assigning_clones: Wrong suggestion with deref

[taiki-e]

Summary

assigning_clones suggests code that fails to compile when the original code has deref.

Mentioning @Kobzol, who implemented this lint in #12077.

Reproducer

I tried this code:

#![allow(dead_code)]
#![warn(clippy::assigning_clones)]

use std::task::{Context, Waker};

fn f(mut prev: Box<Waker>, cx: &mut Context<'_>) -> Box<Waker> {
    *prev = cx.waker().clone();
    prev
}

I expected to see this happen: lint suggests code that can compile

Instead, this happened:

warning: assigning the result of `Clone::clone()` may be inefficient
 --> src/lib.rs:7:5
  |
7 |     *prev = cx.waker().clone();
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `clone_from()`: `prev.clone_from(cx.waker())`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones

Suggested code prev.clone_from(cx.waker()) fails to compile because deref (*prev) is dropped:

#![allow(dead_code)]
#![warn(clippy::assigning_clones)]

use std::task::{Context, Waker};

fn f(mut prev: Box<Waker>, cx: &mut Context<'_>) -> Box<Waker> {
    prev.clone_from(cx.waker());
    prev
}

error[E0308]: mismatched types
   --> src/lib.rs:7:21
    |
7   |     prev.clone_from(cx.waker());
    |          ---------- ^^^^^^^^^^ expected `&Box<Waker>`, found `&Waker`
    |          |
    |          arguments to this method are incorrect
    |
    = note: expected reference `&std::boxed::Box<std::task::Waker>`
               found reference `&std::task::Waker`

The correct code is (*prev).clone_from(cx.waker()); (preserve deref).

#![allow(dead_code)]
#![warn(clippy::assigning_clones)]

use std::task::{Context, Waker};

fn f(mut prev: Box<Waker>, cx: &mut Context<'_>) -> Box<Waker> {
    (*prev).clone_from(cx.waker());
    prev
}

Version

rustc 1.78.0-nightly (9c3ad802d 2024-03-07)
binary: rustc
commit-hash: 9c3ad802d9b9633d60d3a74668eb1be819212d34
commit-date: 2024-03-07
host: aarch64-apple-darwin
release: 1.78.0-nightly
LLVM version: 18.1.0

Additional Labels

@rustbot label +I-suggestion-causes-error

[gwilymk]

I'm seeing a similar issue but where the variable is being assigned to itself. Not sure if this counts as a separate issue.

Example code (taken from one of my projects where it locates the root of the repo)

use std::{env, error::Error, fmt::Display};

#[derive(Debug)]
struct NoJustfile;

impl Display for NoJustfile {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{self:?}")
    }
}
impl Error for NoJustfile {}

fn main() -> Result<(), Box<dyn Error>> {
    let mut path_to_search = env::current_dir()?;

    while !path_to_search.join("justfile").exists() {
        path_to_search = path_to_search
            .parent()
            .ok_or_else(|| Box::new(NoJustfile))?
            .to_owned();
    }

    println!("{}", path_to_search.display());

    Ok(())
}

you get the suggestion:

    Checking test-crate v0.1.0 (/tmp/test-crate)
warning: assigning the result of `ToOwned::to_owned()` may be inefficient
  --> src/main.rs:17:9
   |
17 | /         path_to_search = path_to_search
18 | |             .parent()
19 | |             .ok_or_else(|| Box::new(NoJustfile))?
20 | |             .to_owned();
   | |_______________________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones
   = note: `#[warn(clippy::assigning_clones)]` on by default
help: use `clone_into()`
   |
17 ~         path_to_search
18 +             .parent()
19 ~             .ok_or_else(|| Box::new(NoJustfile))?.clone_into(&mut path_to_search);
   |

warning: `test-crate` (bin "test-crate") generated 1 warning (run `cargo clippy --fix --bin "test-crate"` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.07s

but if you do that you get:

error[E0502]: cannot borrow `path_to_search` as mutable because it is also borrowed as immutable
  --> src/main.rs:20:25
   |
17 |         path_to_search
   |         -------------- immutable borrow occurs here
...
20 |             .clone_into(&mut path_to_search);
   |              ---------- ^^^^^^^^^^^^^^^^^^^ mutable borrow occurs here
   |              |
   |              immutable borrow later used by call

For more information about this error, try `rustc --explain E0502`.
error: could not compile `test-crate` (bin "test-crate") due to 1 previous error

Version:

rustc 1.78.0-nightly (9c3ad802d 2024-03-07)
binary: rustc
commit-hash: 9c3ad802d9b9633d60d3a74668eb1be819212d34
commit-date: 2024-03-07
host: x86_64-unknown-linux-gnu
release: 1.78.0-nightly
LLVM version: 18.1.0

[Kobzol]

Thanks for the report, I'll take a look.

[Kobzol]

#12473 should hopefully fix the original issue from @taiki-e.

The issue from @gwilymk is related to a different problem, and has been found in multiple other issues (see the linked PR).