[cxx-interop] Fixit: std.x -> CxxStdlib.x.

Author: zoecarver

include/swift/AST/DiagnosticsSema.def

@@ -764,6 +764,8 @@ ERROR(expect_compile_time_const,none,
 
 ERROR(sema_no_import,Fatal,
       "no such module '%0'", (StringRef))
+NOTE(did_you_mean_cxxstdlib,none,
+      "did you mean 'CxxStdlib'?'", ())
 ERROR(sema_no_import_target,Fatal,
       "could not find module '%0' for target '%1'; "
       "found: %2, at: %3", (StringRef, StringRef, StringRef, StringRef))

lib/Sema/ImportResolution.cpp

@@ -420,6 +420,15 @@ UnboundImport::getTopLevelModule(ModuleDecl *M, SourceFile &SF) {
 // MARK: Implicit imports
 //===----------------------------------------------------------------------===//
 
+static void tryStdlibFixit(ASTContext &ctx,
+                           StringRef moduleName,
+                           SourceLoc loc) {
+  if (moduleName.startswith("std")) {
+    ctx.Diags.diagnose(loc, diag::did_you_mean_cxxstdlib)
+      .fixItReplaceChars(loc, loc.getAdvancedLoc(3), "CxxStdlib");
+  }
+}
+
 static void diagnoseNoSuchModule(ModuleDecl *importingModule,
                                  SourceLoc importLoc,
                                  ImportPath::Module modulePath,
@@ -438,6 +447,7 @@ static void diagnoseNoSuchModule(ModuleDecl *importingModule,
     if (nonfatalInREPL && ctx.LangOpts.DebuggerSupport)
       diagKind = diag::sema_no_import_repl;
     ctx.Diags.diagnose(importLoc, diagKind, modulePathStr);
+    tryStdlibFixit(ctx, modulePathStr, importLoc);
   }
 
   if (ctx.SearchPathOpts.getSDKPath().empty() &&