`inherent_to_string` doesn't check for unsafe

[morrisonlevi]

Summary

The lint clippy::inherent_to_string still applies even if the function is marked unsafe, which obviously cannot then be impl ToString.

The example provides a motivating reason for why the unsafe is important: the struct is used across an FFI boundary.

Lint Name

inherent_to_string

Reproducer

Example src/lib.rs:

use core::marker::PhantomData;
use libc::{c_char, size_t};

#[repr(C)]
pub struct StringView<'a> {
    ptr: *const c_char,
    len: size_t,
    _marker: PhantomData<&'a [c_char]>,
}

impl<'a> StringView<'a> {
    /// # Safety
    /// The view must follow all requirements of [std::slice::from_raw_parts].
    pub unsafe fn to_string(self) -> String {
        let slice = std::slice::from_raw_parts(self.ptr as *const u8, self.len);
        String::from_utf8_lossy(slice).to_string()
    }
}

And Cargo.toml:

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

[dependencies]
libc = "0.2.147"

I saw this happen:

warning: implementation of inherent method `to_string(&self) -> String` for type `StringView<'a>`
  --> src/lib.rs:14:5
   |
14 | /     pub unsafe fn to_string(self) -> String {
15 | |         let slice = std::slice::from_raw_parts(self.ptr as *const u8, self.len);
16 | |         String::from_utf8_lossy(slice).to_string()
17 | |     }
   | |_____^
   |
   = help: implement trait `Display` for type `StringView<'a>` instead
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#inherent_to_string
   = note: `#[warn(clippy::inherent_to_string)]` on by default

warning: `clippy_inherent_to_string` (lib) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s

I expected no error, because unsafe means this signature is not applicable for the trait ToString.

Version

rustc 1.71.0 (8ede3aae2 2023-07-12)
binary: rustc
commit-hash: 8ede3aae28fe6e4d52b38157d7bfe0d3bceef225
commit-date: 2023-07-12
host: aarch64-apple-darwin
release: 1.71.0
LLVM version: 16.0.5

Additional Labels

No response

[Centri3]

Also doesn't check for extern fns as well

[morrisonlevi]

Thank you for the quick turnaround on this!