PSAvoidTrailingWhitespace: Rule not applied when using formatter + single character lines with trailing whitespace are truncated (#1993)

* Formatter: Added PSAvoidTrailingWhitespace to the list of rules considered by the formatter

* Rules/AvoidTrailingWhitespace: Fixed issue where lines with a single character, followed by multiple white-spaces were truncated when fixed/formatted

* Tests/Rules/AvoidTrailingWhitespace: Added test for usage of Invoke-Formatter with PSAvoidTrailingWhitespace and also checking that single-character lines that have trailing whitespace are not removed

---------

Co-authored-by: Christoph Bergmeister <c.bergmeister@gmail.com>

Author: liamjpeters

Diff for: Engine/Formatter.cs

@@ -47,6 +47,7 @@ public static string Format<TCmdlet>(
                 "PSAvoidUsingDoubleQuotesForConstantString",
                 "PSAvoidSemicolonsAsLineTerminators",
                 "PSAvoidExclaimOperator",
+                "PSAvoidTrailingWhitespace",
             };
 
             var text = new EditableText(scriptDefinition);

Diff for: Rules/AvoidTrailingWhitespace.cs

@@ -54,7 +54,7 @@ public IEnumerable<DiagnosticRecord> AnalyzeScript(Ast ast, string fileName)
                 }
 
                 int startColumnOfTrailingWhitespace = 1;
-                for (int i = line.Length - 2; i > 0; i--)
+                for (int i = line.Length - 2; i >= 0; i--)
                 {
                     if (line[i] != ' ' && line[i] != '\t')
                     {

Diff for: Tests/Rules/AvoidTrailingWhitespace.tests.ps1

@@ -9,6 +9,9 @@ BeforeAll {
 
     $settings = @{
         IncludeRules = @($ruleName)
+        Rules = @{
+            $ruleName = @{}
+        }
     }
 }
 
@@ -34,4 +37,26 @@ Describe "AvoidTrailingWhitespace" {
         $violations = Invoke-ScriptAnalyzer -ScriptDefinition $def -Settings $settings
         Test-CorrectionExtentFromContent $def $violations 1 $Whitespace ''
     }
+
+    It 'Should be used by Invoke-Formatter, when in settings, replacing trailing <Type>' -TestCases $testCases {
+        param (
+            [string] $Whitespace
+        )
+        # Test also guards against regression where single-character lines, with trailing whitespace
+        # would be removed entirely. See issues #1757, #1992
+        $def = @"
+Function Get-Example {
+    'Example'$Whitespace
+}$Whitespace
+"@
+
+        $expected = @"
+Function Get-Example {
+    'Example'
 }
+"@
+        $formatted = Invoke-Formatter -ScriptDefinition $def -Settings $settings
+        $formatted | Should -Be $expected
+    }
+
+}