`PhantomPinned` warns that field is never read, but `PhantomData` does not

[clarfonthey]

I tried this code:

use core::marker::{PhantomData, PhantomPinned};

struct Test {
    pin: PhantomPinned,
    data: PhantomData<()>,
}

fn main() {
    let _ = Test { pin: PhantomPinned, data: PhantomData };
}

This should not show any errors indicating that pin was not read, since its purpose is merely to avoid an Unpin impl. Instead, I get this output:

warning: field `pin` is never read
 --> src/main.rs:4:5
  |
3 | struct Test {
  |        ---- field in this struct
4 |     pin: PhantomPinned,
  |     ^^^
  |
  = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

Note that PhantomData is specially handled to not do this, and I would expect PhantomPinned to act the same.

Meta

rustc --version --verbose:

rustc 1.96.0-nightly (55e86c996 2026-04-02)
binary: rustc
commit-hash: 55e86c996809902e8bbad512cfb4d2c18be446d9
commit-date: 2026-04-02
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.2

[asder8215]

@rustbot claim

This seems to be a clippy problem. I notice in clippy_lints/src/missing_fields_in_debug.rs it avoids emitting a field is not used warning specifically for PhantomData. We'd also have to modify clippy_lints/src/pub_underscore_fields.rs since it also excludes PhantomData from emitting a warning if a field is declared as public but uses a leading underscore.

In both cases, clippy does have access to checking if the field is a PhantomData through LangItem::PhantomData, but from the language_item_table, it doesn't seem like there's a lang item for PhantomPinned and it's not marked with that #[lang = "..."] macro in here just like how PhantomData is.

Is it okay to add PhantomPinned as a lang_item in the language_item_table?

---

Adding on that the comments above PhantomPinned marker trait mention that the marker trait will likely be deprecated and people should use UnsafePinned instead.

[asder8215]

Might as well add these labels since the solution I have in mind for this affects these three areas.

@rustbot label +T-compiler +T-libs +T-clippy

[asder8215]

(this might be more appropriate)

@rustbot label +A-lints +A-macros -T-clippy +L-dead_code

[rustbot]

Error: Parsing relabel command in comment failed: ...'-dead_code' | error: a label delta at >| ' (this mig'...

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[rustbot]

Error: Unknown labels: A-macro

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