22572d0994
const checking: properly compute the set of transient locals For const-checking the MIR of a const/static initializer, we have to know the set of "transient" locals. The reason for this is that creating a mutable (or interior mutable) reference to a transient local is "safe" in the sense that this reference cannot possibly end up in the final value of the const -- even if it is turned into a raw pointer and stored in a union, we will see that pointer during interning and reliably reject it as dangling. So far, we determined the set of transient locals as "locals that have a `StorageDead` somewhere". But that's not quite right -- if we had MIR like ```rust StorageLive(_5); StorageDead(_5); StorageLive(_5); // no further storage annotations for _5 ``` Then we'd consider `_5` to be "transient", but it is not actually transient. We do not currently generate such MIR, but I feel uneasy relying on subtle invariants like this. So this PR implements a proper analysis for computing the set of "transient" locals: a local is "transient" if it is guaranteed dead at all `Return` terminators. Cc `@cjgillot`
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