Allow lambda capture of immutable val fields from mutable class instances#1707
Allow lambda capture of immutable val fields from mutable class instances#1707
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…lds of immutable type
When a lambda captures `this.v` where `v` is an immutable (val) field of
immutable type (e.g. BigInt), the AntiAliasing phase should not reject it
even though `this` has a mutable class type. The captured value is
effectively immutable and cannot become stale.
Added isAccessedOnlyThroughImmutableFields helper function that checks
whether a variable of mutable type is only used through ClassSelector on
val fields whose types are also immutable. Variables passing this check
are excluded from the captured mutable variables set.
Fixes the issue where code like:
case class A(val v: BigInt, var m: BigInt) {
def f(): BigInt => BigInt = x => v
}
was incorrectly rejected with "Illegal capturing of variables with
mutable type: thiss".
Co-authored-by: samuelchassot <14821693+samuelchassot@users.noreply.github.com>
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Pull request overview
Relaxes AntiAliasing’s lambda capture restrictions to permit capturing a mutable receiver (e.g., this) when it is only used to read val fields whose types are immutable, addressing the reported false positive when referencing this.v inside a lambda.
Changes:
- Adjusted lambda capture computation in
AntiAliasingto exempt certain mutable-typed variables when accessed only through immutablevalfields of immutable type. - Added
isAccessedOnlyThroughImmutableFieldshelper to validate the “safe access only” condition. - Added a new extraction benchmark (
ImmutableFieldCapture.scala) covering the allowed capture scenario.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.
| File | Description |
|---|---|
| core/src/main/scala/stainless/extraction/imperative/AntiAliasing.scala | Modifies captured-variable detection for lambdas and adds a helper to treat this.v-style accesses as safe. |
| frontends/benchmarks/extraction/valid/ImmutableFieldCapture.scala | Adds a valid benchmark that exercises immutable val field capture from a mutable class instance. |
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| val allVarsInBody = exprOps.variablesOf(body) | ||
| val safeVars = allVarsInBody.filter(v => isMutableType(v.tpe) && isAccessedOnlyThroughImmutableFields(v, body)) | ||
| val captured = (allVarsInBody -- safeVars).flatMap { v => | ||
| env.targets.get(v.toVal) match { | ||
| case Some(targets) => targets.map(_.receiver) | ||
| case None => Set(v) | ||
| } | ||
| }.filter(vd => isMutableType(vd.tpe)) -- aliasedParams.map(_.toVariable).toSet |
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safeVars currently treats any mutable-typed variable as safe if it is only used as v.<valField> with immutable field type. This also includes reassignable variables (IsVar), which would make captures like var a: A; () => a.v pass the capture check even though a can change over time (not referentially transparent). Consider requiring !v.flags.contains(IsVar) (and/or checking isReferentiallyTransparent on the receiver) before classifying a variable as safe to capture.
| // fields whose types are also immutable (e.g., accessing a val BigInt field | ||
| // of a mutable class). | ||
| val allVarsInBody = exprOps.variablesOf(body) | ||
| val safeVars = allVarsInBody.filter(v => isMutableType(v.tpe) && isAccessedOnlyThroughImmutableFields(v, body)) |
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safeVars calls isAccessedOnlyThroughImmutableFields(v, body) once per variable in body, each time walking the full expression tree. In worst cases this is O(#vars × size(body)) per lambda. Consider a single traversal that accumulates (un)safe variables, or memoize results per variable for the current body, to keep AntiAliasing overhead predictable on large lambdas.
| val safeVars = allVarsInBody.filter(v => isMutableType(v.tpe) && isAccessedOnlyThroughImmutableFields(v, body)) | |
| // Compute, with a single traversal, which variables are used in an unsafe way: | |
| // - direct occurrence as a Variable, or | |
| // - as receiver of a ClassSelector on a mutable or var field. | |
| var directVars = Set.empty[Variable] | |
| var badSelectorVars = Set.empty[Variable] | |
| object lambdaVarUsageCollector extends ConcreteOOSelfTreeTraverser { | |
| override def traverse(e: Expr): Unit = e match { | |
| case cs @ ClassSelector(recv: Variable, _) => | |
| cs.field match { | |
| case Some(vd) if !(vd.flags contains IsVar) && !isMutableType(vd.tpe) => | |
| () // allowed immutable val field; do not treat recv as a direct use | |
| case _ => | |
| badSelectorVars += recv | |
| } | |
| case ClassSelector(recv, _) => | |
| // Non-variable receiver: just continue traversing inside it | |
| traverse(recv) | |
| case v: Variable => | |
| directVars += v | |
| case Operator(es, _) => | |
| es.foreach(traverse) | |
| } | |
| } | |
| lambdaVarUsageCollector.traverse(body) | |
| val safeVars = allVarsInBody.filter { v => | |
| isMutableType(v.tpe) && !directVars(v) && !badSelectorVars(v) | |
| } |
| // Disallow capturing of variables of mutable type, but allow capturing | ||
| // of mutable objects when they are only accessed through immutable (val) | ||
| // fields whose types are also immutable (e.g., accessing a val BigInt field | ||
| // of a mutable class). | ||
| val allVarsInBody = exprOps.variablesOf(body) | ||
| val safeVars = allVarsInBody.filter(v => isMutableType(v.tpe) && isAccessedOnlyThroughImmutableFields(v, body)) | ||
| val captured = (allVarsInBody -- safeVars).flatMap { v => | ||
| env.targets.get(v.toVal) match { | ||
| case Some(targets) => targets.map(_.receiver) | ||
| case None => Set(v) | ||
| } | ||
| }.filter(vd => isMutableType(vd.tpe)) -- aliasedParams.map(_.toVariable).toSet |
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A regression test would help ensure this relaxation doesn’t accidentally allow captures of reassignable variables (var a: A) where the body reads a.v. Adding an extraction/invalid benchmark for that pattern (or extending an existing one) would guard the intended behavior while still allowing the this.v scenario.
AntiAliasing rejects lambdas that reference
this.vwhenvis avalof immutable type, becausethishas mutable class type. The captured value is effectively immutable — the field cannot be reassigned and its type carries no mutable state.Changes
isAccessedOnlyThroughImmutableFields(v, expr): New helper that walks the expression tree and returnstrueiff every occurrence ofvis the receiver of aClassSelectoron avalfield with immutable type. Direct variable uses or accesses throughvar/mutable-typed fields returnfalse.Lambdacase: variables of mutable type that pass the above check are excluded before dealiasing throughenv.targets, so they don't trigger the fatal error.extraction/valid/ImmutableFieldCapture.scalatest covering the reported scenario.Existing rejection cases are unaffected —
varfield accesses (i1365a/b), array captures (i1365c), and illegal lambda effects (LambdaMutation) all correctly fail theisAccessedOnlyThroughImmutableFieldscheck.Warning
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