Add anti-fundamental attribute by ShaddyDC · Pull Request #160391 · rust-lang/rust

ShaddyDC · 2026-08-02T16:33:25Z

This PR introduces the internal #[rustc_anti_fundamental] trait attribute to prevent downstream crates from implementing traits on non-local #[fundamental] types (such as Pin and Box).

We extend the orphan rule to reject implementations for anti-fundamental traits if the head of its Self type is a non-local fundamental type.
We mark Deref, DerefMut, Receiver, CoerceUnsized, and DispatchFromDyn with #[rustc_anti_fundamental], which cleanly resolves orphan rule conflicts and removes the internal PinDerefMutHelper workaround trait in core::pin.

See also #85099
Fixes #148727, #147998 and #148899
Removes hack needed by #145608

AI disclosure

This PR was created with AI assistance. I don't think I have permission to add the llm-assisted label.
It touches soundness, but I created it before the new LLM usage policy came into effect, and I only noticed once I fixed some issues I came across and rebased and wanted to update the PR.
Xiang has agreed to review this PR.

We sketched out the implementation logic by hand together and I then gave instructions to the LLM to implement it. I then iterated it and expanded testing through discussing edge cases and AI review until we were fairly confident in the current implementation.

r? @dingxiangfei2009

rustbot · 2026-08-02T16:33:29Z

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @dingxiangfei2009 (or someone else) some time within the next two weeks.

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This adds a new compiler attribute that prevents non-local fundamental types from receiving implementations of marked traits. This allows addressing soundness problems with traits like DerefMut on Pin and similar wrapper types. The attribute is checked during the orphan check in coherence.

Mark Deref, DerefMut, DispatchFromDyn, CoerceUnsized, and Receiver with #[rustc_anti_fundamental] to prevent downstream crates from implementing these traits on #[fundamental] types like Box and Pin.

dingxiangfei2009 · 2026-09-02T09:21:35Z

@rustbot label +llm-assisted

dingxiangfei2009 · 2026-09-02T09:23:24Z

@bors try

Add anti-fundamental attribute

rust-bors · 2026-09-02T10:23:28Z

☀️ Try build successful (CI)
Build commit: 14450d2 (14450d27a2d0d3b3cb66037c8111ee3da14ed3c2)
Base parent: c25253f (c25253fac451fff60fb5b6c0b2c64d60f628c7cd)

Darksonn · 2026-09-02T11:19:04Z

I'd update the wording here to talk about anti-fundamental instead of saying "special hack".

View changes since the review

Good catch, thanks! Adjusted the wording and changed the link to point to this PR instead

dingxiangfei2009 · 2026-09-02T15:56:55Z

@craterbot check

craterbot · 2026-09-02T15:57:26Z

👌 Experiment pr-160391 created and queued.
🤖 Automatically detected try build 14450d2
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

We marked DerefMut anti-fundamental, so the PinHelper indirection is no longer needed to prevent downstream users from implementing DerefMut on Pin.

dingxiangfei2009 · 2026-09-03T13:24:29Z

+///    it wraps a local type as in (2)).
+///     - e.g., `Box<LocalType>` or `&Pin<LocalType>` is rejected if the trait
+///       is `#[rustc_anti_fundamental]`.
+///     - This lets the standard library reserve control over traits like `Deref`


I feel that we don't need bullet points to structure the motivation, it would be more readable if this is just a prose.

View changes since the review

dingxiangfei2009 · 2026-09-03T13:26:54Z

+    loop {
+        ty = infcx.shallow_resolve(ty);
+        let norm_ty = match lazily_normalize_ty(ty)? {
+            norm if norm.is_ty_var() => ty,


Huh, if norm is a type var, ty can't be normalized to the slightest bit I suppose. So it reads a bit off to me if we call it norm_ty... Wdyt?

View changes since the review

dingxiangfei2009 · 2026-09-03T13:27:45Z

+/// Checks the head of the Self type for a non-local fundamental type.
+///
+/// If the head is a reference (`&` / `&mut`), we unwrap it and inspect the pointee type.
+/// This ensures that wrapping a fundamental type in a reference (such as `&Pin<LocalType>`)


Suggested change

/// This ensures that wrapping a fundamental type in a reference (such as `&Pin<LocalType>`)

/// This ensures that wrapping a fundamental type in a reference, say a `&Pin<LocalType>`,

View changes since the review

theemathas · 2026-09-03T14:00:34Z

@craterbot cancel

See #162233

craterbot · 2026-09-03T14:00:46Z

🗑️ Experiment pr-160391 deleted!

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

rustbot assigned dingxiangfei2009 Aug 2, 2026

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ShaddyDC force-pushed the anti-fundamental branch from 96b570b to eec2923 Compare August 2, 2026 16:51

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ShaddyDC force-pushed the anti-fundamental branch from eec2923 to 8bc0866 Compare August 2, 2026 18:19

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ShaddyDC added 2 commits September 1, 2026 22:10

Annotate std traits with #[rustc_anti_fundamental]

aa2681c

Mark Deref, DerefMut, DispatchFromDyn, CoerceUnsized, and Receiver with #[rustc_anti_fundamental] to prevent downstream crates from implementing these traits on #[fundamental] types like Box and Pin.

ShaddyDC force-pushed the anti-fundamental branch from 8bc0866 to 1663605 Compare September 2, 2026 08:35

rustbot added the llm-assisted An LLM-assisted PR as defined by the LLM policy. Requires ahead-of-time consent by assignee. label Sep 2, 2026

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rust-bors Bot pushed a commit that referenced this pull request Sep 2, 2026

Auto merge of #160391 - ShaddyDC:anti-fundamental, r=<try>

14450d2

Add anti-fundamental attribute

Darksonn reviewed Sep 2, 2026

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craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 2, 2026

Remove PinDerefMutHelper

b6158f7

We marked DerefMut anti-fundamental, so the PinHelper indirection is no longer needed to prevent downstream users from implementing DerefMut on Pin.

ShaddyDC force-pushed the anti-fundamental branch from 1663605 to b6158f7 Compare September 3, 2026 11:11

theemathas mentioned this pull request Sep 3, 2026

Crater rollup #162233

Draft

dingxiangfei2009 reviewed Sep 3, 2026

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craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Sep 3, 2026

	/// This ensures that wrapping a fundamental type in a reference (such as `&Pin<LocalType>`)
	/// This ensures that wrapping a fundamental type in a reference, say a `&Pin<LocalType>`,

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