Make pin!() more foolproof.#158061
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Previously, the soundness argument for the `pin!()` macro relied on very complicated conditions on what coercions are possible. As a result, it was not obvious whether the macro was sound or not, and the macro was potentially fragile to changes in how coercions work. Therefore, we change the `pin!()` macro so that it relies on less subtle properties of coercions. In particular: * The soundness argument now only relies on enumerating the possible types that a coercion could produce, as opposed to relying on details about how coercion expectations are propagated. * The soundness argument now only requires reasoning about possible coercions between a single pair of types defined in the standard library, as opposed to reasoning about two pairs of types, with some involved types being defined by the user. Fixes <rust-lang#153438>
Darksonn
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Jun 18, 2026
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| | expected type parameter `T`, found `&mut T` | ||
| | expected `&mut PinMacroHelper<T>`, found `&mut PinMacroHelper<&mut T>` | ||
| | arguments to this function are incorrect |
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Could/should we omit the helper type from error messages similar to https://github.com/rust-lang/rust/pull/145608/changes#diff-c712eee556d0df70dfc5c8c66c34fa13fc94a30853854f632f5dd84acfd9c973?
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I'm not familiar with diagnostics code. Am I supposed to add code in this function to do the check?
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Previously, the soundness argument for the
pin!()macro relied on very complicated conditions on what coercions are possible. As a result, it was not obvious whether the macro was sound or not, and the macro was potentially fragile to changes in how coercions work.Therefore, we change the
pin!()macro so that it relies on less subtle properties of coercions. In particular:Fixes #153438. This PR supersedes the proposed FCP on that issue.
cc @dianne
r? types