redundant_clone: false positive using ToOwned

[nicomem]

The following code :

pub fn own<T, U>(v: T) -> U
where
    T: ToOwned<Owned = U>,
{
    v.to_owned()
}

generates the following lint:

warning: redundant clone
 --> src/main.rs:5:6
  |
5 |     v.to_owned()
  |      ^^^^^^^^^^^ help: remove this
  |
  = note: `#[warn(clippy::redundant_clone)]` on by default
note: this value is dropped without further use
 --> src/main.rs:5:5
  |
5 |     v.to_owned()
  |     ^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone

However, removing the .to_owned leads to a compilation error since the return types are different.

May be related to #5700

Meta

• cargo clippy -V: clippy 0.1.52 (e9920ef 2021-02-11)
• rustc -Vv:

  rustc 1.52.0-nightly (e9920ef77 2021-02-11)
  binary: rustc
  commit-hash: e9920ef7749d11fc71cc32ca4ba055bcfeaab945
  commit-date: 2021-02-11
  host: x86_64-unknown-linux-gnu
  release: 1.52.0-nightly
  LLVM version: 11.0.1
  

[camsteffen]

The problem is that your function takes a value instead of a reference. But it would be better if redundant_clone was more clear about that. So this is an enhancement rather than a false positive.

[camsteffen]

In fact, it really should be needless_pass_by_value instead.

[humb1t]

@camsteffen - I had similar issue and if I change function to take a reference it would still cause complains from clippy.

[humb1t]

    9         pub fn new<P: AsRef<Path> + ToOwned<Owned = PathBuf>>(root_path: &P) -> Self {
    8             Configuration {
    7                 #[allow(clippy::redundant_clone)]
    6                 root_path: root_path.to_owned(),
    5                 stream_read_ahead: 10,
    4                 stream_buffer_size: CHUNK_BYTES_SIZE,
    3             }
    2         }

[pitaj]

@humb1t I wasn't able to reproduce your issue (playground)

Perhaps it's been fixed. Can you provide a complete example?

[humb1t]

@pitaj looks like it was fixed. Thank you!

[flip1995]

Just noticed that the original example still triggers the lint.

[Kimundi]

Lint still triggers on current rust playground (1.70), another short example:

fn example<T: ToOwned<Owned = String>>(v: T) -> String {
    v.to_owned()
}