Ignore PhantomData not in Drop impl for dropck

[SoniEx2]

Proposal

Simply ignore PhantomData for dropck, allowing the following to compile: (note that it compiles if you take out the String.)

use core::cell::Cell;
use core::marker::PhantomData;

struct Dropper<T>(T);

impl<T> Drop for Dropper<T> {
    fn drop(&mut self) {}
}

struct Foo<'a>(PhantomData<Dropper<Cell<&'a Foo<'a>>>>, String);

fn main() {
  fn make_selfref<'a>(x: &'a Foo<'a>){}
  let x = Foo(PhantomData, String::new());
  make_selfref(&x);
}

---

This is an user-facing change but mostly we just wanna see a crater run for this. Ignoring it "not in Drop impl" basically boils down to how it's currently used with #[may_dangle] and we don't wanna break that, but the above code is not unsound or anything so it seems like forbidding it is an oversight.

see also rust-lang/rust#102810

Mentors or Reviewers

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Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[oli-obk]

Closing as this is a language change proposal, so the compiler team can't make decisions on this MCP. As mentioned in the zulip thread, please start a new thread in the lang team zulip stream