musl (i.e. the MIPS architecture features; should be the same with other mips-* targets), rustc says the following:

    soft-float               - Does not support floating point instructions.

Note that for other features considered experimental, this is noted in their description, for example:

    mips5_32r2               - Subset of MIPS-V that is also in MIPS32r2 [highly experimental].
    mips64r6                 - Mips64r6 ISA Support [experimental].

However, when compiling with -Ctarget-feature=+soft-float, the following warning is emitted:

   Compiling breakwater-parser v0.21.0
warning: unknown and unstable feature specified for `-Ctarget-feature`: `soft-float`
  |
  = note: it is still passed through to the codegen backend, but use of this feature might be unsound and the behavior of this feature can change in the future
  = help: consider filing a feature request

warning: `breakwater-parser` (lib) generated 1 warning

The feature should therefore be denoted as experimental in the docs.

Note: I think that this feature is on by default for the target CPU mips32r2; my particular system has a MIPS 24Kc which does not have an FPU (the variant 24Kf would), but is officially listed by MIPS’s documentation as supporting Release 2 of the MIPS32 base (so the target CPU should be correct). I’m simply adding the feature to avoid any spurious illegal instruction exceptions.

Start by reproducing rustc +stable --target mips-unknown-linux-musl --print target-features and compare the soft-float entry with the other MIPS feature descriptions. Locate the target-feature definitions that generate this CLI listing, mark soft-float as experimental, and verify the displayed description matches the issue's expected wording.