Add PHP lint check for expressions which can be statically evaluated

Summary:
This is pretty coarse and could be refined, but I often do this when testing:

  lang=diff
  - if (some_complicated_condition())
  + if (true || some_complicated_condition())

aran has caught me not only doing it but sending out diffs with it like 30
times. Catch it in lint instead.

Test Plan:
Unit test, added a "true || $junk" to the code and linted it.

Reviewed By: aran
Reviewers: aran, jungejason, tuomaspelkonen, pad
CC: aran
Differential Revision: 447

src/lint/linter/xhpast/ArcanistXHPASTLinter.php

@@ -172,25 +172,70 @@ class ArcanistXHPASTLinter extends ArcanistLinter {
       '>'   => true,
     );
 
+    static $logical = array(
+      '||'  => true,
+      '&&'  => true,
+    );
+
     foreach ($expressions as $expr) {
       $operator = $expr->getChildByIndex(1)->getConcreteString();
-      if (empty($operators[$operator])) {
+      if (!empty($operators[$operator])) {
-        continue;
+        $left = $expr->getChildByIndex(0)->getSemanticString();
+        $right = $expr->getChildByIndex(2)->getSemanticString();
+
+        if ($left == $right) {
+          $this->raiseLintAtNode(
+            $expr,
+            self::LINT_TAUTOLOGICAL_EXPRESSION,
+            'Both sides of this expression are identical, so it always '.
+            'evaluates to a constant.');
+        }
       }
 
-      $left = $expr->getChildByIndex(0)->getSemanticString();
+      if (!empty($logical[$operator])) {
-      $right = $expr->getChildByIndex(2)->getSemanticString();
+        $left = $expr->getChildByIndex(0)->getSemanticString();
+        $right = $expr->getChildByIndex(2)->getSemanticString();
 
-      if ($left == $right) {
+        // NOTE: These will be null to indicate "could not evaluate".
-        $this->raiseLintAtNode(
+        $left = $this->evaluateStaticBoolean($left);
-          $expr,
+        $right = $this->evaluateStaticBoolean($right);
-          self::LINT_TAUTOLOGICAL_EXPRESSION,
+
-          'Both sides of this expression are identical, so it always '.
+        if (($operator == '||' && ($left === true || $right === true)) ||
-          'evaluates to a constant.');
+            ($operator == '&&' && ($left === false || $right === false))) {
+          $this->raiseLintAtNode(
+            $expr,
+            self::LINT_TAUTOLOGICAL_EXPRESSION,
+            'The logical value of this expression is static. Did you forget '.
+            'to remove some debugging code?');
+        }
       }
     }
   }
 
+
+  /**
+   * Statically evaluate a boolean value from an XHP tree.
+   *
+   * TODO: Improve this and move it to XHPAST proper?
+   *
+   * @param  string The "semantic string" of a single value.
+   * @return mixed  ##true## or ##false## if the value could be evaluated
+   *                statically; ##null## if static evaluation was not possible.
+   */
+  private function evaluateStaticBoolean($string) {
+    switch (strtolower($string)) {
+      case '0':
+      case 'null':
+      case 'false':
+        return false;
+      case '1':
+      case 'true':
+        return true;
+    }
+    return null;
+  }
+
+
   protected function lintHashComments($root) {
     $tokens = $root->getTokens();
     foreach ($tokens as $token) {

src/lint/linter/xhpast/__tests__/data/tautological-expressions.lint-test

@@ -15,7 +15,14 @@ if ($x == $y) {
 // See xhpast 0.54 -> 0.55.
 return $a->sub->value < $b->sub->value;
 
+$skip_cache = true || $some_complicated_expression;
+$skip_cache = $a || $b;
+$skip_cache = false && something();
+$skip_cache = f();
+
 ~~~~~~~~~~
 error:3:5
 error:6:5
 error:9:5
+error:18:15
+error:20:15