[Caching] Do not infer default on disk cas path when parsing options

Currently, swift-frontend will always try to infer a default CASPath if
no `-cas-path` is passed. The function `getDefaultOnDiskCASPath` called
to infer the default path can fatal error in some environment where no
caching directory and home directory, no matter if caching is used or
not.

Remove the inferred path during parsing since that is not strictly
necessary. If caching is enabled and no CASPath is passed, error can be
issued during CAS construction time.

rdar://158899187

Author: cachemeifyoucan

lib/Frontend/CompilerInvocation.cpp

@@ -778,8 +778,6 @@ static bool ParseCASArgs(CASOptions &Opts, ArgList &Args,
   Opts.CacheSkipReplay |= Args.hasArg(OPT_cache_disable_replay);
   if (const Arg *A = Args.getLastArg(OPT_cas_path))
     Opts.CASOpts.CASPath = A->getValue();
-  else if (Opts.CASOpts.CASPath.empty())
-    Opts.CASOpts.CASPath = llvm::cas::getDefaultOnDiskCASPath();
 
   if (const Arg *A = Args.getLastArg(OPT_cas_plugin_path))
     Opts.CASOpts.PluginPath = A->getValue();

lib/Frontend/Frontend.cpp

@@ -466,6 +466,11 @@ bool CompilerInstance::setupCASIfNeeded(ArrayRef<const char *> Args) {
     return false;
 
   const auto &Opts = getInvocation().getCASOptions();
+  if (Opts.CASOpts.CASPath.empty() && Opts.CASOpts.PluginPath.empty()) {
+    Diagnostics.diagnose(SourceLoc(), diag::error_cas_initialization,
+                         "no CAS options provided");
+    return true;
+  }
   auto MaybeDB = Opts.CASOpts.getOrCreateDatabases();
   if (!MaybeDB) {
     Diagnostics.diagnose(SourceLoc(), diag::error_cas_initialization,

test/CAS/cas_output_backend.swift

@@ -13,6 +13,9 @@
 // RUN: echo "\"-disable-implicit-concurrency-module-import\"" >> %t/MyApp.cmd
 // RUN: echo "\"-parse-stdlib\"" >> %t/MyApp.cmd
 
+// RUN: not %target-swift-frontend -c -cache-compile-job %s -o %t/test.o @%t/MyApp.cmd 2>&1 | %FileCheck %s --check-prefix=NOT-CONFIG
+// NOT-CONFIG: error: CAS cannot be initialized from the specified '-cas-*' options: no CAS options provided
+
 // RUN: %target-swift-frontend -c -cache-compile-job -cas-path %t/cas %s -o %t/test.o @%t/MyApp.cmd
 // RUN: %cache-tool -cas-path %t/cas -cache-tool-action print-output-keys -- \
 // RUN:   %target-swift-frontend -c -cache-compile-job -cas-path %t/cas %s -o %t/test.o @%t/MyApp.cmd > %t/cache_key.json