Don't suggest feature-gated paths on stable

[QuineDot]

This might well be a duplicate, but I couldn't find an existing bug. I ran into this with c_int.

Given the following program:

fn main() {
    let _: c_int;
}

The output (with no dependencies) is

 --> src/main.rs:2:12
  |
2 |     let _: c_int;
  |            ^^^^^ not found in this scope
  |
help: consider importing one of these items
  |
1 | use core::ffi::c_int;
  |
1 | use std::os::raw::c_int;

Ideally the output should look like:

 --> src/main.rs:2:12
  |
2 |     let _: c_int;
  |            ^^^^^ not found in this scope
  |
help: consider importing one of these items
  |
1 | use std::os::raw::c_int;

as core::ffi::c_int is unstable:

use core::ffi::c_int;
fn main() {
    let _: c_int;
}

Output:

error[E0658]: use of unstable library feature 'core_ffi_c'
 --> src/main.rs:1:5
  |
1 | use core::ffi::c_int;
  |     ^^^^^^^^^^^^^^^^
  |
  = note: see issue #94501 <https://github.com/rust-lang/rust/issues/94501> for more information

error[E0658]: use of unstable library feature 'core_ffi_c'
 --> src/main.rs:3:12
  |
3 |     let _: c_int;
  |            ^^^^^
  |
  = note: see issue #94501 <https://github.com/rust-lang/rust/issues/94501> for more information

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

Stable compiler version 1.63.0. This particular example works on Beta and Nightly, but presumably there are or will be other things in the standard library with a stable path and an unstable path.

[QuineDot]

This is indeed still happening with other unstable paths.

fn main() -> Result<(), Box<dyn Error>> { Ok(()) }

error E0405: cannot find trait `Error` in this scope
 --> src/main.rs:1:33
  |
1 | fn main() -> Result<(), Box<dyn Error>> { Ok(()) }
  |                                 ^^^^^ not found in this scope
  |
help: consider importing one of these items
  |
1 | use core::error::Error;

+use core::error::Error;
 fn main() -> Result<(), Box<dyn Error>> { Ok(()) }

error E0658: use of unstable library feature 'error_in_core'
 --> src/main.rs:1:5
  |
1 | use core::error::Error;
  |     ^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #103765 <https://github.com/rust-lang/rust/issues/103765> for more information