Reserve <{...}> for interpolation syntax

Ban a balanced set of `<{...}>` delimiters for a
potential future interpolation syntax.

Sources/_RegexParser/Regex/AST/AST.swift

@@ -53,6 +53,9 @@ extension AST {
     /// Comments, non-semantic whitespace, etc
     case trivia(Trivia)
 
+    /// Intepolation `<{...}>`, currently reserved for future use.
+    case interpolation(Interpolation)
+
     case atom(Atom)
 
     case customCharacterClass(CustomCharacterClass)
@@ -78,6 +81,7 @@ extension AST.Node {
     case let .quantification(v):        return v
     case let .quote(v):                 return v
     case let .trivia(v):                return v
+    case let .interpolation(v):         return v
     case let .atom(v):                  return v
     case let .customCharacterClass(v):  return v
     case let .empty(v):                 return v
@@ -128,7 +132,7 @@ extension AST.Node {
     case .group, .conditional, .customCharacterClass, .absentFunction:
       return true
     case .alternation, .concatenation, .quantification, .quote, .trivia,
-        .empty:
+        .empty, .interpolation:
       return false
     }
   }
@@ -192,6 +196,16 @@ extension AST {
     }
   }
 
+  public struct Interpolation: Hashable, _ASTNode {
+    public let contents: String
+    public let location: SourceLocation
+
+    public init(_ contents: String, _ location: SourceLocation) {
+      self.contents = contents
+      self.location = location
+    }
+  }
+
   public struct Empty: Hashable, _ASTNode {
     public let location: SourceLocation
 

Sources/_RegexParser/Regex/AST/Atom.swift

@@ -822,7 +822,7 @@ extension AST.Node {
     case .alternation, .concatenation, .group,
         .conditional, .quantification, .quote,
         .trivia, .customCharacterClass, .empty,
-        .absentFunction:
+        .absentFunction, .interpolation:
       return nil
     }
   }

Sources/_RegexParser/Regex/Parse/CaptureList.swift

@@ -103,7 +103,7 @@ extension AST.Node {
         break
       }
 
-    case .quote, .trivia, .atom, .customCharacterClass, .empty:
+    case .quote, .trivia, .atom, .customCharacterClass, .empty, .interpolation:
       break
     }
   }

Sources/_RegexParser/Regex/Parse/LexicalAnalysis.swift

@@ -589,6 +589,26 @@ extension Source {
     return AST.Quote(str.value, str.location)
   }
 
+  /// Try to consume an interpolation sequence.
+  ///
+  ///     Interpolation -> '<{' String '}>'
+  ///
+  mutating func lexInterpolation() throws -> AST.Interpolation? {
+    let contents = try recordLoc { src -> String? in
+      try src.tryEating { src in
+        guard src.tryEat(sequence: "<{") else { return nil }
+        _ = src.lexUntil { $0.isEmpty || $0.starts(with: "}>") }
+        guard src.tryEat(sequence: "}>") else { return nil }
+
+        // Not currently supported. We error here instead of during Sema to
+        // get a better error for something like `(<{)}>`.
+        throw ParseError.unsupported("interpolation")
+      }
+    }
+    guard let contents = contents else { return nil }
+    return .init(contents.value, contents.location)
+  }
+
   /// Try to consume a comment
   ///
   ///     Comment -> '(?#' [^')']* ')'

Sources/_RegexParser/Regex/Parse/Parse.swift

@@ -222,6 +222,13 @@ extension Parser {
         result.append(.quote(quote))
         continue
       }
+
+      // Interpolation -> `lexInterpolation`
+      if let interpolation = try source.lexInterpolation() {
+        result.append(.interpolation(interpolation))
+        continue
+      }
+
       //     Quantification  -> QuantOperand Quantifier?
       if let operand = try parseQuantifierOperand() {
         if let (amt, kind, trivia) =

Sources/_RegexParser/Regex/Parse/Sema.swift

@@ -395,6 +395,11 @@ extension RegexValidator {
       // These are Oniguruma specific.
       throw error(.unsupported("absent function"), at: a.location)
 
+    case .interpolation(let i):
+      // This is currently rejected in the parser for better diagnostics, but
+      // reject here too until we get runtime support.
+      throw error(.unsupported("interpolation"), at: i.location)
+
     case .quote, .trivia, .empty:
       break
     }

Sources/_RegexParser/Regex/Printing/DumpAST.swift

@@ -101,6 +101,10 @@ extension AST.Trivia {
   }
 }
 
+extension AST.Interpolation {
+  public var _dumpBase: String { "interpolation <\(contents)>" }
+}
+
 extension AST.Empty {
   public var _dumpBase: String { "" }
 }

Sources/_RegexParser/Regex/Printing/PrintAsCanonical.swift

@@ -97,6 +97,9 @@ extension PrettyPrinter {
     case let .trivia(t):
       output(t._canonicalBase)
 
+    case let .interpolation(i):
+      output(i._canonicalBase)
+
     case let .atom(a):
       output(a._canonicalBase)
 
@@ -178,6 +181,12 @@ extension AST.Quote {
   }
 }
 
+extension AST.Interpolation {
+  var _canonicalBase: String {
+    "<{\(contents)}>"
+  }
+}
+
 extension AST.Group.Kind {
   var _canonicalBase: String {
     switch self {

Sources/_StringProcessing/Regex/ASTConversion.swift

@@ -137,6 +137,9 @@ extension AST.Node {
       case let .trivia(v):
         return .trivia(v.contents)
 
+      case .interpolation:
+        throw Unsupported("TODO: interpolation")
+
       case let .atom(v):
         switch v.kind {
         case .scalarSequence(let seq):

Tests/RegexTests/ParseTests.swift

@@ -804,6 +804,20 @@ extension RegexTests {
       #"a(?#. comment)b"#,
       concat("a", "b"))
 
+    // MARK: Interpolation
+
+    // These are literal as there's no closing '}>'
+    parseTest("<{", concat("<", "{"))
+    parseTest("<{a", concat("<", "{", "a"))
+    parseTest("<{a}", concat("<", "{", "a", "}"))
+    parseTest("<{<{}", concat("<", "{", "<", "{", "}"))
+
+    // Literal as escaped
+    parseTest(#"\<{}>"#, concat("<", "{", "}", ">"))
+
+    // A quantification
+    parseTest(#"<{2}"#, exactly(2, of: "<"))
+
     // MARK: Quantification
 
     parseTest("a*", zeroOrMore(of: "a"))
@@ -2574,6 +2588,15 @@ extension RegexTests {
     diagnosticTest("|\u{360}", .confusableCharacter("|\u{360}"))
     diagnosticTest(" \u{361}", .confusableCharacter(" \u{361}"))
 
+    // MARK: Interpolation (currently unsupported)
+
+    diagnosticTest("<{}>", .unsupported("interpolation"))
+    diagnosticTest("<{...}>", .unsupported("interpolation"))
+    diagnosticTest("<{)}>", .unsupported("interpolation"))
+    diagnosticTest("<{}}>", .unsupported("interpolation"))
+    diagnosticTest("<{<{}>", .unsupported("interpolation"))
+    diagnosticTest("(<{)}>", .unsupported("interpolation"))
+
     // MARK: Character properties
 
     diagnosticTest(#"\p{Lx}"#, .unknownProperty(key: nil, value: "Lx"))