[ASTGen] Reduce usage of generate(choices:)

`generate(choices:)` is not good because it expects "ASTNode". Not all
`SyntaxChildChoices` can generate ASTNode.

* `generate(codeBlockItem:)` to manually switch over the cases
* switch on `IfExpr.ElseBody` insdie `makeIfStmt()`, now supports `else
  if` case

Author: rintaro

lib/ASTGen/Sources/ASTGen/ASTGen.swift

@@ -241,7 +241,14 @@ extension ASTGenVisitor {
   }
 
   public func generate(codeBlockItem node: CodeBlockItemSyntax) -> ASTNode {
-    generate(choices: node.item)
+    switch node.item {
+    case .decl(let node):
+      return .decl(self.generate(decl: node))
+    case .stmt(let node):
+      return .stmt(self.generate(stmt: node))
+    case .expr(let node):
+      return .expr(self.generate(expr: node))
+    }
   }
 
   public func generate(arrayElement node: ArrayElementSyntax) -> BridgedExpr {
@@ -275,16 +282,6 @@ extension ASTGenVisitor {
     return self.generate(expr: node)
   }
 
-  /// DO NOT introduce another usage of this. Not all choices can produce 'ASTNode'.
-  @inline(__always)
-  func generate(optional node: (some SyntaxChildChoices)?) -> ASTNode? {
-    guard let node else {
-      return nil
-    }
-
-    return self.generate(choices: node)
-  }
-
   @inline(__always)
   func generate(optional node: GenericParameterClauseSyntax?) -> BridgedGenericParamList? {
     guard let node else {
@@ -329,6 +326,15 @@ extension ASTGenVisitor {
 
     return self.generate(precedenceGroupNameList: node)
   }
+
+  /// Generate AST from an optional node.
+  /// This is `node.map(body)` but with `self.generate(...)` syntax.
+  /// Usage:
+  ///   self.generate(optional: node.initializer, generate(expr:))
+  @inline(__always)
+  func generate<T: SyntaxProtocol, U>(optional node: T?, _ body: (T) -> U) -> U? {
+    return node.map(body)
+  }
 }
 
 extension Collection {

lib/ASTGen/Sources/ASTGen/Stmts.swift

@@ -75,7 +75,14 @@ extension ASTGenVisitor {
       condition: conditions.first!.castToExpr,
       thenStmt: self.generate(codeBlock: node.body).asStmt,
       elseLoc: node.elseKeyword.bridgedSourceLoc(in: self),
-      elseStmt: (self.generate(optional: node.elseBody)?.castToStmt).asNullable
+      elseStmt: self.generate(optional: node.elseBody) {
+        switch $0 {
+        case .codeBlock(let node):
+          return self.generate(codeBlock: node).asStmt
+        case .ifExpr(let node):
+          return self.makeIfStmt(node).asStmt
+        }
+      }.asNullable
     )
   }
 

test/ASTGen/verify-parse.swift

@@ -76,8 +76,8 @@ func test5(_ b: Swift.Bool) -> Int {
   return if b { 0 } else { 1 }
 }
 
-func test6(_ b: Bool) -> Int {
-  let x = if b { 0 } else { 1 }
+func test6(_ b1: Bool, b2: Bool) -> Int {
+  let x = if b1 { 0 } else if b2 { 1 } else { 2 }
   return x
 }