[clangd] Add some logging to explain why textual fallback navigation failed

[nridge]

Summary

Depends on D76451.

[nridge]

Applies on top of D76451.

[sammccall]

This seems fine-grained even for vlog. Is this for debugging, can it be dlog()? (i.e. available in asserts-enabled builds behind -debug or -debug-only=XRefs.cpp)

[nridge]

The intent is to be able to run a production build on a production codebase and see why a particular instance of navigation failed. However, it's not super important that this land; if you don't think it's useful for others, I can just run with it applied locally.

[nridge]

Abandoning as per discussion. Running this locally has led me to propose D90134.

clang-tools-extra/clangd/XRefs.cpp

 }
 
 } // namespace
 
 std::vector<LocatedSymbol>
 locateSymbolTextually(const SpelledWord &Word, ParsedAST &AST,
                       const SymbolIndex *Index, const std::string &MainFilePath,
                       ASTNodeKind NodeKind) {
+  if (!Index)
+    return {};
+
   // Don't use heuristics if this is a real identifier, or not an
   // identifier.
   // Exception: dependent names, because those may have useful textual
   // matches that AST-based heuristics cannot find.
-  if ((Word.ExpandedToken && !isDependentName(NodeKind)) ||
-      !Word.LikelyIdentifier || !Index)
+  if (Word.ExpandedToken && !isDependentName(NodeKind)) {
+    vlog("Textual navigation: word {0} under cursor is a real token and not "
+         "dependent",
+         Word.Text);
     return {};
+  }
+  if (!Word.LikelyIdentifier) {
+    vlog("Textual navigation: word {0} under cursor is unlikely to be an "
+         "identifier",
+         Word.Text);
+    return {};
+  }
   // We don't want to handle words in string literals. It'd be nice to whitelist
   // comments instead, but they're not retained in TokenBuffer.
   if (Word.PartOfSpelledToken &&
       isStringLiteral(Word.PartOfSpelledToken->kind()))
     return {};
 
   const auto &SM = AST.getSourceManager();
   // Look up the selected word in the index.
if (Sym.Definition) {
       }
       DefLoc = *MaybeDefLoc;
     }
 
     if (ScoredResults.size() >= 3) {
       // If we have more than 3 results, don't return anything,
       // as confidence is too low.
       // FIXME: Alternatively, try a stricter query?
+      vlog("Textual navigation: more than 3 matching index results, confidence "
+           "is too low");
       TooMany = true;
       return;
     }
 
     LocatedSymbol Located;
     Located.Name = (Sym.Name + Sym.TemplateSpecializationArgs).str();
     Located.PreferredDeclaration = bool(Sym.Definition) ? DefLoc : DeclLoc;
     Located.Definition = DefLoc;