Allow CustomConsuming types to match w/ zero width (swiftlang#479)

* Allow CustomConsuming types to match w/ zero width

We previously asserted if a custom consuming type matches with zero
width, but that isn't necessary or good. A custom type can implement
a lookaround assertion or act as a tracer.

* Rename Processor.advance(to:) to resume(at:)

Since the given index doesn’t need to advance, this name is less
misleading.

Author: natecook1000

Sources/_StringProcessing/Engine/Processor.swift

@@ -153,11 +153,15 @@ extension Processor {
     return true
   }
 
-  mutating func advance(to nextIndex: Input.Index) {
-    assert(nextIndex >= bounds.lowerBound)
-    assert(nextIndex <= bounds.upperBound)
-    assert(nextIndex > currentPosition)
-    currentPosition = nextIndex
+  /// Continue matching at the specified index.
+  ///
+  /// - Precondition: `bounds.contains(index) || index == bounds.upperBound`
+  /// - Precondition: `index >= currentPosition`
+  mutating func resume(at index: Input.Index) {
+    assert(index >= bounds.lowerBound)
+    assert(index <= bounds.upperBound)
+    assert(index >= currentPosition)
+    currentPosition = index
   }
 
   func doPrint(_ s: String) {
@@ -344,7 +348,7 @@ extension Processor {
         signalFailure()
         return
       }
-      advance(to: nextIndex)
+      resume(at: nextIndex)
       controller.step()
 
     case .assertBy:
@@ -372,7 +376,7 @@ extension Processor {
           return
         }
         registers[valReg] = val
-        advance(to: nextIdx)
+        resume(at: nextIdx)
         controller.step()
       } catch {
         abort(error)

Tests/RegexBuilderTests/RegexDSLTests.swift

@@ -1015,6 +1015,42 @@ class RegexDSLTests: XCTestCase {
       XCTAssertEqual(str.wholeMatch(of: parser)?.1, version)
     }
   }
+  
+  func testZeroWidthConsumer() throws {
+    struct Trace: CustomConsumingRegexComponent {
+      typealias RegexOutput = Void
+      var label: String
+      init(_ label: String) { self.label = label }
+
+      static var traceOutput = ""
+
+      func consuming(_ input: String, startingAt index: String.Index, in bounds: Range<String.Index>) throws -> (upperBound: String.Index, output: Void)? {
+        print("Matching '\(label)'", to: &Self.traceOutput)
+        print(input, to: &Self.traceOutput)
+        let dist = input.distance(from: input.startIndex, to: index)
+        print(String(repeating: " ", count: dist) + "^", to: &Self.traceOutput)
+        return (index, ())
+      }
+    }
+    
+    let regex = Regex {
+      OneOrMore(.word)
+      Trace("end of key")
+      ":"
+      Trace("start of value")
+      OneOrMore(.word)
+    }
+    XCTAssertNotNil("hello:goodbye".firstMatch(of: regex))
+    XCTAssertEqual(Trace.traceOutput, """
+      Matching 'end of key'
+      hello:goodbye
+           ^
+      Matching 'start of value'
+      hello:goodbye
+            ^
+      
+      """)
+  }
 }
 
 extension Unicode.Scalar {