Fix source location tracking in lexUntil

The `predicate` may independently advance the
location before bailing, and we don't want that
to affect the recorded location of the result. We
probably ought to replace `lexUntil` with a better
API.

Author: hamishknight

Sources/_RegexParser/Regex/Parse/LexicalAnalysis.swift

@@ -503,13 +503,22 @@ extension Source {
   private mutating func lexUntil(
     _ predicate: (inout Source) throws -> Bool
   ) rethrows -> Located<String> {
+    // We track locations outside of recordLoc, as the predicate may advance the
+    // input when we hit the end, and we don't want that to affect the location
+    // of what was lexed in the `result`. We still want the recordLoc call to
+    // attach locations to any thrown errors though.
+    // TODO: We should find a better way of doing this, `lexUntil` seems full
+    // of footguns.
+    let start = currentPosition
+    var end = currentPosition
+    var result = ""
     try recordLoc { src in
-      var result = ""
       while try !predicate(&src) {
         result.append(src.eat())
+        end = src.currentPosition
       }
-      return result
     }
+    return .init(result, start ..< end)
   }
 
   private mutating func lexUntil(eating end: String) throws -> Located<String> {

Tests/RegexTests/ParseTests.swift

@@ -2195,6 +2195,14 @@ extension RegexTests {
       $0.as(AST.Atom.self)!.as(AST.Atom.Scalar.self)!.location
     })
 
+    rangeTest(#"\u{  65 58 }"#, range(5 ..< 7), at: {
+      $0.as(AST.Atom.self)!.as(AST.Atom.ScalarSequence.self)!.scalars[0].location
+    })
+
+    rangeTest(#"\u{  65 58 }"#, range(8 ..< 10), at: {
+      $0.as(AST.Atom.self)!.as(AST.Atom.ScalarSequence.self)!.scalars[1].location
+    })
+
     // MARK: References
 
     rangeTest(#"\k<a+2>"#, range(3 ..< 6), at: {