499 enum support in mir-semantics #521
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jberthold
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Apr 4, 2025
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| syntax Int ::= #lookupDiscriminant ( TypeInfo , VariantIdx ) [function, total] | ||
| | #lookupDiscrAux ( Discriminants , VariantIdx ) [function] | ||
| // -------------------------------------------------------------------- | ||
| rule #lookupDiscriminant(typeInfoEnumType(_, _, DISCRIMINANTS), IDX) => #lookupDiscrAux(DISCRIMINANTS, IDX) | ||
| requires isInt(#lookupDiscrAux(DISCRIMINANTS, IDX)) [preserves-definedness] | ||
| rule #lookupDiscriminant(_OTHER, _) => 0 [owise, preserves-definedness] // default 0. May be undefined behaviour, though. |
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Instead of this default returning 0 , we could signal undefined behaviour - if it was clear what exactly constitutes undefined behaviour here, the discussion on rust-lang/rust#91095 remains unresolved IIUC.
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I think this is consistent with miri which is probably good to start with. But I think signalling UB would be good for a future version
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dkcumming
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Apr 10, 2025
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| syntax Int ::= #lookupDiscriminant ( TypeInfo , VariantIdx ) [function, total] | ||
| | #lookupDiscrAux ( Discriminants , VariantIdx ) [function] | ||
| // -------------------------------------------------------------------- | ||
| rule #lookupDiscriminant(typeInfoEnumType(_, _, DISCRIMINANTS), IDX) => #lookupDiscrAux(DISCRIMINANTS, IDX) | ||
| requires isInt(#lookupDiscrAux(DISCRIMINANTS, IDX)) [preserves-definedness] | ||
| rule #lookupDiscriminant(_OTHER, _) => 0 [owise, preserves-definedness] // default 0. May be undefined behaviour, though. |
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I think this is consistent with miri which is probably good to start with. But I think signalling UB would be good for a future version
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Aggregatevalues, which now carry the variant indexAggregates forstructs andenums now have a correctTyfield (stillTyUnknownfor tuples, though)RValue::Discriminantis implemented:enum, it looks up the discriminant and returns it as au128(discriminant type information unavailable)Aggregates, it will always return0(see related discussion inrustc: Decide on when MIR Discriminant() operation is UB rust-lang/rust#91095)This is a first step. The data returned by
Discriminantis actually not consistent (bit width and signedness do not match the indicatedTy), which will cause issues if the obtained discriminant is used in any way other than as aSwitchIntargument.Fixes #499