Initial implementation of a @_cdecl attribute to export top-level functions to C.

There's an immediate need for this in the core libs, and we have most of the necessary pieces on hand to make it easy to implement. This is an unpolished initial implementation, with the following limitations, among others:

- It doesn't support bridging error conventions,
- It relies on ObjC interop,
- It doesn't check for symbol name collisions,
- It has an underscored name with required symbol name `@cdecl("symbol_name")`, awaiting official bikeshed painting.

Author: jckarter

Diff for: include/swift/AST/Attr.def

@@ -239,6 +239,9 @@ SIMPLE_DECL_ATTR(indirect, Indirect,
 SIMPLE_DECL_ATTR(warn_unqualified_access, WarnUnqualifiedAccess,
                  OnFunc /*| OnVar*/ | LongAttribute, 61)
 
+DECL_ATTR(_cdecl, CDecl,
+          OnFunc | LongAttribute | UserInaccessible, 62)
+
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS
 #undef SIMPLE_DECL_ATTR

Diff for: include/swift/AST/Attr.h

@@ -571,6 +571,24 @@ class SILGenNameAttr : public DeclAttribute {
   }
 };
 
+/// Defines the @_cdecl attribute.
+class CDeclAttr : public DeclAttribute {
+public:
+  CDeclAttr(StringRef Name, SourceLoc AtLoc, SourceRange Range, bool Implicit)
+    : DeclAttribute(DAK_CDecl, AtLoc, Range, Implicit),
+      Name(Name) {}
+
+  CDeclAttr(StringRef Name, bool Implicit)
+    : CDeclAttr(Name, SourceLoc(), SourceRange(), /*Implicit=*/true) {}
+
+  /// The symbol name.
+  const StringRef Name;
+
+  static bool classof(const DeclAttribute *DA) {
+    return DA->getKind() == DAK_CDecl;
+  }
+};
+
 /// Defines the @_semantics attribute.
 class SemanticsAttr : public DeclAttribute {
 public:

Diff for: include/swift/AST/DiagnosticsSema.def

@@ -742,6 +742,14 @@ ERROR(no_objc_tagged_pointer_not_class_protocol,sema_tcd,none,
 ERROR(swift_native_objc_runtime_base_not_on_root_class,sema_tcd,none,
       "@_swift_native_objc_runtime_base_not_on_root_class can only be applied "
       "to root classes", ())
+
+ERROR(cdecl_not_at_top_level,sema_tcd,none,
+      "@_cdecl can only be applied to global functions", ())
+ERROR(cdecl_empty_name,sema_tcd,none,
+      "@_cdecl symbol name cannot be empty", ())
+ERROR(cdecl_throws,sema_tcd,none,
+      "raising errors from @_cdecl functions is not supported", ())
+
 ERROR(attr_methods_only,sema_tcd,none,
       "only methods can be declared %0", (DeclAttribute))
 ERROR(access_control_in_protocol,sema_tcd,none,
@@ -2344,7 +2352,7 @@ ERROR(objc_name_func_mismatch,sema_objc,none,
       (bool, unsigned, bool, unsigned, bool, bool))
 
 // If you change this, also change enum ObjCReason
-#define OBJC_ATTR_SELECT "select{marked dynamic|marked @objc|marked @IBOutlet|marked @NSManaged|a member of an @objc protocol|implicitly @objc|an @objc override}"
+#define OBJC_ATTR_SELECT "select{marked @_cdecl|marked dynamic|marked @objc|marked @IBOutlet|marked @NSManaged|a member of an @objc protocol|implicitly @objc|an @objc override}"
 
 ERROR(objc_invalid_on_var,sema_objc,none,
       "property cannot be %" OBJC_ATTR_SELECT "0 "

Diff for: include/swift/Serialization/ModuleFormat.h

@@ -42,7 +42,7 @@ const unsigned char MODULE_DOC_SIGNATURE[] = { 0xE2, 0x9C, 0xA8, 0x07 };
 
 /// Serialized module format major version number.
 ///
-/// Always 0 for Swift 1.0.
+/// Always 0 for Swift 1.x and 2.x.
 const uint16_t VERSION_MAJOR = 0;
 
 /// Serialized module format minor version number.
@@ -51,7 +51,7 @@ const uint16_t VERSION_MAJOR = 0;
 /// To ensure that two separate changes don't silently get merged into one
 /// in source control, you should also update the comment to briefly
 /// describe what change you made.
-const uint16_t VERSION_MINOR = 222; // Last change: @_fixed_layout
+const uint16_t VERSION_MINOR = 223; // Last change: @_cdecl
 
 using DeclID = Fixnum<31>;
 using DeclIDField = BCFixed<31>;
@@ -1203,6 +1203,13 @@ namespace decls_block {
     BCFixed<1>, // implicit flag
     BCBlob      // _silgen_name
   >;
+
+  using CDeclDeclAttrLayout = BCRecordLayout<
+    CDecl_DECL_ATTR,
+    BCFixed<1>, // implicit flag
+    BCBlob      // _silgen_name
+  >;
+
 
   using AlignmentDeclAttrLayout = BCRecordLayout<
     Alignment_DECL_ATTR,

Diff for: lib/AST/ASTContext.cpp

@@ -1875,7 +1875,10 @@ void AbstractFunctionDecl::setForeignErrorConvention(
 
 Optional<ForeignErrorConvention>
 AbstractFunctionDecl::getForeignErrorConvention() const {
-  if (!isObjC() || !isBodyThrowing()) return None;
+  if (!isObjC() && !getAttrs().hasAttribute<CDeclAttr>())
+    return None;
+  if (!isBodyThrowing())
+    return None;
   auto &conventionsMap = getASTContext().Impl.ForeignErrorConventions;
   auto it = conventionsMap.find(this);
   if (it == conventionsMap.end()) return None;

Diff for: lib/AST/Attr.cpp

@@ -327,6 +327,11 @@ void DeclAttribute::print(ASTPrinter &Printer,
     if (cast<AutoClosureAttr>(this)->isEscaping())
       Printer << "(escaping)";
     break;
+      
+  case DAK_CDecl:
+    Printer << "@_cdecl(\"" << cast<CDeclAttr>(this)->Name << "\")";
+    break;
+
   case DAK_ObjC: {
     if (Options.PrintForSIL && isImplicit())
       break;
@@ -421,6 +426,8 @@ StringRef DeclAttribute::getAttrName() const {
     return "_silgen_name";
   case DAK_Alignment:
     return "_alignment";
+  case DAK_CDecl:
+    return "_cdecl";
   case DAK_SwiftNativeObjCRuntimeBase:
     return "_swift_native_objc_runtime_base";
   case DAK_Semantics:

Diff for: lib/AST/Verifier.cpp

@@ -2249,8 +2249,9 @@ struct ASTNodeBase {};
         abort();
       }
 
-      if (AFD->getForeignErrorConvention() && !AFD->isObjC()) {
-        Out << "foreign error convention on non-@objc function\n";
+      if (AFD->getForeignErrorConvention()
+          && !AFD->isObjC() && !AFD->getAttrs().hasAttribute<CDeclAttr>()) {
+        Out << "foreign error convention on non-@objc, non-@_cdecl function\n";
         AFD->dump(Out);
         abort();
       }

Diff for: lib/IRGen/Linking.cpp

@@ -220,7 +220,7 @@ void LinkEntity::mangle(raw_ostream &buffer) const {
 
   //   entity ::= declaration                     // other declaration
   case Kind::Function:
-    // As a special case, functions can have external asm names.
+    // As a special case, functions can have manually mangled names.
     if (auto AsmA = getDecl()->getAttrs().getAttribute<SILGenNameAttr>()) {
       buffer << AsmA->Name;
       return;

Diff for: lib/Parse/ParseDecl.cpp

@@ -563,6 +563,7 @@ bool Parser::parseNewDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc,
     break;
   }
 
+  case DAK_CDecl:
   case DAK_SILGenName: {
     if (!consumeIf(tok::l_paren)) {
       diagnose(Loc, diag::attr_expected_lparen, AttrName,
@@ -599,9 +600,16 @@ bool Parser::parseNewDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc,
       diagnose(Loc, diag::attr_only_at_non_local_scope, AttrName);
     }
 
-    if (!DiscardAttribute)
-      Attributes.add(new (Context) SILGenNameAttr(AsmName.getValue(), AtLoc,
+    if (!DiscardAttribute) {
+      if (DK == DAK_SILGenName)
+        Attributes.add(new (Context) SILGenNameAttr(AsmName.getValue(), AtLoc,
+                                                AttrRange, /*Implicit=*/false));
+      else if (DK == DAK_CDecl)
+        Attributes.add(new (Context) CDeclAttr(AsmName.getValue(), AtLoc,
                                                AttrRange, /*Implicit=*/false));
+      else
+        llvm_unreachable("out of sync with switch");
+    }
 
     break;
   }