Fix issue with when a CR/LF split occurs at a SubText's underlying span start (#80911)

I hit an exception in the SourceText system when inserting a copilot
edit for C# content in a razor file when the file uses newlines only.
This ended up causing a CR/LF split in the subtext class, which had some
bogus logic for when that happened.

Added a very simple unit test that demonstrated previous failure.

Author: ToddGrun

src/Compilers/Core/CodeAnalysisTest/Text/CompositeTextTests.cs

@@ -63,6 +63,8 @@ private static IEnumerable<string> GetLinesTexts(TextLineCollection textLines)
                 sourceTextsBuilder.AddRange(sourceTexts);
 
                 var compositeText = (CompositeText)CompositeText.ToSourceText(sourceTextsBuilder, sourceText, adjustSegments: false);
+                sourceTextsBuilder.Free();
+
                 yield return (sourceText, compositeText);
             }
         }

src/Compilers/Core/CodeAnalysisTest/Text/SubTextTests.cs

@@ -0,0 +1,52 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using Microsoft.CodeAnalysis.Text;
+using Xunit;
+
+namespace Microsoft.CodeAnalysis.UnitTests.Text
+{
+    public class SubTextTests
+    {
+        [Theory]
+        [InlineData("abcdefghijkl")]
+        [InlineData(["\r\r\r\r\r\r\r\r\r\r\r\r"])]
+        [InlineData(["\n\n\n\n\n\n\n\n\n\n\n\n"])]
+        [InlineData(["\r\n\r\n\r\n\r\n\r\n\r\n"])]
+        [InlineData(["\n\r\n\r\n\r\n\r\n\r\n\r"])]
+        [InlineData(["a\r\nb\r\nc\r\nd\r\n"])]
+        [InlineData(["\ra\n\rb\n\rc\n\rd\n"])]
+        [InlineData(["\na\r\nb\r\nc\r\nd\r"])]
+        [InlineData(["ab\r\ncd\r\nef\r\n"])]
+        [InlineData(["ab\r\r\ncd\r\r\nef"])]
+        [InlineData(["ab\n\n\rcd\n\n\ref"])]
+        [InlineData(["ab\u0085cdef\u2028ijkl\u2029op"])]
+        [InlineData(["\u0085\u2028\u2029\u0085\u2028\u2029\u0085\u2028\u2029\u0085\u2028\u2029"])]
+        public void SubTextTestAllPossibleSubstrings(string contents)
+        {
+            var fullStringText = SourceText.From(contents);
+            for (var start = 0; start < contents.Length; start++)
+            {
+                for (var end = start + 1; end <= contents.Length; end++)
+                {
+                    var stringText = SourceText.From(contents[start..end]);
+                    var subText = new SubText(fullStringText, new TextSpan(start, length: end - start));
+
+                    Assert.Equal(stringText.Length, subText.Length);
+                    for (var i = 0; i < stringText.Length; i++)
+                    {
+                        Assert.Equal(stringText.Lines.IndexOf(i), subText.Lines.IndexOf(i));
+                    }
+
+                    Assert.Equal(stringText.Lines.Count, subText.Lines.Count);
+                    for (var i = 0; i < stringText.Lines.Count; i++)
+                    {
+                        Assert.Equal(stringText.Lines[i].ToString(), subText.Lines[i].ToString());
+                        Assert.Equal(stringText.Lines[i].EndIncludingLineBreak, subText.Lines[i].EndIncludingLineBreak);
+                    }
+                }
+            }
+        }
+    }
+}

src/Compilers/Core/Portable/Text/SubText.cs

@@ -103,7 +103,6 @@ private sealed class SubTextLineInfo : TextLineCollection
             private readonly SubText _subText;
             private readonly int _startLineNumberInUnderlyingText;
             private readonly int _lineCount;
-            private readonly bool _startsWithinSplitCRLF;
             private readonly bool _endsWithinSplitCRLF;
 
             public SubTextLineInfo(SubText subText)
@@ -116,14 +115,6 @@ public SubTextLineInfo(SubText subText)
                 _startLineNumberInUnderlyingText = startLineInUnderlyingText.LineNumber;
                 _lineCount = (endLineInUnderlyingText.LineNumber - _startLineNumberInUnderlyingText) + 1;
 
-                var underlyingSpanStart = _subText.UnderlyingSpan.Start;
-                if (underlyingSpanStart == startLineInUnderlyingText.End + 1 &&
-                    underlyingSpanStart == startLineInUnderlyingText.EndIncludingLineBreak - 1)
-                {
-                    Debug.Assert(_subText.UnderlyingText[underlyingSpanStart - 1] == '\r' && _subText.UnderlyingText[underlyingSpanStart] == '\n');
-                    _startsWithinSplitCRLF = true;
-                }
-
                 var underlyingSpanEnd = _subText.UnderlyingSpan.End;
                 if (underlyingSpanEnd == endLineInUnderlyingText.End + 1 &&
                     underlyingSpanEnd == endLineInUnderlyingText.EndIncludingLineBreak - 1)
@@ -151,7 +142,7 @@ public override TextLine this[int lineNumber]
                         // Special case splitting the CRLF at the end as the UnderlyingText doesn't view the position
                         // after between the \r and \n as on a new line whereas this subtext doesn't contain the \n
                         // and needs to view that position as on a new line.
-                        return TextLine.FromSpanUnsafe(_subText, new TextSpan(_subText.UnderlyingSpan.End, 0));
+                        return TextLine.FromSpanUnsafe(_subText, new TextSpan(_subText.UnderlyingSpan.Length, 0));
                     }
 
                     var underlyingTextLine = _subText.UnderlyingText.Lines[lineNumber + _startLineNumberInUnderlyingText];
@@ -210,12 +201,6 @@ public override int IndexOf(int position)
                 var underlyingPosition = position + _subText.UnderlyingSpan.Start;
                 var underlyingLineNumber = _subText.UnderlyingText.Lines.IndexOf(underlyingPosition);
 
-                if (_startsWithinSplitCRLF && position != 0)
-                {
-                    // The \n contributes a line to the count in this subtext, but not in the UnderlyingText.
-                    underlyingLineNumber += 1;
-                }
-
                 return underlyingLineNumber - _startLineNumberInUnderlyingText;
             }
         }