Report variable writes whose value never reaches a use, per array offset

[ondrejmirtes]

Follow-up to #6330 (merged): the two refinements listed there, the unset() false negative, and a few things found while dogfooding.

Value flow (Psalm's "better unused variable detection")

A read is no longer a use by itself. A write is used iff its value reaches a sink — a call argument, a condition, a return, echo, throw, a property write… — directly or through the writes it is computed into. So $a = 5; $a = $a + 1;, $s .= … chains, $i++ runs and Psalm's article example are reported at every write, while $a = 5; $a = $a + 1; sink($a); stays quiet.

• ExpressionContext carries a value-flow target. AssignHandler, AssignOpHandler and the inc/dec handlers build the target write up front (VariableFlowBuilder::writeSite()) and walk the value in enterValueFlow(); pure combinators (arithmetic/concat/comparisons, casts, unary ops, ternary and match arms, literal arrays, interpolated strings, ??'s right side) keep the target via enterDeepKeepingValueFlow(), everything else drops it — enterDeep() for nested sub-expressions, withoutValueFlow() for same-depth sinks (&&/|| right operands, piped calls, closure uses, assignment-target sub-walks).
• VariableHandler::composeResult() emits a read flow tagged with the target's id instead of a plain read. VariableLivenessResolver turns those into dependency edges between writes during its backward pass and resolves the used set as a worklist fixpoint from the sinks (resolveDependencies()); VariableWritesNode exposes both answers as isRead() (some read of the value exists) and isUsed() (the value reaches a sink).
• $a = $b = $c + 1 and $a = ($b OP= …): the inner write's inputs are handed to the enclosing expression as a VariableInputFlow — copied to the outer target, or treated as consumed when the enclosing expression is a sink. $a = $b = 1; sink($a); still reports $b (its variable is never read).
• UnusedParametersCheck and UnusedClosureUsesRule ask isUsed() too, so a parameter or by-value capture that only feeds unused values is reported.
• Assignments to $this and superglobals are never write sites, so $_GET['x'] = $value; keeps $value observable.

The message tells the two apart: … is never read. when nothing reads the written value at all, … is never used. when it is read, but only by expressions computing other values that never reach a sink.

Array offsets — literals and $a['k'] = …

• Items of a literal assigned straight to a variable become child writes of the whole write (constant keys, implicit indices after explicit ones, spreads/unknown keys as unknown offsets). They are observed together with the whole write in the liveness pass and resolved through it.
• $a['k'] = … is an offset write that kills only the earlier writes of offset 'k'; $a['k']['j'] = … extends offset 'k' (reads it, kills nothing); $a[$i] = … / $a[] = … are unknown-offset writes, and are reported when a constant-offset read is the only thing after them ($a[$i] = 1; $a['x'] = 2; return $a['x'];).
• The receiver of an offset access is read as a container (whole-variable writes only); $a['k'] reads offset 'k' plus unknown-offset writes; a dynamic offset reads everything; $a['k'] OP= and $a['k'] ??= read the offset first.
• The redundant-assignment check covers constant-offset writes: $a = [1, 2]; $a[0] = 1; reports Offset $a[0] is assigned value 1 but it already has that value. (nested offsets and coerced keys included; append, dynamic keys, by-reference items and PHPDoc-only certainty are excluded).
• unset($x) / unset($a['k']) discard the reaching writes without reading them (VariableFlow::discard()), so $x = 1; unset($x); is reported too — unless releasing the value has side effects (objects, resources, arrays that may contain them), where the unset() counts as a read so $cache = new Cache(); unset($cache); stays quiet.

Messages: Value assigned to variable $x is never used., Value assigned to $a['k'] is never read. (also Value of $a['n'] after ++ …, Foreach value $a['x'] …), Offset 'k' of array assigned to variable $a is never used. (array.unusedOffset) — an item is reported on its own only when the array as a whole is used, otherwise the assignment is.

Verification

• Rule test: 25 tests / 214 expectations; new fixtures unused-variable-value-flow.php, unused-variable-offsets.php, unused-variable-redundant-offsets.php, unused-variable-offset-overwrite.php, unused-variable-destructor.php, plus unused-input-value-flow.php for the parameter and closure-use rules. The #12012 case now reports exactly the lines the reporter asked for.
• Two genuine bugs in fixtures surfaced and are now expected: bug-10847's loop appends to the iterated $overloads instead of $processedOverloads, and array-destructuring-array-dim-fetch builds $barcodes for nothing.
• On the current tip: the three rule tests green (UnusedVariableRuleTest, UnusedClosureUsesRuleTest, UnusedFunctionParametersRuleTest), make phpstan clean, --group levels green — the one new expectation file (arrayDimFetches-4.json) is part of this PR.
• Slevomat dogfood and the perf A/B were done on an earlier revision of this branch (before Report values assigned to variables that are never read #6330 landed as the flow-tree engine): 54 reports = the base's 48 + 6 new, all true positives; the perf difference was inside the noise. Neither has been re-measured on the current tip.
• The integration-tests jobs are red because the downstream projects run with bleeding edge and now see the unused-variable/parameter reports (the same as on the base PR); doctrine/collections fails on a property type error that reproduces identically on plain 2.3.x (ArrayCollection::$elements does not accept array<int|TKey, T>), not on this branch.

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