Ensure empty AttributedSubstrings produce no runs (#996)

Author: jmschonfeld

Sources/FoundationEssentials/AttributedString/AttributedString+Runs.swift

@@ -47,8 +47,7 @@ extension AttributedString {
                     _runOffset: _guts.runs.count,
                     runIndex: _guts.runs.endIndex.base,
                     stringIndex: _strBounds.upperBound)
-            } else if _strBounds.upperBound == _guts.string.startIndex {
-                assert(stringLowerBound == stringUpperBound)
+            } else if _strBounds.upperBound == _strBounds.lowerBound {
                 end = start
             } else {
                 let last = _guts.runs.index(atUTF8Offset: _strBounds.upperBound.utf8Offset - 1).index
@@ -62,7 +61,7 @@ extension AttributedString {
             assert(start._runIndex != nil && start._stringIndex != nil)
             assert(end._runIndex != nil && end._stringIndex != nil)
             assert(start._stringIndex!.utf8Offset <= _strBounds.lowerBound.utf8Offset)
-            assert(end._stringIndex!.utf8Offset >= _strBounds.upperBound.utf8Offset)
+            assert(end == start || end._stringIndex!.utf8Offset >= _strBounds.upperBound.utf8Offset)
             self._bounds = start ..< end
         }
     }

Tests/FoundationEssentialsTests/AttributedString/AttributedStringTests.swift

@@ -43,6 +43,21 @@ final class TestAttributedString: XCTestCase {
         for _ in AttributedString().runs {
             XCTFail("Empty AttributedString should not enumerate any attributes")
         }
+        
+        do {
+            let str = AttributedString("Foo")
+            for _ in str[str.startIndex ..< str.startIndex].runs {
+                XCTFail("Empty AttributedSubstring should not enumerate any attributes")
+            }
+        }
+        
+        do {
+            let str = AttributedString("Foo", attributes: AttributeContainer.testInt(2))
+            let i = str.index(afterCharacter: str.startIndex)
+            for _ in str[i ..< i].runs {
+                XCTFail("Empty AttributedSubstring should not enumerate any attributes")
+            }
+        }
     }
 
     func verifyAttributes<T>(_ runs: AttributedString.Runs.AttributesSlice1<T>, string: AttributedString, expectation: [(String, T.Value?)]) where T.Value : Sendable {