`f64::copysign` claims support since `1.35.0` in `core`

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
68/100
Issue type
Documentation
Clarity
Mostly clear
Activity status
Quiet
Tech stack
rust
Domain
documentation

Research direction

Start at the linked core::primitive.f64::copysign documentation and inspect how its availability version is recorded. Reproduce the shown no_std cargo check, then correct the documented version so it matches core support and verify the example no longer contradicts the API.

Written by the indexing model from the issue text.

Description

A-docs T-libs
Location (URL)

https://doc.rust-lang.org/core/primitive.f64.html#method.copysign

Summary

#131304 added f64::copysign to core which is long after 1.35.0. The following code using the much newer cargo 1.76.0 in a no_std environment causes a compilation failure:

#![no_std]
fn example() {
    _ = 0f64.copysign(0f64);
}

[zack@laptop src]$ cargo -V
cargo 1.76.0 (c84b36747 2024-01-18)
[zack@laptop src]$ cargo check
    Checking water v0.1.0 (/home/zack/projects/water)
error[E0599]: no method named `copysign` found for type `f64` in the current scope
 --> src/lib.rs:3:14
  |
3 |     _ = 0f64.copysign(0f64);
  |              ^^^^^^^^ method not found in `f64`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `water` (lib) due to 1 previous error

Commenting out #![no_std] fixes the problem showing that while the method was defined in std since 1.35.0, it wasn't added to core until far later:

[zack@laptop src]$ cargo check
    Checking water v0.1.0 (/home/zack/projects/water)
warning: function `example` is never used
 --> src/lib.rs:2:4
  |
2 | fn example() {
  |    ^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: `water` (lib) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
Dominant language
Rust
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Merged PRs (30d)
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