Parser: Accept @cdecl with an indentifier for the C name

Begin accepting the attribute in the form of `@cdecl(cName)`, using an
identifier instead of a string.

For ease of landing this change we still accept the string form. We
should stop accepting it before making this feature available in
production.

include/swift/AST/DiagnosticsParse.def

@@ -1532,6 +1532,9 @@ ERROR(attr_expected_comma,none,
 ERROR(attr_expected_string_literal,none,
 "expected string literal in '%0' attribute", (StringRef))
 
+ERROR(attr_expected_cname,none,
+      "expected C identifier in '%0' attribute", (StringRef))
+
 ERROR(attr_expected_option_such_as,none,
       "expected '%0' option such as '%1'", (StringRef, StringRef))
 

lib/Parse/ParseDecl.cpp

@@ -3014,7 +3014,45 @@ ParserStatus Parser::parseNewDeclAttribute(DeclAttributes &Attributes,
     break;
   }
 
-  case DeclAttrKind::CDecl:
+  case DeclAttrKind::CDecl: {
+    if (!AttrName.starts_with("_") &&
+
+        // Backwards support for @cdecl("stringId"). Remove before enabling in
+        // production so we accept only the identifier format.
+        lookahead<bool>(1, [&](CancellableBacktrackingScope &) {
+           return Tok.isNot(tok::string_literal);
+        })) {
+
+      std::optional<StringRef> CName;
+      if (consumeIfAttributeLParen()) {
+        // Custom C name.
+        if (Tok.isNot(tok::identifier)) {
+          diagnose(Loc, diag::attr_expected_cname, AttrName);
+          return makeParserSuccess();
+        }
+
+        CName = Tok.getText();
+        consumeToken(tok::identifier);
+        AttrRange = SourceRange(Loc, Tok.getRange().getStart());
+
+        if (!consumeIf(tok::r_paren)) {
+          diagnose(Loc, diag::attr_expected_rparen, AttrName,
+                   DeclAttribute::isDeclModifier(DK));
+          return makeParserSuccess();
+        }
+      } else {
+        AttrRange = SourceRange(Loc);
+      }
+
+      Attributes.add(new (Context) CDeclAttr(CName.value_or(StringRef()), AtLoc,
+                                             AttrRange, /*Implicit=*/false,
+                                             /*isUnderscored*/false));
+      break;
+    }
+
+    // Leave legacy @_cdecls to the logic expecting a string.
+    LLVM_FALLTHROUGH;
+  }
   case DeclAttrKind::Expose:
   case DeclAttrKind::SILGenName: {
     if (!consumeIfAttributeLParen()) {

test/PrintAsObjC/cdecl-official.swift

@@ -28,8 +28,8 @@
 // CHECK: #endif
 
 /// My documentation
-@cdecl("simple")
+@cdecl(simple)
-func a_simple(x: Int, bar y: Int) -> Int { return x }
+func a0_simple(x: Int, bar y: Int) -> Int { return x }
 // CHECK-LABEL: // My documentation
 // CHECK-LABEL: SWIFT_EXTERN ptrdiff_t simple(ptrdiff_t x, ptrdiff_t y) SWIFT_NOEXCEPT SWIFT_WARN_UNUSED_RESULT;
 

test/attr/attr_cdecl_official.swift

@@ -6,7 +6,23 @@
 
 // REQUIRES: swift_feature_CDecl
 
-@cdecl("cdecl_foo") func foo(x: Int) -> Int { return x }
+@cdecl(cdecl_foo) func foo(x: Int) -> Int { return x }
+
+@cdecl(not an identifier) func invalidName() {}
+// expected-error @-1 {{expected ')' in 'cdecl' attribute}}
+// expected-error @-2 {{expected declaration}}
+
+@cdecl() func emptyParen() {}
+// expected-error @-1 {{expected C identifier in 'cdecl' attribute}}
+// expected-error @-2 {{expected declaration}}
+
+@cdecl(42) func aNumber() {}
+// expected-error @-1 {{expected C identifier in 'cdecl' attribute}}
+// expected-error @-2 {{expected declaration}}
+
+@cdecl(a:selector:) func selectordName() {}
+// expected-error @-1 {{expected ')' in 'cdecl' attribute}}
+// expected-error @-2 {{expected declaration}}
 
 @cdecl("") // expected-error{{@cdecl symbol name cannot be empty}}
 func emptyName(x: Int) -> Int { return x }