fix for #4518: (no-trailing-whitespace) flags leading empty line as error

src/rules/noTrailingWhitespaceRule.ts

@@ -22,6 +22,8 @@ import * as Lint from "../index";
 
 import { getTemplateRanges } from "./noConsecutiveBlankLinesRule";
 
+const ZERO_WIDTH_NO_BREAK_SPACE = 0xfeff;
+
 const OPTION_IGNORE_COMMENTS = "ignore-comments";
 const OPTION_IGNORE_JSDOC = "ignore-jsdoc";
 const OPTION_IGNORE_TEMPLATE_STRINGS = "ignore-template-strings";
@@ -87,7 +89,11 @@ function walk(ctx: Lint.WalkContext<Options>) {
     for (const line of getLineRanges(sourceFile)) {
         // \s matches any whitespace character (equal to [\r\n\t\f\v ])
         const match = text.substr(line.pos, line.contentLength).match(/\s+$/);
-        if (match !== null && !(ctx.options.ignoreBlankLines && match.index === 0)) {
+        if (
+            match !== null &&
+            !(ctx.options.ignoreBlankLines && match.index === 0) &&
+            (match[0] !== String.fromCharCode(ZERO_WIDTH_NO_BREAK_SPACE))
+        ) {
             possibleFailures.push({
                 end: line.pos + line.contentLength,
                 pos: line.pos + match.index!,

test/rules/no-trailing-whitespace/zero-width-no-break-space/test.ts.lint

@@ -0,0 +1,4 @@
+
+// This file starts with a zero width no-break-space.
+// See http://www.fileformat.info/info/unicode/char/feff/index.htm
+const a = 3;

test/rules/no-trailing-whitespace/zero-width-no-break-space/test2.ts.lint

@@ -0,0 +1,5 @@
+ 
+~~ [trailing whitespace]
+// This file starts with a zero width no-break-space followed by a trailing space.
+// See http://www.fileformat.info/info/unicode/char/feff/index.htm
+const a = 3;

test/rules/no-trailing-whitespace/zero-width-no-break-space/tslint.json

@@ -0,0 +1,5 @@
+{
+  "rules": {
+    "no-trailing-whitespace": true
+  }
+}