Swift SIL: add a StringParser utility for simple parsing tasks

Author: eeckstein

SwiftCompilerSources/Sources/SIL/Utils.swift

@@ -163,6 +163,73 @@ extension LazyFilterSequence : CollectionLikeSequence,
                                FormattedLikeArray, CustomStringConvertible, CustomReflectable
                                where Base: CollectionLikeSequence {}
 
+//===----------------------------------------------------------------------===//
+//                            String parsing
+//===----------------------------------------------------------------------===//
+
+public struct StringParser {
+  private var s: Substring
+  private let originalLength: Int
+  
+  private mutating func consumeWhitespace() {
+    s = s.drop { $0.isWhitespace }
+  }
+
+  public init(_ string: String) {
+    s = Substring(string)
+    originalLength = string.count
+  }
+  
+  mutating func isEmpty() -> Bool {
+    consumeWhitespace()
+    return s.isEmpty
+  }
+
+  public mutating func consume(_ str: String) -> Bool {
+    consumeWhitespace()
+    if !s.starts(with: str) { return false }
+    s = s.dropFirst(str.count)
+    return true
+  }
+
+  public mutating func consumeInt(withWhiteSpace: Bool = true) -> Int? {
+    if withWhiteSpace {
+      consumeWhitespace()
+    }
+    var intStr = ""
+    s = s.drop {
+      if $0.isNumber {
+        intStr.append($0)
+        return true
+      }
+      return false
+    }
+    return Int(intStr)
+  }
+  
+  public mutating func consumeIdentifier() -> String? {
+    consumeWhitespace()
+    var name = ""
+    s = s.drop {
+      if $0.isLetter {
+        name.append($0)
+        return true
+      }
+      return false
+    }
+    return name.isEmpty ? nil : name
+  }
+  
+  public func throwError(_ message: StaticString) throws -> Never {
+    throw ParsingError(message: message, position: originalLength - s.count)
+  }
+}
+
+public struct ParsingError : Error {
+  public let message: StaticString
+  public let position: Int
+}
+
 //===----------------------------------------------------------------------===//
 //                            Bridging Utilities
 //===----------------------------------------------------------------------===//

include/swift/SIL/SILBridging.h

@@ -136,6 +136,11 @@ typedef struct {
   SwiftObject obj;
 } BridgedMultiValueResult;
 
+typedef struct {
+  const unsigned char * _Nullable message;
+  SwiftInt position;
+} BridgedParsingError;
+
 // Must be in sync with SILInstruction::MemoryBehavior
 // TODO: do this less hacky.
 typedef enum {