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644041b3a4
This was previously implemented by analyzing the AIR prior to the ZIR `make_ptr_const` instruction. This solution was highly delicate, and in particular broke down whenever there was a second `alloc` between the `store` and `alloc` instructions, which is especially common in destructure statements. Sema now uses a different strategy to detect whether a `const` is comptime-known. When the `alloc` is created, Sema begins tracking all pointers and stores which refer to that allocation in temporary local state. If any store is not comptime-known or has a higher runtime index than the allocation, the allocation is marked as being runtime-known. When we reach the `make_ptr_const` instruction, if the allocation is not marked as runtime-known, it must be comptime-known. Sema will use the set of `store` instructions to re-initialize the value in comptime memory. We optimize for the common case of a single `store` instruction by not creating a comptime alloc in this case, instead directly plucking the result value from the instruction. Resolves: #16083