std-instead-of-core: core not in scope

Using the following flags

--force-warn clippy::std-instead-of-core

this code:

// This test's filename is... a bit verbose. But it ensures we suggest the correct code when `Ord`
// is not in scope.
#![no_main]
#![no_implicit_prelude]
//@no-rustfix
extern crate std;

use std::cmp::{self, Eq, Ordering, PartialEq, PartialOrd};
use std::option::Option::{self, Some};
use std::todo;

// lint

#[derive(Eq, PartialEq)]
struct A(u32);

impl cmp::Ord for A {
    fn cmp(&self, other: &Self) -> Ordering {
        todo!();
    }
}

impl PartialOrd for A {
    //~^ non_canonical_partial_ord_impl
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        // NOTE: This suggestion is wrong, as `Ord` is not in scope. But this should be fine as it isn't
        // automatically applied
        todo!();
    }
}

#[derive(Eq, PartialEq)]
struct B(u32);

impl B {
    fn cmp(&self, other: &Self) -> Ordering {
        todo!();
    }
}

impl cmp::Ord for B {
    fn cmp(&self, other: &Self) -> Ordering {
        todo!();
    }
}

impl PartialOrd for B {
    //~^ non_canonical_partial_ord_impl
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        // This calls `B.cmp`, not `Ord::cmp`!
        Some(self.cmp(other))
    }
}

caused the following diagnostics:

    Checking _non_canonical_partial_ord_impl_fully_qual v0.1.0 (/tmp/icemaker_global_tempdir.paNtiJF6jVLU/icemaker_clippyfix_tempdir.MkAby3WtvAcW/_non_canonical_partial_ord_impl_fully_qual)
warning: used import from `std` instead of `core`
 --> src/lib.rs:8:5
  |
8 | use std::cmp::{self, Eq, Ordering, PartialEq, PartialOrd};
  |     ^^^ help: consider importing the item from `core`: `core`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#std_instead_of_core
  = note: requested on the command line with `--force-warn clippy::std-instead-of-core`

warning: used import from `std` instead of `core`
 --> src/lib.rs:9:5
  |
9 | use std::option::Option::{self, Some};
  |     ^^^ help: consider importing the item from `core`: `core`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#std_instead_of_core

warning: `_non_canonical_partial_ord_impl_fully_qual` (lib) generated 2 warnings (run `cargo clippy --fix --lib -p _non_canonical_partial_ord_impl_fully_qual` to apply 2 suggestions)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.28s

However after applying these diagnostics, the resulting code:

// This test's filename is... a bit verbose. But it ensures we suggest the correct code when `Ord`
// is not in scope.
#![no_main]
#![no_implicit_prelude]
//@no-rustfix
extern crate std;

use core::cmp::{self, Eq, Ordering, PartialEq, PartialOrd};
use core::option::Option::{self, Some};
use std::todo;

// lint

#[derive(Eq, PartialEq)]
struct A(u32);

impl cmp::Ord for A {
    fn cmp(&self, other: &Self) -> Ordering {
        todo!();
    }
}

impl PartialOrd for A {
    //~^ non_canonical_partial_ord_impl
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        // NOTE: This suggestion is wrong, as `Ord` is not in scope. But this should be fine as it isn't
        // automatically applied
        todo!();
    }
}

#[derive(Eq, PartialEq)]
struct B(u32);

impl B {
    fn cmp(&self, other: &Self) -> Ordering {
        todo!();
    }
}

impl cmp::Ord for B {
    fn cmp(&self, other: &Self) -> Ordering {
        todo!();
    }
}

impl PartialOrd for B {
    //~^ non_canonical_partial_ord_impl
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        // This calls `B.cmp`, not `Ord::cmp`!
        Some(self.cmp(other))
    }
}

no longer compiled:

    Checking _non_canonical_partial_ord_impl_fully_qual v0.1.0 (/tmp/icemaker_global_tempdir.paNtiJF6jVLU/icemaker_clippyfix_tempdir.MkAby3WtvAcW/_non_canonical_partial_ord_impl_fully_qual)
error[E0433]: failed to resolve: use of unresolved module or unlinked crate `core`
 --> src/lib.rs:8:5
  |
8 | use core::cmp::{self, Eq, Ordering, PartialEq, PartialOrd};
  |     ^^^^ use of unresolved module or unlinked crate `core`
  |
  = help: if you wanted to use a crate named `core`, use `cargo add core` to add it to your `Cargo.toml`

error[E0433]: failed to resolve: use of unresolved module or unlinked crate `core`
 --> src/lib.rs:9:5
  |
9 | use core::option::Option::{self, Some};
  |     ^^^^ use of unresolved module or unlinked crate `core`
  |
  = help: if you wanted to use a crate named `core`, use `cargo add core` to add it to your `Cargo.toml`

error[E0432]: unresolved import `core`
 --> src/lib.rs:8:5
  |
8 | use core::cmp::{self, Eq, Ordering, PartialEq, PartialOrd};
  |     ^^^^ use of unresolved module or unlinked crate `core`
  |
  = help: if you wanted to use a crate named `core`, use `cargo add core` to add it to your `Cargo.toml`

Some errors have detailed explanations: E0432, E0433.
For more information about an error, try `rustc --explain E0432`.
error: could not compile `_non_canonical_partial_ord_impl_fully_qual` (lib test) due to 3 previous errors
warning: build failed, waiting for other jobs to finish...
error: could not compile `_non_canonical_partial_ord_impl_fully_qual` (lib) due to 3 previous errors

Version:

rustc 1.92.0-nightly (4a54b26d3 2025-10-07)
binary: rustc
commit-hash: 4a54b26d30dac43778afb0e503524b763fce0eee
commit-date: 2025-10-07
host: x86_64-unknown-linux-gnu
release: 1.92.0-nightly
LLVM version: 21.1.2

Reproduce the issue with --force-warn clippy::std-instead-of-core using the shown no_implicit_prelude example, then trace the std-instead-of-core lint and its suggestion handling. The fix is complete when applying the reported suggestions leaves the test compiling and preserves correct suggestions when Ord is not in scope.