UFCS ignoring unstable library feature requirements?

[iliekturtles]

In the context of #![no_std] libraries the core::num::Float trait is marked as unstable but has stable methods, including powi. To use the stable methods glob imports or UFCS both work:

Glob import members of the num module:

#![no_std]
#![crate_type="lib"]

use core::num::*;
//use core::num::Float; // error: use of unstable library feature 'core_float': stable
                        // interface is via `impl f{32,64}` in later crates (see issue #32110)

fn foo() -> f32 {
    3.0f32.powi(2)
}

Use UFCS:

#![no_std]
#![crate_type="lib"]

fn bar() -> f32 {
    <f32 as core::num::Float>::powi(3.0f32, 2)
}

Shouldn't the UFCS variant give the unstable library feature error? Is there another way to use the member call syntax without the glob import?

[hanna-kruppe]

The latter (UFCS) is particularly nasty because it allows stable code to depend on there being a trait at that path with that method.

[alexcrichton]

Thanks for the report! I think this may e a duplicate of #30209, though, so closing in favor of that.

[hanna-kruppe]

Ugh how did I miss that before telling @iliekturtles to file this issue? =/