Report values assigned to variables that are never read

[ondrejmirtes]

Reports local variables whose assigned value is never read afterwards on any path (variable.unused, level 4, behind the new unusedVariable bleeding-edge toggle).

⚠️ The last commit enables the toggle for everyone on purpose, so that CI and the downstream dogfooding projects surface false positives — revert Temporarily enable unusedVariable for everyone before merging.

How it works

A write is dead iff no path from it reaches a read of the written value — so $a = 1; $a = 2; echo $a; reports line 1 and if (c) { $a = 1; } return; reports the write, while for ($i = 0; $i < 3; $i++), back-edge reads in loops, $x[] = ...; return $x; etc. do not.

• Every source-level write site of a local variable (assignment, compound assignment, inc/dec, offset write on an array/string, list() item, foreach value/key, catch variable) becomes an immutable VariableWrite with a VariableWrittenExpr marker in the scope's expression types (__phpstanVariableWritten($x, <id>), listed as a compositional virtual key, PHP + turbo mirror). A new write of the same variable kills the earlier markers explicitly (assignVariable(..., write:, supersededMarkerExprs:) — the scope is told what to kill, it never consults engine state); merges keep markers as Maybe, so at any point they say which writes still reach it.
• The one place a source-level read is priced (VariableHandler) records the reaching writes as read in an immutable, persistent VariableWritesFrame (one per function-like body; NodeScopeResolver holds the stack and swaps the top after each with*() transition; arrow functions share the enclosing frame; processNodes isolates the stack). compact(), get_defined_vars(), extract(), eval, include read everything; goto makes the frame opaque; by-ref parameters/uses, global, static and reference aliases are untracked.
• The frame is emitted as a VariableWritesNode after each *ReturnStatementsNode; UnusedVariableRule reports the unread, tracked writes (catch variables only where non-capturing catches exist; $_-prefixed names are exempt).

Engine fixes needed on the way: generalizeWith() now carries markers planted only by a later loop pass (a branch dead while the variable was still null), createConditionalExpressions() no longer records certainty-No conditionals for virtual nodes (a later narrowing could have erased a marker), ClosureTypeResolver's body walk gets its own throwaway frame, and reads walked in consume-stored mode (arguments of a nullsafe call's plain twin) count as reads.

The first commit removes the 30 genuine dead stores the rule found in src/ and tests/.

Verification

• Rule test with 31 cases (Psalm's valid/invalid catalogue as the trap list; fail-first verified), make tests green turbo-off and turbo-on, make phpstan clean in both modes, cs/lint clean.
• Dogfood on slevomat (bleeding edge, final build): 48 reports = 41 true positives (two real bugs among them: a duplicate list() target and an offset write on an array the closure had already captured by value) + 7 writes in branches PHPStan itself proves unreachable (always-false is_int() guard, impossible isset(), dead catch) — left as is, consistent with the existing *.alwaysFalse / catch.neverThrown diagnostics.
• Performance (locally built phars, fork + turbo, cold cache, ABBA pairs): slevomat +0.7 % user CPU (5 pairs, t = 0.8, n.s.); self-analysis +1.55 % (8 pairs, sd 0.82, t = 6.4); on src/Type alone the tracking is ≈ 3.7 % (reads ≈ 2.5 %, scope markers ≈ 1–2.7 %).

Follow-ups (not in this PR)

• Psalm's value-flow refinement ($b = $b + 1 chains that never reach a sink): reads in pure RHS positions become dependency edges between writes, the rule computes a fixpoint.
• Unused array-literal offsets ($a = ['a' => 1, 'b' => 2] with only $a['b'] read) — bounded per-offset sub-writes.
• Known false negative: $x = 1; unset($x); (unset() walks the variable as a read).
• Whether f($raw = true) (value consumed, variable never read) and writes in PHPStan-proven-dead branches should be reported is a policy call.

Closes phpstan/phpstan#12789
Closes phpstan/phpstan#12012
Closes phpstan/phpstan#11483
Closes phpstan/phpstan#10202

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