Fix NullReferenceException in AvoidAssignmentToAutomaticVariable rule when assigning a .Net property and only look at the LHS (#1008)

* Fix NullReferenceException in AvoidAssignmentToAutomaticVariable rule when assigning a net property to a .Net property

* when variableExpressionAst is null, then continue loop to reduce nesting

* fix indentation for cleaner diff

* Fix issue #1013 as well by making sure the rule looks only the LHS

* Fix issue #1012 as well

* Simplify test that checks that PSSA does not throw to address PR review

Author: bergmeister

Rules/AvoidAssignmentToAutomaticVariable.cs

@@ -44,9 +44,10 @@ public IEnumerable<DiagnosticRecord> AnalyzeScript(Ast ast, string fileName)
             if (ast == null) throw new ArgumentNullException(Strings.NullAstErrorMessage);
 
             IEnumerable<Ast> assignmentStatementAsts = ast.FindAll(testAst => testAst is AssignmentStatementAst, searchNestedScriptBlocks: true);
-            foreach (var assignmentStatementAst in assignmentStatementAsts)
+            foreach (AssignmentStatementAst assignmentStatementAst in assignmentStatementAsts)
             {
-                var variableExpressionAst = assignmentStatementAst.Find(testAst => testAst is VariableExpressionAst, searchNestedScriptBlocks: false) as VariableExpressionAst;
+                var variableExpressionAst = assignmentStatementAst.Left.Find(testAst => testAst is VariableExpressionAst && testAst.Parent == assignmentStatementAst, searchNestedScriptBlocks: false) as VariableExpressionAst;
+                if (variableExpressionAst == null) { continue; }
                 var variableName = variableExpressionAst.VariablePath.UserPath;
                 if (_readOnlyAutomaticVariables.Contains(variableName, StringComparer.OrdinalIgnoreCase))
                 {

Tests/Rules/AvoidAssignmentToAutomaticVariable.tests.ps1

@@ -63,6 +63,20 @@ Describe "AvoidAssignmentToAutomaticVariables" {
             $warnings.Count | Should -Be 0
         }
 
+        It "Does not throw a NullReferenceException when using assigning a .Net property to a .Net property (Bug in 1.17.0 - issue 1007)" {
+            Invoke-ScriptAnalyzer -ScriptDefinition '[foo]::bar = [baz]::qux' -ErrorAction Stop
+        }
+
+        It "Does not flag properties of a readonly variable (issue 1012)" {
+            [System.Array] $warnings = Invoke-ScriptAnalyzer -ScriptDefinition '$Host.PrivateData["ErrorBackgroundColor"] = "Black"'
+            $warnings.Count | Should -Be 0
+        }
+
+        It "Does not flag RHS of variable assignment (Bug in 1.17.0, issue 1013)" {
+            [System.Array] $warnings = Invoke-ScriptAnalyzer -ScriptDefinition '[foo]::bar = $true'
+            $warnings.Count | Should -Be 0
+        }
+
         It "Setting Variable <VariableName> throws exception in applicable PowerShell version to verify the variables is read-only" -TestCases $testCases_ReadOnlyVariables {
             param ($VariableName, $ExpectedSeverity, $OnlyPresentInCoreClr)