unnecessary use of `to_string` for a type with custom `ToString` trait

[xphoniex]

Summary

I'm programmatically commiting a patch to a git repo and then writing the hash (a git2::Oid) in a file:

let commit = util::commit(...);
fs::write(path, commit);

if I use above code, I'll get bytes written to the file whereas I wanted the string of hash, so naturally I change it to fs::write(path, commit.to_string()), however that's throwing a lint error:

warning: unnecessary use of `to_string`
   --> ...
    |
810 |             commit.to_string(),
    |             ^^^^^^^^^^^^^^^^^^ help: use: `commit`
    |
    = note: `#[warn(clippy::unnecessary_to_owned)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_to_owned

solution is to use format!(...) instead, but that's pretty ugly.

Lint Name

unnecessary_to_owned

Version

rustc 1.61.0 (fe5b13d68 2022-05-18)
binary: rustc
commit-hash: fe5b13d681f25ee6474be29d748c65adcd91f69e
commit-date: 2022-05-18
host: x86_64-unknown-linux-gnu
release: 1.61.0
LLVM version: 14.0.0

Additional Labels

No response

[Jarcho]

Note to anyone working on this. to_string should only be linted when the type is string-like. e.g. implements Deref<Target=str/String> or AsRef<str/String>.

[kartva]

@rustbot claim

[kartva]

@xphoniex

I wasn't able to replicate your report with rust v1.61.0 locally nor v1.63.0 (playground for v1.63.0 - Tools - Clippy )
Am I perhaps missing any relevant trait implementations of Oid?

[rcgoodfellow]

Hitting this too. A simple repro is

main.rs

fn main() {
    let id = uuid::Uuid::new_v4();

    std::fs::write("bad", &id).unwrap();
    std::fs::write("good", id.to_string()).unwrap();
}

Cargo.toml

[package]
name = "poo"
version = "0.1.0"
edition = "2021"

[dependencies]
uuid = { version = "1.0.0", features = [ "serde", "v4" ] }

Demo

$ cargo run ; cat good; echo ""; cat bad; echo "";
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/poo`
e9d1adb8-a9b2-4864-a2d2-d8095cba34a9
�ѭ���Hd���	\�4�

$ cargo --version
cargo 1.63.0 (fd9c4297c 2022-07-01)

$ cargo clippy
warning: unnecessary use of `to_string`
 --> src/main.rs:5:28
  |
5 |     std::fs::write("good", id.to_string()).unwrap();
  |                            ^^^^^^^^^^^^^^ help: use: `id`
  |
  = note: `#[warn(clippy::unnecessary_to_owned)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_to_owned

warning: `poo` (bin "poo") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.40s

[xphoniex]

@DesmondWillowbrook

Hey, I'd already done the preliminary work and prepared a fix. You can skip this issue.

[xphoniex]

@rcgoodfellow

Yes, this is the same exact issue. PR #9329 fixes this.

[kartva]

@rustbot release