Add unsafe expression syntax under an experimental feature flag unsafeExpression

Author: DougGregor

CodeGeneration/Sources/SyntaxSupport/ExperimentalFeatures.swift

@@ -21,6 +21,7 @@ public enum ExperimentalFeature: String, CaseIterable {
   case coroutineAccessors
   case valueGenerics
   case abiAttribute
+  case unsafeExpression
 
   /// The name of the feature as it is written in the compiler's `Features.def` file.
   public var featureName: String {
@@ -41,6 +42,8 @@ public enum ExperimentalFeature: String, CaseIterable {
       return "ValueGenerics"
     case .abiAttribute:
       return "ABIAttribute"
+    case .unsafeExpression:
+      return "WarnUnsafe"
     }
   }
 
@@ -63,6 +66,8 @@ public enum ExperimentalFeature: String, CaseIterable {
       return "value generics"
     case .abiAttribute:
       return "@abi attribute"
+    case .unsafeExpression:
+      return "'unsafe' expression"
     }
   }
 

CodeGeneration/Sources/SyntaxSupport/ExprNodes.swift

@@ -180,6 +180,23 @@ public let EXPR_NODES: [Node] = [
     ]
   ),
 
+  Node(
+    kind: .unsafeExpr,
+    base: .expr,
+    experimentalFeature: .unsafeExpression,
+    nameForDiagnostics: "'unsafe' expression",
+    children: [
+      Child(
+        name: "unsafeKeyword",
+        kind: .token(choices: [.keyword(.unsafe)])
+      ),
+      Child(
+        name: "expression",
+        kind: .node(kind: .expr)
+      ),
+    ]
+  ),
+
   Node(
     kind: .binaryOperatorExpr,
     base: .expr,

CodeGeneration/Sources/SyntaxSupport/SyntaxNodeKind.swift

@@ -299,6 +299,7 @@ public enum SyntaxNodeKind: String, CaseIterable, IdentifierConvertible, TypeCon
   case unresolvedAsExpr
   case unresolvedIsExpr
   case unresolvedTernaryExpr
+  case unsafeExpr
   case valueBindingPattern
   case variableDecl
   case versionComponent