Tracking issue for release notes of #154647: change `c_double` to `f32` on `avr` targets

[rustbot]

This issue tracks the release notes text for #154647.

cc @folkertdev, @tgross35 -- original issue/PR authors and assignees for drafting text

See the forge.rust-lang.org chapter about release notes for an overview of how the release team makes use of these tracking issues.

Release notes text

This section should be edited to specify the correct category(s) for the change, with succinct description(s) of what changed. Some things worth considering:

• Does this need an additional compat notes section?
• Was this a libs stabilization that should have additional headers to list new APIs under Stabilized APIs and Const Stabilized APIs?

# Compatibility Notes
- On `avr` targets, C's `double` type is 32-bit by default, so [change `c_double` to `f32` on `avr` targets to match](https://github.com/rust-lang/rust/pull/154647). This is a breaking change, but necessary to make `c_double` match C's double.

Tip

Use the previous releases for inspiration on how to write the release notes text and which categories to pick.

Release blog section

If this change is notable enough for inclusion in the blog post then this section should be edited to contain a draft for the blog post. Otherwise leave it empty.

On `avr` targets, C's `double` type is 32-bit by default. Rust's `core::ffi::c_double` was previously incorrectly 64-bit. In this release, we've changed `core::ffi::c_double` to match C's double on AVR. This is a breaking change, but necessary to make `c_double` match C's double.

Note

If a blog post section is required the release-blog-post label should be added (@rustbot label +release-blog-post) to this issue as otherwise it may be missed by the release team.

[Mark-Simulacrum]

Also nominating for libs-api since this (afaict) wasn't FCPd despite being a stable affecting change + a breaking change, albeit a correctness fix.

[Amanieu]

We discussed this in the @rust-lang/libs-api meeting and are happy with this change: it's a correctness fix to match C, which is the entire point of c_double. This should have gone through FCP, but I'm just going to start one here as a formality.

@rfcbot merge libs-api

[rust-rfcbot]

Team member @Amanieu has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @the8472

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rust-rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔