Handle decl modifier keywords in isStartOfSwiftDecl

Previously we would always return true if we
encountered a decl modifier keyword, however this
could cause us to incorrectly consider SIL's
usage of such keywords, e.g 'private', to be the
start of a Swift decl.

Adjust the logic to check the next token for the
start of a Swift decl.

Author: hamishknight

lib/Parse/ParseDecl.cpp

@@ -3350,6 +3350,35 @@ bool Parser::isStartOfSwiftDecl() {
     return isStartOfSwiftDecl();
   }
 
+  // If we have a decl modifying keyword, check if the next token is a valid
+  // decl start. This is necessary to correctly handle Swift keywords that are
+  // shared by SIL, e.g 'private' in 'sil private @foo :'. We need to make sure
+  // this isn't considered a valid Swift decl start.
+  if (Tok.isKeyword()) {
+    auto DAK = DeclAttribute::getAttrKindFromString(Tok.getText());
+    if (DAK != DAK_Count && DeclAttribute::isDeclModifier(DAK)) {
+      BacktrackingScope backtrack(*this);
+      consumeToken();
+
+      // Eat paren after modifier name; e.g. private(set)
+      if (consumeIf(tok::l_paren)) {
+        while (Tok.isNot(tok::r_brace, tok::eof, tok::pound_endif)) {
+          if (consumeIf(tok::r_paren))
+            break;
+
+          // If we found the start of a decl while trying to skip over the
+          // paren, then we have something incomplete like 'private('. Return
+          // true for better recovery.
+          if (isStartOfSwiftDecl())
+            return true;
+
+          skipSingle();
+        }
+      }
+      return isStartOfSwiftDecl();
+    }
+  }
+
   // Otherwise, the only hard case left is the identifier case.
   if (Tok.isNot(tok::identifier)) return true;
 

test/Parse/identifiers.swift

@@ -32,9 +32,9 @@ func <#some name#>() {} // expected-error {{editor placeholder in source file}}
 class switch {} // expected-error {{keyword 'switch' cannot be used as an identifier here}} expected-note {{if this name is unavoidable, use backticks to escape it}} {{7-13=`switch`}}
 struct Self {} // expected-error {{keyword 'Self' cannot be used as an identifier here}} expected-note {{if this name is unavoidable, use backticks to escape it}} {{8-12=`Self`}}
 protocol enum {} // expected-error {{keyword 'enum' cannot be used as an identifier here}} expected-note {{if this name is unavoidable, use backticks to escape it}} {{10-14=`enum`}}
-protocol test { // expected-note{{in declaration of 'test'}}
-  associatedtype public // expected-error {{keyword 'public' cannot be used as an identifier here}} expected-note {{if this name is unavoidable, use backticks to escape it}} {{18-24=`public`}} expected-error {{consecutive declarations on a line must be separated by ';'}}
-} // expected-error{{expected declaration}}
+protocol test {
+  associatedtype public // expected-error {{keyword 'public' cannot be used as an identifier here}} expected-note {{if this name is unavoidable, use backticks to escape it}} {{18-24=`public`}}
+}
 func _(_ x: Int) {} // expected-error {{keyword '_' cannot be used as an identifier here}} // expected-note {{if this name is unavoidable, use backticks to escape it}} {{6-7=`_`}}
 
 // SIL keywords are tokenized as normal identifiers in non-SIL mode.