PhantomData<T> no longer dropck?

[matklad]

Consider this code:

use std::marker::PhantomData;

struct Pending<T> {
    phantom: PhantomData<T>,
}

fn pending<T>(value: T) -> Pending<T> {
    Pending {
        phantom: PhantomData,
    }
}

struct Inspector<'a> {
    value: &'a String,
}

impl Drop for Inspector<'_> {
    fn drop(&mut self) {
        eprintln!("drop inspector {}", self.value)
    }
}

fn main() {
    let p: Pending<Inspector>;
    let s = "hello".to_string();
    p = pending(Inspector { value: &s });
}

Playground

I believe it should not compile (by failing dropcheck). It, however, compiles and runs on 1.42 and 1.31. On 1.24 it indeed fails as I expect.

The equivalent code, where PhantomData<T> is replaced with T rightfully fails to compiles: Playground. So, dropchk somehow observes the difference between T and PhantomData<T>?

Either I misunderstand how PhantomData<T> is supposed to work, or this is a stable-to-stable regression and a soundless hole.

[jonas-schievink]

It, however, compiles and runs on 1.42 and 1.31.

It fails to compile with the 2015 edition on 1.31, so it's probably caused by NLL.

I suspect the issue is something like: PhantomData never "needs drop", so your struct also doesn't, so no Drop terminator is created that could access the borrowed value.

[jonas-schievink]

Adding an unrelated field that has a destructor makes the error appear again

[tesuji]

Regression from 1.36: https://rust.godbolt.org/z/hnEi6A

[jonas-schievink]

cc @matthewjasper

[spastorino]

Assigning P-high to this bug and leaving the nomination, this could have been P-critical too. This was discussed as part of our pre-triage meeting in Zulip.