[clang] [clang][DependencyScanning] Logging Dependency Scanning Events (PR #195896)

[Qiongsi Wu via cfe-commits]

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@@ -71,14 +75,19 @@ bool InMemoryModuleCache::tryToDropPCM(llvm::StringRef Filename) {
   auto &PCM = I->second;
   assert(PCM.Buffer && "PCM to remove is scheduled to be built...");
 
-  if (PCM.IsFinal)
+  if (PCM.IsFinal) {
+    Logger.log() << "pcm_not_dropped: " << Filename;
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qiongsiwu wrote:

I think the reason we are not reporting the error here is that we have lost the exact information to report detailed diagnostics here (most of the information is in, say `readControlBlocks`). I think it is reasonable to not diagnose anything here, and diagnose closer to the root cause of such issues.

I went through all the cases that I can find where a finalized pcm cannot be dropped. We have good diagnostics for most of them. I found one missing case that is fixed by https://github.com/llvm/llvm-project/pull/209857. 

https://github.com/llvm/llvm-project/pull/195896