Implicit Copy Lint

What it does

An allow-by-default lint to highlight implicit uses of the Copy trait, which has relevance in some kinds of unsafe code. Using it in deny mode would force the programmer to write .clone() everywhere they want to copy a value.

Lint Name

implicit_copy

Category

correctness

Advantage

Among other things, I suggested it in UnsafeCell should implement the Copy trait #25053 , as a potential solution. There are other cases in unsafe code.

Drawbacks

It will almost always be on allow; the use cases are very rare.

Example

Given a hypothetical Copy impl for UnsafeCell:

unsafe fn footgun(cell: UnsafeCell<u32>) {
  *cell.get() += 2
}

fn ouch_i_shot_my_foot() {
  let x = UnsafeCell::new(2);
  unsafe { footgun(x); }
  assert_eq(unsafe { *x.get() }, 4); // kaboom 
}

An example of how this would help:

// user code
unsafe fn footgun(cell: UnsafeCell<u32>) {
  *cell.get() += 2
}

#[deny(implicit_copy)]
fn ouch_i_shot_my_foot() {
  let x = UnsafeCell::new(2);
  unsafe { footgun(x); } // lint complains here!
  // programmer now has a strong hint that footgun should take an &UnsafeCell instead
  assert_eq(unsafe { *x.get() }, 4); // no kaboom 
}

// meanwhile this is possible
#[derive(Copy, Clone)]
struct MyCleverThing(UnsafeCell<u64>);

Start by reviewing existing Rust Clippy lint implementations and how correctness lints are registered and tested. Define the behavior from the issue's examples: detect implicit Copy uses, support the implicit_copy name and correctness category, and verify the deny-mode diagnostics without changing explicit clone behavior.