Add folding for the arrow operator

Author: DougGregor

Sources/SwiftOperatorPrecedence/OperatorPrecedence+Folding.swift

@@ -80,7 +80,11 @@ extension OperatorPrecedence {
       return "CastingPrecedence"
     }
 
-    // FIXME: Handle all of the language-defined precedence relationships.
+    // The arrow operator has fixed precedence.
+    if expr.is(ArrowExprSyntax.self) {
+      return "FunctionArrowPrecedence"
+    }
+
     return nil
   }
 
@@ -155,6 +159,16 @@ extension OperatorPrecedence {
         )
     }
 
+    // An arrow expression (->).
+    if let arrowExpr = op.as(ArrowExprSyntax.self) {
+      return ExprSyntax(
+        InfixOperatorExprSyntax(
+          leftOperand: lhs,
+          operatorOperand: ExprSyntax(arrowExpr),
+          rightOperand: rhs)
+      )
+    }
+
     // FIXME: Fallback that we should never need
     fatalError("Unknown binary operator")
   }

Tests/SwiftOperatorPrecedenceTest/OperatorPrecedence.swift

@@ -130,6 +130,14 @@ public class OperatorPrecedenceTests: XCTestCase {
     try opPrecedence.assertExpectedFold("a as c == nil", "((a as c) == nil)")
   }
 
+  func testArrowExpr() throws {
+    let opPrecedence = OperatorPrecedence.standardOperators
+    try opPrecedence.assertExpectedFold(
+      "a = b -> c -> d",
+      "(a = (b -> (c -> d)))"
+    )
+  }
+
   func testParsedLogicalExprs() throws {
     let logicalOperatorSources =
     """