Detect use of static inside pure function

config.xsd

@@ -172,6 +172,7 @@
             <xs:element name="ImpureFunctionCall" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="ImpureMethodCall" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="ImpurePropertyAssignment" type="IssueHandlerType" minOccurs="0" />
+            <xs:element name="ImpureStaticVariable" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="InaccessibleClassConstant" type="IssueHandlerType" minOccurs="0" />
             <xs:element name="InaccessibleMethod" type="MethodIssueHandlerType" minOccurs="0" />
             <xs:element name="InaccessibleProperty" type="IssueHandlerType" minOccurs="0" />

docs/running_psalm/issues.md

@@ -400,6 +400,20 @@ function filterOdd(int $i, A $a) : ?int {
 }
 ```
 
+### ImpureStaticVariable
+
+Emitted when attempting to use a static variable from a function or method marked as pure
+
+```php
+/** @psalm-pure */
+function addCumulative(int $left) : int {
+    /** @var int */
+    static $i = 0;
+    $i += $left;
+    return $left;
+}
+```
+
 ### InaccessibleClassConstant
 
 Emitted when a public/private class constant is not accessible from the calling context

src/Psalm/Internal/Analyzer/StatementsAnalyzer.php

@@ -1296,6 +1296,18 @@ class StatementsAnalyzer extends SourceAnalyzer implements StatementsSource
     {
         $codebase = $this->getCodebase();
 
+        if ($context->mutation_free) {
+            if (IssueBuffer::accepts(
+                new \Psalm\Issue\ImpureStaticVariable(
+                    'Cannot use a static variable in a mutation-free context',
+                    new CodeLocation($this, $stmt)
+                ),
+                $this->getSuppressedIssues()
+            )) {
+                // fall through
+            }
+        }
+
         foreach ($stmt->vars as $var) {
             if (!is_string($var->var->name)) {
                 continue;

src/Psalm/Issue/ImpureStaticVariable.php

@@ -0,0 +1,6 @@
+<?php
+namespace Psalm\Issue;
+
+class ImpureStaticVariable extends CodeIssue
+{
+}

tests/PureAnnotationTest.php

@@ -245,6 +245,17 @@ class PureAnnotationTest extends TestCase
                     }',
                 'error_message' => 'ImpureMethodCall',
             ],
+            'useOfStaticMakesFunctionImpure' => [
+                '<?php
+                    /** @psalm-pure */
+                    function addCumulative(int $left) : int {
+                        /** @var int */
+                        static $i = 0;
+                        $i += $left;
+                        return $left;
+                    }',
+                'error_message' => 'ImpureStaticVariable',
+            ],
         ];
     }
 }