aux.rs

#![crate_type = "lib"]
#![feature(staged_api)]
#![stable(feature = "the_tortoise_and_the_unstable_function", since = "1.0.0")]

#[stable(feature = "the_tortoise_and_the_unstable_function", since = "1.0.0")]
pub struct Foo;

#[stable(feature = "the_tortoise_and_the_unstable_function", since = "1.0.0")]
pub struct Bar;

#[stable(feature = "the_tortoise_and_the_unstable_function", since = "1.0.0")]
impl std::ops::Deref for Foo {
    type Target = Bar;

    fn deref(&self) -> &Self::Target {
        &Bar
    }
}

impl Foo {
    #[unstable(feature = "tortoise_on_foo", issue = "none")]
    pub fn tortoise(&self) -> u32 {
        4
    }
}

impl Bar {
    #[stable(feature = "the_tortoise_and_the_unstable_function", since = "1.0.0")]
    pub fn tortoise(&self) -> u32 {
        3
    }
}

main.rs

// aux-build:aux.rs

extern crate aux;
use aux::Foo;

fn main() {
    let f = Foo;
    assert_eq!(f.tortoise(), 4);
}

warning: an associated function with this name may be added to the standard library in the future
 --> src/test/ui/typeck/probe-deref-before-unstable-with-flag.rs:8:18
  |
8 |     assert_eq!(f.tortoise(), 4);
  |                  ^^^^^^^^
  |
  = note: `#[warn(unstable_name_collisions)]` on by default
  = warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
  = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
  = help: call with fully qualified syntax `Bar::tortoise(...)` to keep using the current method
  = help: add `#![feature(tortoise_on_foo)]` to the crate attributes to enable `Foo::tortoise`

warning: 1 warning emitted

or, with -Zpick-stable-methods-before-any-unstable=no:

error[E0658]: use of unstable library feature 'tortoise_on_foo'
 --> src/test/ui/typeck/probe-deref-before-unstable-with-flag.rs:8:18
  |
8 |     assert_eq!(f.tortoise(), 4);
  |                  ^^^^^^^^
  |
  = help: add `#![feature(tortoise_on_foo)]` to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.