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Encountered while processing https://github.com/rust-lang/rust/blob/8c93a79e387f5197e8b8fe73b3d87d2b101b7c4a/src/test/run-pass/move-guard-const.rs
- Value is moved in a match guard, which fails and falls to an arm where the value is still belived to be valid.
Ideal 1: Model match common codegen as a else-if chain.
> For each arm: Make a split scope, expand patterns, codegen guard, then switch to the other scope and immediately complete it.
- Problem: A move can be bypassed.
- Solution: Treat a bypass path as a second arm to the `if`
- Problem 2: Arms can be mutually exclusive, and hence can move in both with no lower-time error.
- Post-validation can solve that.
Idea 2: Refactor state tracking to instead store a COW state stack what can have the state removed for later use.
- Push/pop state around each pattern
- Unify apply after patterns in an arm
- Pop state after processing the condition
- Unify before processing each arm
- Problem: Doens't concretely determine if something is moved? (Will use drop
flags when not needed)
Idea 3: When processing a match guard, enter a mode where all moves of above values are conditional.
> Flag on the Scope::Split used for the match pattern
> Flag is set/cleared around processing the arm guard
> Mutating the state within this scope creates a drop flag
- Problem: Structurally, allocating the drop flag is a challeng
> Solution: Use a new scope type (with no split, just applies backwards)
Overall problem: Use-after-free still possible (but has to be invalid code with the above fixes)
- Solution: Dump the entire concept and just add a "NoMutate" scope type that errors if state changes across it
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