[Frontend] Add -warn-on-editor-placeholder

This hidden frontend option lets us be more lax
when type-checking in the presence of editor
placeholders by treating them as holes during
constraint solving.

Author: hamishknight

include/swift/Basic/LangOptions.h

@@ -128,6 +128,9 @@ namespace swift {
     /// Should potential unavailability on enum cases be downgraded to a warning?
     bool WarnOnPotentiallyUnavailableEnumCase = false;
 
+    /// Should the editor placeholder error be downgraded to a warning?
+    bool WarnOnEditorPlaceholder = false;
+
     /// Maximum number of typo corrections we are allowed to perform.
     /// This is disabled by default until we can get typo-correction working within acceptable performance bounds.
     unsigned TypoCorrectionLimit = 0;

include/swift/Option/FrontendOptions.td

@@ -436,6 +436,10 @@ def warn_on_potentially_unavailable_enum_case : Flag<["-"],
   "warn-on-potentially-unavailable-enum-case">,
   HelpText<"Downgrade potential unavailability of enum case to a warning">;
 
+def warn_on_editor_placeholder : Flag<["-"],
+  "warn-on-editor-placeholder">,
+  HelpText<"Downgrade the editor placeholder error to a warning">;
+
 def report_errors_to_debugger : Flag<["-"], "report-errors-to-debugger">,
   HelpText<"Deprecated, will be removed in future versions.">;
 

lib/Frontend/CompilerInvocation.cpp

@@ -426,6 +426,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.WarnOnPotentiallyUnavailableEnumCase |=
       Args.hasArg(OPT_warn_on_potentially_unavailable_enum_case);
+  Opts.WarnOnEditorPlaceholder |= Args.hasArg(OPT_warn_on_editor_placeholder);
+
   if (auto A = Args.getLastArg(OPT_enable_access_control,
                                OPT_disable_access_control)) {
     Opts.EnableAccessControl

lib/Parse/Lexer.cpp

@@ -2145,9 +2145,10 @@ void Lexer::tryLexEditorPlaceholder() {
     if (Ptr[0] == '<' && Ptr[1] == '#')
       break;
     if (Ptr[0] == '#' && Ptr[1] == '>') {
-      // Found it. Flag it as error (or warning, if in playground mode) for the
-      // rest of the compiler pipeline and lex it as an identifier.
-      if (LangOpts.Playground) {
+      // Found it. Flag it as error (or warning, if in playground mode or we've
+      // been asked to warn) for the rest of the compiler pipeline and lex it
+      // as an identifier.
+      if (LangOpts.Playground || LangOpts.WarnOnEditorPlaceholder) {
         diagnose(TokStart, diag::lex_editor_placeholder_in_playground);
       } else {
         diagnose(TokStart, diag::lex_editor_placeholder);