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feat(rt): Handle interior mutability during writeback #810

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13 changes: 13 additions & 0 deletions kmir/src/kmir/kdist/mir-semantics/rt/data.md
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Original file line number Diff line number Diff line change
Expand Up @@ -248,6 +248,19 @@ A `Deref` projection in the projections list changes the target of the write ope
rule <k> #traverseProjection(_, VAL, .ProjectionElems, _) ~> #readProjection(false) => VAL ... </k>
rule <k> #traverseProjection(_, VAL, .ProjectionElems, _) ~> (#readProjection(true) => #writeMoved ~> VAL) ... </k>

// interior mutability permits projected writeback
rule <k> #traverseProjection(toLocal(I), _ORIGINAL, .ProjectionElems, CONTEXTS)
~> #writeProjection(NEW)
=> #forceSetLocal(local(I), #buildUpdate(NEW, CONTEXTS))
...
</k>
<locals> LOCALS </locals>
requires CONTEXTS =/=K .Contexts
andBool 0 <=Int I andBool I <Int size(LOCALS)
andBool isTypedLocal(LOCALS[I])
andBool #allowsInteriorMutation(lookupTy(tyOfLocal({LOCALS[I]}:>TypedLocal)))
[priority(40), preserves-definedness]

rule <k> #traverseProjection(toLocal(I), _ORIGINAL, .ProjectionElems, CONTEXTS)
~> #writeProjection(NEW)
=> #setLocalValue(place(local(I), .ProjectionElems), #buildUpdate(NEW, CONTEXTS))
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17 changes: 17 additions & 0 deletions kmir/src/kmir/kdist/mir-semantics/rt/types.md
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,8 @@ requires "value.md"

module RT-TYPES
imports BOOL
imports INT
imports STRING
imports MAP
imports K-EQUAL

Expand Down Expand Up @@ -101,6 +103,21 @@ To make this function total, an optional `MaybeTy` is used.
rule elemTy( _ ) => TyUnknown [owise]
```

## Interior Mutability

Rust models interior mutability with `UnsafeCell`. When the runtime determines whether a projected write can observe mutation through a shared reference, it relies on `#allowsInteriorMutation` to detect that marker inside Stable MIR type metadata.

```k
syntax Bool ::= #allowsInteriorMutation(TypeInfo) [function, total]
// ------------------------------------------------------
// Stable MIR serializes struct names as either `mirString` or a plain `String`.
rule #allowsInteriorMutation(typeInfoStructType(mirString(NAME), _, _, _))
=> findString(NAME, "UnsafeCell", 0) =/=Int -1
rule #allowsInteriorMutation(typeInfoStructType(NAME:String, _, _, _))
=> findString(NAME, "UnsafeCell", 0) =/=Int -1
rule #allowsInteriorMutation(_) => false [owise]
```

## Static and Dynamic Metadata for Types

References to data on the heap or stack may require metadata, most commonly the size of slices, which is not statically known.
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