Fix bugs in builder raw literal analysis

There were two bugs in the way StringLiteralExpr.init(… content: …) determined the necessary raw literal separator:

1. It assumed that only interpolations in a raw literal would require # signs, when in fact all escape sequences do.

2. It did not handle certain edge cases correctly.

Rewrite the `StringLiteralExpr.requiresEscaping(_:)` helper method to implement the correct logic, and simplify the implementation as a result.

Author: beccadax

Sources/SwiftSyntaxBuilder/ConvenienceInitializers.swift

@@ -241,34 +241,28 @@ extension StringLiteralExpr {
   }
 
   private static func requiresEscaping(_ content: String) -> (Bool, poundCount: Int) {
-    var state: PoundState = .none
+    var countingPounds = false
     var consecutivePounds = 0
     var maxPounds = 0
     var requiresEscaping = false
 
     for c in content {
-      switch c {
-      case "#":
+      switch (countingPounds, c) {
+      // Normal mode: scanning for characters that can be followed by pounds.
+      case (false, "\""), (false, "\\"):
+        countingPounds = true
+        requiresEscaping = true
+      case (false, _):
+        continue
+
+      // Special mode: counting a sequence of pounds until we reach its end.
+      case (true, "#"):
         consecutivePounds += 1
-      case "\"":
-        state = .afterQuote
-        consecutivePounds = 0
-      case "\\":
-        state = .afterBackslash
-        consecutivePounds = 0
-      case "(" where state == .afterBackslash:
         maxPounds = max(maxPounds, consecutivePounds)
-        fallthrough
-      default:
+      case (true, _):
+        countingPounds = false
         consecutivePounds = 0
-        state = .none
-      }
-
-      if state == .afterQuote {
-        maxPounds = max(maxPounds, consecutivePounds)
       }
-
-      requiresEscaping = requiresEscaping || state != .none
     }
 
     return (requiresEscaping, poundCount: maxPounds)

Tests/SwiftSyntaxBuilderTest/StringLiteralTests.swift

@@ -81,6 +81,21 @@ final class StringLiteralTests: XCTestCase {
     ##"""
     #"\"#
     """##)
+
+    AssertBuildResult(StringLiteralExpr(content: ##"\#n"##),
+    ##"""
+    ##"\#n"##
+    """##)
+
+    AssertBuildResult(StringLiteralExpr(content: ##"\#\"##),
+    ##"""
+    ##"\#\"##
+    """##)
+
+    AssertBuildResult(StringLiteralExpr(content: ##"\#"##),
+    ##"""
+    ##"\#"##
+    """##)
   }
 
   func testNewlines() {

Tests/SwiftSyntaxMacrosTest/MacroSystemTests.swift

@@ -66,7 +66,7 @@ final class MacroSystemTests: XCTestCase {
       _ = ({ () -> Bool in
         print("hello")
         return true
-      }, #"{ () -> Bool in\\n  print("hello")\\n  return true\\n}"#)
+      }, #"{ () -> Bool in\\#n  print("hello")\\#n  return true\\#n}"#)
       """
     )
   }