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CTFE/Miri engine Pointer type overhaul This fixes the long-standing problem that we are using `Scalar` as a type to represent pointers that might be integer values (since they point to a ZST). The main problem is that with int-to-ptr casts, there are multiple ways to represent the same pointer as a `Scalar` and it is unclear if "normalization" (i.e., the cast) already happened or not. This leads to ugly methods like `force_mplace_ptr` and `force_op_ptr`. Another problem this solves is that in Miri, it would make a lot more sense to have the `Pointer::offset` field represent the full absolute address (instead of being relative to the `AllocId`). This means we can do ptr-to-int casts without access to any machine state, and it means that the overflow checks on pointer arithmetic are (finally!) accurate. To solve this, the `Pointer` type is made entirely parametric over the provenance, so that we can use `Pointer<AllocId>` inside `Scalar` but use `Pointer<Option<AllocId>>` when accessing memory (where `None` represents the case that we could not figure out an `AllocId`; in that case the `offset` is an absolute address). Moreover, the `Provenance` trait determines if a pointer with a given provenance can be cast to an integer by simply dropping the provenance. I hope this can be read commit-by-commit, but the first commit does the bulk of the work. It introduces some FIXMEs that are resolved later. Fixes https://github.com/rust-lang/miri/issues/841 Miri PR: https://github.com/rust-lang/miri/pull/1851 r? `@oli-obk` |
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| README.md | ||
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.