[`check_result_unit_err`]: Recommended fix not possible in `no_std`

[FawazTirmizi]

Description

This lint currently suggests implementing a custom Error type, as seen in the below example:

Running Clippy on a project with the following lib.rs:

#![no_std]

use core::result::Result;

pub fn ret_err() -> Result<(),()> {
    if true { Ok(()) } else { Err(()) }
}

will yield the following warning:

warning: this returns a `Result<_, ()>`
 --> src/lib.rs:6:1
  |
6 | pub fn ret_err() -> Result<(),()> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: use a custom `Error` type instead
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#result_unit_err
  = note: `#[warn(clippy::result_unit_err)]` on by default

However, Error is not available in no_std without enabling the error_in_core feature (tracking issue).

As discussed in #9671, this lint should be suggesting returning a non-unit type as the lint checks that Err is a unit, not that it doesn't implement Error. I think the fix here is to just change the lint to first recommend returning a non-unit type, then suggest one that implements Error, mentioning the feature gate if on no_std. I'd be happy to implement this or whatever other solution people would prefer.

Version

rustc 1.66.0-nightly (a6b7274a4 2022-10-10)
binary: rustc
commit-hash: a6b7274a462829f8ef08a1ddcdcec7ac80dbf3e1
commit-date: 2022-10-10
host: x86_64-unknown-linux-gnu
release: 1.66.0-nightly
LLVM version: 15.0.2

Additional Labels

No response