Only add pure to functions with params

src/Psalm/Internal/Analyzer/FunctionLikeAnalyzer.php

@@ -584,6 +584,7 @@ abstract class FunctionLikeAnalyzer extends SourceAnalyzer
             && !$this->inferred_impure
             && ($this->function instanceof Function_
                 || $this->function instanceof ClassMethod)
+            && $storage->params
         ) {
             $manipulator = FunctionDocblockManipulator::getForFunction(
                 $project_analyzer,

src/Psalm/Issue/ImpureVariable.php

@@ -0,0 +1,8 @@
+<?php
+namespace Psalm\Issue;
+
+class ImpureVariable extends CodeIssue
+{
+    const ERROR_LEVEL = -1;
+    const SHORTCODE = 235;
+}

tests/FileManipulation/PureAnnotationAdditionTest.php

@@ -116,6 +116,33 @@ class PureAnnotationAdditionTest extends FileManipulationTest
                 ['MissingPureAnnotation'],
                 true,
             ],
+            'dontAddWhenReferencingThis' => [
+                '<?php
+                    class A {
+                        public int $a = 5;
+
+                        /**
+                         * @psalm-pure
+                         */
+                        public function foo() : self {
+                            return $this;
+                        }
+                    }',
+                '<?php
+                    class A {
+                        public int $a = 5;
+
+                        /**
+                         * @psalm-pure
+                         */
+                        public function foo() : self {
+                            return $this;
+                        }
+                    }',
+                '7.4',
+                ['MissingPureAnnotation'],
+                true,
+            ],
         ];
     }
 }