Remove trailing whitespace on first line of multiline string. (#1236)

Remove trailing whitespace on first line of multiline string.

Normalizes, but retains, backslashes on the first ignored line of a multiline string.
(There should probably be a fix to remove them.)

CHANGELOG.md

@@ -3,6 +3,8 @@
 * Add `tall-style` experiment flag to enable the in-progress unstable new
   formatting style (#1253).
 * Format extension types.
+* Normalize ignored whitespace and "escaped whitespace" on first line
+  of multiline string literals. (#1235)
 
 ## 2.3.3
 

lib/src/source_visitor.dart

@@ -4158,6 +4158,8 @@ class SourceVisitor extends ThrowingAstVisitor {
     _endBody(rightBracket, forceSplit: nodes.isNotEmpty);
   }
 
+  static final _lineTerminatorRE = RegExp(r'\r\n?|\n');
+
   /// Writes the string literal [string] to the output.
   ///
   /// Splits multiline strings into separate chunks so that the line splitter
@@ -4167,21 +4169,96 @@ class SourceVisitor extends ThrowingAstVisitor {
     // comments are written first.
     writePrecedingCommentsAndNewlines(string);
 
-    // Split each line of a multiline string into separate chunks.
-    var lines = string.lexeme.split(_formatter.lineEnding!);
+    var lines = string.lexeme.split(_lineTerminatorRE);
     var offset = string.offset;
+    var firstLine = lines.first;
+    if (lines.length > 1) {
+      // Special case for multiline string which contains
+      // at least one newline.
+      _writeStringFirstLine(firstLine, string, offset: offset);
+    } else {
+      _writeText(firstLine, string, offset: offset);
+    }
+    offset += firstLine.length;
 
-    _writeText(lines.first, string, offset: offset);
-    offset += lines.first.length;
-
-    for (var line in lines.skip(1)) {
+    for (var i = 1; i < lines.length; i++) {
+      var line = lines[i];
       builder.writeNewline(flushLeft: true, nest: true);
       offset++;
       _writeText(line, string, offset: offset, mergeEmptySplits: false);
       offset += line.length;
     }
   }
 
+  /// Writes the first line of a multi-line string.
+  ///
+  /// If the string is a multiline string, and it has only whitespace
+  /// and escaped whitespace before a first line break,
+  /// omit the non-escaped trailing whitespace.
+  /// Normalize escaped non-final whitspace to spaces.
+  ///
+  /// More specifically:
+  /// If a multiline string literal contains at least one line-break
+  /// (a CR, LF or CR+LF) as part of the source character content
+  /// (characters inside interpolation expressions do not count),
+  /// and the source characters from the starting quote to the first
+  /// line-break contains only the characters space, tab and backslash,
+  /// with no two adjacent backslashes, then that part of the string source,
+  /// including the following line break, is excluded from particiapting
+  /// code points to the string value.
+  ///
+  /// This function normalizes such excluded character sequences
+  /// to just the back-slashes, separated by space characters.
+  void _writeStringFirstLine(String line, Token string, {required int offset}) {
+    // Detect leading whitespace on the first line of multiline strings.
+    var quoteStart = line.startsWith('r') ? 1 : 0;
+    var quoteEnd = quoteStart + 3;
+    var backslashCount = 0;
+    if (line.length > quoteEnd &&
+        (line.startsWith("'''", quoteStart) ||
+            line.startsWith('"""', quoteStart))) {
+      // Start of a multiline string literal.
+      // Check if rest of the line is whitespace, possibly preceded by
+      // backslash, or has a single trailing backslash preceding the newline.
+      // Count the backslashes.
+      var cursor = quoteEnd;
+      const backslash = 0x5c;
+      const space = 0x20;
+      const tab = 0x09;
+
+      do {
+        var char = line.codeUnitAt(cursor);
+        if (char == backslash) {
+          cursor += 1;
+          backslashCount++;
+          if (cursor >= line.length) {
+            break;
+          }
+          char = line.codeUnitAt(cursor);
+        }
+        if (char != space && char != tab) break;
+        cursor++;
+      } while (cursor < line.length);
+      if (cursor == line.length) {
+        // No invalid character sequence found before end of line.
+        // Normalize the ignored "escaped" whitespace which has no
+        // effect on string content.
+        var firstLineText = line.substring(0, quoteEnd);
+        if (backslashCount > 0) {
+          var buffer = StringBuffer(firstLineText);
+          buffer.write(r'\');
+          while (--backslashCount > 0) {
+            buffer.write(r' \');
+          }
+          firstLineText = buffer.toString();
+        }
+        _writeText(firstLineText, string, offset: offset);
+        return;
+      }
+    }
+    _writeText(line, string, offset: offset);
+  }
+
   /// Write the comma token following [node], if there is one.
   void _writeCommaAfter(AstNode node) {
     token(node.commaAfter);

test/whitespace/multiline_string_first_list.stmt

@@ -0,0 +1,117 @@
+40 columns                              |
+>>> trailing all-space multiline string  (×20 = space, ×09 = tab)
+var x = '''×20×09×20×09
+×20×09
+×09×20''';
+<<<
+var x = '''
+×20×09
+×09×20''';
+>>> single trailing space
+var x = '''×20
+z''';
+<<<
+var x = '''
+z''';
+>>> single trailing tab
+var x = '''×09
+z''';
+<<<
+var x = '''
+z''';
+>>> trailing all-space multiline raw string
+var x = r'''×20×09×20×09
+×20×09
+×09×20''';
+<<<
+var x = r'''
+×20×09
+×09×20''';
+>>> no trailing characters, nothing happens
+var x = '''
+×20×09''';
+<<<
+var x = '''
+×20×09''';
+>>> no line break, nothing happnes
+var x = '''×20×09''';
+<<<
+var x = '''×20×09''';
+>>> line break not part of string.
+var x = '''×20×09×20×09${
+''}×20×09''';
+<<<
+var x = '''×20×09×20×09${''}×20×09''';
+>>> "escapes" allowed, not removed, but normalized
+var x = '''×20×09\×20\×09×20×09
+''';
+<<<
+var x = '''\×20\
+''';
+>>> single escaped space
+var x = '''\×20
+z''';
+<<<
+var x = '''\
+z''';
+>>> single escaped tab
+var x = '''\×09
+z''';
+<<<
+var x = '''\
+z''';
+>>> single trailing escape
+var x = '''\
+z''';
+<<<
+var x = '''\
+z''';
+>>> final "escape" allowed too, not removed, but normalized
+var x = '''×20×09\×20\×09×20×09\
+''';
+<<<
+var x = '''\×20\×20\
+''';
+>>> "escape" allowed in raw strings, not removed, but normalized
+var x = r'''×20×09\×20\×09×20×09\
+''';
+<<<
+var x = r'''\×20\×20\
+''';
+>>> A "double-escape" is not an escaped whitspace
+var x = '''×20×09\\×20
+''';
+<<<
+var x = '''×20×09\\×20
+''';
+>>> Non-whitispace character zero-content part on first line
+var x = '''   ${''}×20×09
+''';
+<<<
+var x = '''   ${''}×20×09
+''';
+>>> interpolations do not start a new "first line"
+var x = '''×20×09
+${''}×20×09
+''';
+<<<
+var x = '''
+${''}×20×09
+''';
+>>> Works with any line break - U+000A
+var x = '''×20×0a×20''';
+<<<
+var x = '''
+×20''';
+>>> Works with any line break - U+000D
+var x = '''×20×0d×20''';
+<<<
+var x = '''
+×20''';
+>>> Works with any line break - U+000D U+000A
+// First linebreak is not \r\n.
+var x = '''×20×0d×0az×20''';
+<<<
+// First linebreak is not \r\n.
+var x = '''
+z×20''';