Add suggestion to .to_owned() used on Cow when borrowing

[m4rch3n1ng]

fixes #144792
supersedes #144925 with the review comments addressed

the tests suggested from @Kivooeo from #144925 (comment) didn't work entirely, because these tests failed due to error [E0308] mismatched types, which actually already provides a suggestion, that actually makes the code compile:

$ cargo check
error[E0308]: mismatched types
 --> src/main.rs:3:5
  |
1 | fn test_cow_suggestion() -> String {
  |                             ------ expected `std::string::String` because of return type
2 |     let os_string = std::ffi::OsString::from("test");
3 |     os_string.to_string_lossy().to_owned()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `String`, found `Cow<'_, str>`
  |
  = note: expected struct `std::string::String`
               found enum `std::borrow::Cow<'_, str>`
help: try using a conversion method
  |
3 |     os_string.to_string_lossy().to_owned().to_string()
  |                                           ++++++++++++

now this suggestion is of course not good or efficient code, but via clippy with -Wclippy::nursery lint group you can actually get to the correct code, so i don't think this is too much of an issue:

the clippy suggestions

$ cargo clippy -- -Wclippy::nursery
warning: this `to_owned` call clones the `Cow<'_, str>` itself and does not cause its contents to become owned
 --> src/main.rs:3:5
  |
3 |     os_string.to_string_lossy().to_owned().to_string()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/beta/index.html#suspicious_to_owned
  = note: `#[warn(clippy::suspicious_to_owned)]` on by default
help: depending on intent, either make the `Cow` an `Owned` variant
  |
3 |     os_string.to_string_lossy().into_owned().to_string()
  |                                 ++
help: or clone the `Cow` itself
  |
3 -     os_string.to_string_lossy().to_owned().to_string()
3 +     os_string.to_string_lossy().clone().to_string()
  |
$ # apply first suggestion
$ cargo c -- -Wclippy::nursery
warning: redundant clone
 --> src/main.rs:3:45
  |
3 |     os_string.to_string_lossy().into_owned().to_string()
  |                                             ^^^^^^^^^^^^ help: remove this
  |
note: this value is dropped without further use
 --> src/main.rs:3:5
  |
3 |     os_string.to_string_lossy().into_owned().to_string()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/beta/index.html#redundant_clone
  = note: `-W clippy::redundant-clone` implied by `-W clippy::nursery`
  = help: to override `-W clippy::nursery` add `#[allow(clippy::redundant_clone)]`

the actual error that we are looking for is error [E0515], cannot return value referencing local variable, which was only present in the original issue due to automatic type inference assuming you want to return a cloned Cow<'_, str>, which is of course not possible. this is why i took the original test functions and turned them into closures, where it's less obvious that it's trying to return the wrong type.

r? davidtwco

(because you reviewed the last attempt)
(also, let me know if i should squash this down to one commit and add myself as the co-contributor)

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[ada4a]

(just a drive-by comment) in order for a code block inside Details to work correctly, you need to leave an empty line before and after the code block