Updates from master #869
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Updates from master #869
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#### Context Motivated by [iterator-simple-fail.rs](https://github.com/runtimeverification/mir-semantics/blob/3d763c00389dcdb45a68523d6515c6d6fc8b2928/kmir/src/tests/integration/data/prove-rs/iterator-simple-fail.rs#L1-L7) (which should pass). The type [std::mem::MaybeUninit](https://doc.rust-lang.org/std/mem/union.MaybeUninit.html) is a union that represents a potentially uninitialised location in memory. This union has two fields, first `()`, and second [std::mem::ManuallyDrop<T>](https://doc.rust-lang.org/std/mem/struct.ManuallyDrop.html) which represents the initialised data. When [converting an array to an iterator](https://github.com/rust-lang/rust/blob/a2545fd6fc66b4323f555223a860c451885d1d2b/library/core/src/array/iter.rs#L57-L70) a `Transmute` cast is invoked for the array element type (`T` from `[T; N]`) into `std::mem::MaybeUninit<T>`. #### This PR This PR implements the cast `CastKind::Transmute` of `T` into `std::mem:MaybeUninit<T>`. The logic of the semantics and saftey of this cast for us is: - that there is a `Value` to be cast as we cannot construct a `Value` that is not initialised; - that the type being cast from `T` is the same as the type of the unions second field `std::mem::ManuallyDrop<T>`; - we can then then create a union that is constructed with the second field, instantiating the `Value` in a struct; - otherwise we error so that the cast does not `thunk`. Tests are added to show the passing cases for `transmute` and `transmute_unchecked`, and the failing case where the types are not valid for the transmute as explained above. A follow up PR will handle converting the elements of the array when an array, this PR is just for a single element.
dkcumming
approved these changes
Nov 27, 2025
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CastKind::TransmuteforT -> std::mem::MaybeUninit<T>(CastKind::TransmuteforT -> std::mem::MaybeUninit<T>#863) - single element case