Fix SubText ctor parameter verification. (#78989)

* Fix SubText ctor parameter verification.

This ctor currently throws given a zero length span at the end (and only the end) of the subtext. This evidently became far more prevalent with this (fairly) recent change to optimize newline information in our sourcetext classes. (#74728)

It doesn't make to allow zero length spans at all locations but at the end of the subtext, so the fix is just to allow construction in that case too.

Big props to Manish Vasani for providing a very actionable repro for this.

Fixes #78985.

Note there is a potentially related issue outlined in #76225 that looks attributable to the PR above too. It possible that this addresses that issue too, but I'm not completely convinced.

Author: ToddGrun

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

@@ -58,6 +58,29 @@ public void TestSubTextSpanMid()
             Assert.Equal("o W", subText.ToString());
         }
 
+        [Fact]
+        public void TestSubTextSubTextStart()
+        {
+            var text = SourceText.From("Hello World");
+            var subText = text.GetSubText(new TextSpan(0, 5));
+            var subSubText = subText.GetSubText(new TextSpan(0, 0));
+
+            Assert.Equal("Hello", subText.ToString());
+            Assert.Equal(0, subSubText.Length);
+        }
+
+        [Fact]
+        [WorkItem(78989, "https://github.com/dotnet/roslyn/pull/78989")]
+        public void TestSubTextSubTextEnd()
+        {
+            var text = SourceText.From("Hello World");
+            var subText = text.GetSubText(6);
+            var subSubText = subText.GetSubText(subText.Length);
+
+            Assert.Equal("World", subText.ToString());
+            Assert.Equal(0, subSubText.Length);
+        }
+
         [Fact]
         public void TestChangedText()
         {

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

@@ -22,9 +22,9 @@ public SubText(SourceText text, TextSpan span)
                 throw new ArgumentNullException(nameof(text));
             }
 
+            // span.Start and span.End are valid wrt eachother by nature of being passed in as a TextSpan,
+            // so there is no need to verify span.Start against text.Length or span.End against zero.
             if (span.Start < 0
-                || span.Start >= text.Length
-                || span.End < 0
                 || span.End > text.Length)
             {
                 throw new ArgumentOutOfRangeException(nameof(span));