Inconsistent handling of to_string() removal suggestion between field and method access

Summary

I have 2 values to pass to a function. The first is borrowed (indirectly) from a field in the second, and the function needs a mutable reference of the second. So I am making a copy (to_string()) of the first, to avoid the conflicting borrows. But clippy suggests removing to_string(), which would result in a compiler error.

However, this suggestion only happens if the field is borrowed via a method. If done directly with the field instead of an accessor method, clippy does not make the (invalid) suggestion.

Lint Name

unnecessary_to_owned

Reproducer

Here is a complete example:

struct Something {
    value: String,
    // other fields
}

impl Something {
    fn value(&self) -> &str {
        &self.value
    }
}

fn use_it(_value: &str, _thing: &mut Something) {
    // do stuff
}

fn main() {
    let mut thing = Something { value: "abc".to_string() };

    // This (using field `value` directly) is fine, does not generate a warning:
    use_it(&thing.value.to_string(), &mut thing);

    // But this (using method `value()` to read field `value`) generates a warning:
    use_it(&thing.value().to_string(), &mut thing);

    // Using the suggestion results in a compiler error because of the immutable and mutable borrows together:
    // use_it(thing.value(), &mut thing);
}

The first call to use_it (using the field directly) does NOT result in a warning. But the second call results in:

warning: unnecessary use of `to_string`
  --> src/main.rs:23:12
   |
23 |     use_it(&thing.value().to_string(), &mut thing);
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `thing.value()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_to_owned
   = note: `#[warn(clippy::unnecessary_to_owned)]` on by default

And the third call to use_it with the suggested change (commented out) would result in a compiler error because of the conflicting borrows.

Version

rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: x86_64-apple-darwin
release: 1.81.0
LLVM version: 18.1.7

Additional Labels

@rustbot label +I-suggestion-causes-error

Start by running the complete Rust reproducer and inspecting the unnecessary_to_owned lint, comparing the direct field access with the accessor-method case. Done means the lint no longer emits a suggestion that causes the shown conflicting-borrow compiler error, while preserving valid suggestions and adding or updating coverage for both forms.