[Parse] Add parsing for @execution(concurrent | caller) attribute in type context

Author: xedin

include/swift/AST/DiagnosticsParse.def

@@ -1683,6 +1683,14 @@ ERROR(attr_isolated_expected_rparen,none,
 ERROR(attr_isolated_expected_kind,none,
       "expected 'any' as the isolation kind", ())
 
+ERROR(attr_execution_expected_lparen,none,
+      "expected '(' after '@execution'",
+      ())
+ERROR(attr_execution_expected_rparen,none,
+      "expected ')' after execution behavior", ())
+ERROR(attr_execution_expected_kind,none,
+      "expected 'concurrent' or 'caller' as the execution behavior", ())
+
 ERROR(attr_private_import_expected_rparen,none,
       "expected ')' after function name for @_private", ())
 ERROR(attr_private_import_expected_sourcefile, none,

lib/Parse/ParseDecl.cpp

@@ -4788,6 +4788,50 @@ ParserStatus Parser::parseTypeAttribute(TypeOrCustomAttr &result,
     return makeParserSuccess();
   }
 
+  case TypeAttrKind::Execution: {
+    SourceLoc lpLoc = Tok.getLoc(), behaviorLoc, rpLoc;
+    if (!consumeIfNotAtStartOfLine(tok::l_paren)) {
+      if (!justChecking) {
+        diagnose(Tok, diag::attr_execution_expected_lparen);
+        // TODO: should we suggest removing the `@`?
+      }
+      return makeParserError();
+    }
+
+    bool invalid = false;
+    std::optional<ExecutionKind> behavior;
+    if (isIdentifier(Tok, "concurrent")) {
+      behaviorLoc = consumeToken(tok::identifier);
+      behavior = ExecutionKind::Concurrent;
+    } else if (isIdentifier(Tok, "caller")) {
+      behaviorLoc = consumeToken(tok::identifier);
+      behavior = ExecutionKind::Caller;
+    } else {
+      if (!justChecking) {
+        diagnose(Tok, diag::attr_execution_expected_kind);
+      }
+      invalid = true;
+      consumeIf(tok::identifier);
+    }
+
+    if (justChecking && !Tok.is(tok::r_paren))
+      return makeParserError();
+    if (parseMatchingToken(tok::r_paren, rpLoc,
+                           diag::attr_execution_expected_rparen,
+                           lpLoc))
+      return makeParserError();
+
+    if (invalid)
+      return makeParserError();
+    assert(behavior);
+
+    if (!justChecking) {
+      result = new (Context) ExecutionTypeAttr(AtLoc, attrLoc, {lpLoc, rpLoc},
+                                               {*behavior, behaviorLoc});
+    }
+    return makeParserSuccess();
+  }
+
   case TypeAttrKind::Opened: {
     // Parse the opened existential ID string in parens
     SourceLoc beginLoc = Tok.getLoc(), idLoc, endLoc;