Summary

Clippy prefers that I make the following change:

- unsafe { &*(&**thing as *const T as *const U) } + unsafe { &*(ptr::addr_of!(**thing) as *const U) }

I consider this a false positive because the suggested code is only more brittle compared to the original code:

• In the original code, it's clear without even showing the definition of thing that **thing is of type T, as otherwise we could not cast from &T to *const T. It's unlikely that you could have written too few or too many * and still get something that compiles, so this is "type safe".

• In the suggested code, addr_of! will take whatever almost arbitrary place expression you give it and take its address, that then gets casted to *const U. If you got too many or too few * in there, this code will silently or not-so-silently or not-so-harmlessly do the wrong thing. If you got the right number of * today but tomorrow thing changes type, that code becomes almost certainly wrong without any compile error.

This use case is minimized from real-world code in https://github.com/dtolnay/cargo-tally/blob/1.0.2/src/arena.rs#L81.

Lint Name

borrow_as_ptr

Reproducer

#![deny(clippy::pedantic)] #![allow(clippy::cast_ptr_alignment, clippy::ptr_as_ptr)] #[derive(Debug)] #[repr(C)] struct Thing { _i: i32 } fn main() { let thing = &Box::new(1i32); let reference = unsafe { &*(&**thing as *const i32 as *const Thing) }; println!("{:?}", reference); }

$ cargo clippy error: borrow as raw pointer  --> src/main.rs:11:33  | 11 | let reference = unsafe { &*(&**thing as *const i32 as *const Thing) };  | ^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::ptr::addr_of!(**thing)`  | note: the lint level is defined here  --> src/main.rs:1:9  | 1 | #![deny(clippy::pedantic)]  | ^^^^^^^^^^^^^^^^  = note: `#[deny(clippy::borrow_as_ptr)]` implied by `#[deny(clippy::pedantic)]`  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#borrow_as_ptr

Version

rustc 1.60.0-nightly (ad46af247 2022-01-14) binary: rustc commit-hash: ad46af24713115e7b9b258346e66b9b2d14eacfc commit-date: 2022-01-14 host: x86_64-unknown-linux-gnu release: 1.60.0-nightly LLVM version: 13.0.0 

Additional Labels

No response