[Parser] Go back to parsing macro expansions as expressions where possible

The change to eagerly parsing macro expansions as declarations rather
than expressions caused a regression for expressions that start with a
macro expansion, e.g., `#fileID == "Module/filename.swift"`. Rather
than perform extensive lookahead to disambiguate, go back to parsing
as a macro expansion expression except in places where only a
declaration is allowed (e.g., a member list).

To accommodate this for freestanding macro implementations, generalize
the `CodeItemMacro` and `DeclarationMacro` expansion operations to
deal with an arbitrary `FreestandingMacroExpansionSyntax`, as we had
already done for `ExpressionMacro`.

Fixes #1292.

Author: DougGregor

Sources/SwiftParser/Declarations.swift

@@ -32,13 +32,12 @@ extension TokenConsumer {
   mutating func atStartOfDeclaration(
     isAtTopLevel: Bool = false,
     allowInitDecl: Bool = true,
-    allowRecovery: Bool = false,
-    preferPoundAsExpression: Bool = false
+    allowRecovery: Bool = false
   ) -> Bool {
     if self.at(anyIn: PoundDeclarationStart.self) != nil {
-      // If we are in a context where we prefer # to be an expression,
-      // treat it as one if it's not at the start of the line.
-      if preferPoundAsExpression && self.at(.pound) && !self.currentToken.isAtStartOfLine {
+      // Don't treat freestanding macro expansions as declarations. They'll be
+      // parsed as expressions.
+      if self.at(.pound) {
         return false
       }
 

Sources/SwiftParser/Statements.swift

@@ -959,7 +959,7 @@ extension Parser {
       .poundIfKeyword, .poundEndifKeyword, .poundElseKeyword,
       .poundElseifKeyword,
     ])
-      && !self.atStartOfStatement() && !self.atStartOfDeclaration(preferPoundAsExpression: true)
+      && !self.atStartOfStatement() && !self.atStartOfDeclaration()
     {
       let parsedExpr = self.parseExpression()
       if hasMisplacedTry && !parsedExpr.is(RawTryExprSyntax.self) {

Sources/SwiftSyntaxMacros/MacroProtocols/CodeItemMacro.swift

@@ -13,8 +13,11 @@ import SwiftSyntax
 public protocol CodeItemMacro: FreestandingMacro {
   /// Expand a macro described by the given freestanding macro expansion
   /// declaration within the given context to produce a set of declarations.
-  static func expansion<Context: MacroExpansionContext>(
-    of node: MacroExpansionDeclSyntax,
+  static func expansion<
+    Node: FreestandingMacroExpansionSyntax,
+    Context: MacroExpansionContext
+  >(
+    of node: Node,
     in context: Context
   ) throws -> [CodeBlockItemSyntax]
 }

Sources/SwiftSyntaxMacros/MacroProtocols/DeclarationMacro.swift

@@ -14,9 +14,10 @@ public protocol DeclarationMacro: FreestandingMacro {
   /// Expand a macro described by the given freestanding macro expansion
   /// declaration within the given context to produce a set of declarations.
   static func expansion<
+    Node: FreestandingMacroExpansionSyntax,
     Context: MacroExpansionContext
   >(
-    of node: MacroExpansionDeclSyntax,
+    of node: Node,
     in context: Context
   ) throws -> [DeclSyntax]
 }

Sources/SwiftSyntaxMacros/MacroSystem.swift

@@ -127,14 +127,14 @@ class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
     for item in node {
       // Expand declaration macros that were parsed as macro expansion
       // expressions in this context.
-      if case let .decl(declItem) = item.item,
-        let declExpansion = declItem.as(MacroExpansionDeclSyntax.self),
-        let macro = macroSystem.macros[declExpansion.macro.text]
+      if case let .expr(exprItem) = item.item,
+        let exprExpansion = exprItem.as(MacroExpansionExprSyntax.self),
+        let macro = macroSystem.macros[exprExpansion.macro.text]
       {
         do {
           if let macro = macro as? DeclarationMacro.Type {
             let expandedItemList = try macro.expansion(
-              of: declExpansion,
+              of: exprExpansion,
               in: context
             )
             newItems.append(
@@ -144,7 +144,7 @@ class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
             )
           } else if let macro = macro as? ExpressionMacro.Type {
             let expandedExpr = try macro.expansion(
-              of: declExpansion,
+              of: exprExpansion,
               in: context
             )
             newItems.append(CodeBlockItemSyntax(item: .init(expandedExpr)))

Tests/SwiftParserTest/ExpressionTests.swift

@@ -749,7 +749,7 @@ final class ExpressionTests: XCTestCase {
       "#keyPath((b:1️⃣)2️⃣",
       diagnostics: [
         DiagnosticSpec(locationMarker: "1️⃣", message: "expected value in tuple"),
-        DiagnosticSpec(locationMarker: "2️⃣", message: "expected ')' to end macro expansion"),
+        DiagnosticSpec(locationMarker: "2️⃣", message: "expected ')' to end macro expansion expression"),
       ]
     )
   }
@@ -896,6 +896,10 @@ final class ExpressionTests: XCTestCase {
   }
 
   func testMacroExpansionExpression() {
+    AssertParse(
+      #"#file == $0.path"#
+    )
+
     AssertParse(
       #"let a = #embed("filename.txt")"#
     )

Tests/SwiftParserTest/translated/MultilinePoundDiagnosticArgRdar41154797Tests.swift

@@ -22,7 +22,7 @@ final class MultilinePoundDiagnosticArgRdar41154797Tests: XCTestCase {
       """##,
       diagnostics: [
         DiagnosticSpec(message: #"expected '"""' to end string literal"#),
-        DiagnosticSpec(message: "expected ')' to end macro expansion"),
+        DiagnosticSpec(message: "expected ')' to end macro expansion expression"),
       ]
     )
   }

Tests/SwiftParserTest/translated/PoundAssertTests.swift

@@ -49,7 +49,7 @@ final class PoundAssertTests: XCTestCase {
       #assert(1️⃣, "error message")
       """#,
       diagnostics: [
-        DiagnosticSpec(message: "expected value in macro expansion")
+        DiagnosticSpec(message: "expected value in macro expansion expression")
       ]
     )
   }
@@ -63,7 +63,7 @@ final class PoundAssertTests: XCTestCase {
       """,
       diagnostics: [
         DiagnosticSpec(
-          message: "expected ')' to end macro expansion",
+          message: "expected ')' to end macro expansion expression",
           notes: [
             NoteSpec(message: "to match this opening '('")
           ]
@@ -81,7 +81,7 @@ final class PoundAssertTests: XCTestCase {
       """#,
       diagnostics: [
         DiagnosticSpec(
-          message: "expected ')' to end macro expansion",
+          message: "expected ')' to end macro expansion expression",
           notes: [
             NoteSpec(message: "to match this opening '('")
           ]

Tests/SwiftSyntaxMacrosTest/MacroSystemTests.swift

@@ -198,8 +198,11 @@ extension SimpleDiagnosticMessage: FixItMessage {
 }
 
 public struct ErrorMacro: DeclarationMacro {
-  public static func expansion<Context: MacroExpansionContext>(
-    of node: MacroExpansionDeclSyntax,
+  public static func expansion<
+    Node: FreestandingMacroExpansionSyntax,
+    Context: MacroExpansionContext
+  >(
+    of node: Node,
     in context: Context
   ) throws -> [DeclSyntax] {
     guard let firstElement = node.argumentList.first,
@@ -227,8 +230,11 @@ public struct ErrorMacro: DeclarationMacro {
 }
 
 struct DefineBitwidthNumberedStructsMacro: DeclarationMacro {
-  static func expansion<Context: MacroExpansionContext>(
-    of node: MacroExpansionDeclSyntax,
+  static func expansion<
+    Node: FreestandingMacroExpansionSyntax,
+    Context: MacroExpansionContext
+  >(
+    of node: Node,
     in context: Context
   ) throws -> [DeclSyntax] {
     guard let firstElement = node.argumentList.first,