[clangd] Omit type hints that are too long

[nridge]

clang-tools-extra/clangd/InlayHints.cpp

346

I'm open to suggestions for what the length limit should be, here I made a fairly arbitrary choice.

[nridge]

(The relevant issue is https://github.com/clangd/clangd/issues/824)

[kadircet]

Thanks! I was having some discussions with @sammccall about what we really want here. We think folding the arguments & dropping qualifiers until hint fits under the limit would be the most ideal but there's no rush. A stepping stone would be to just truncate all the template arguments (e.g. tuple<foo<x>, foo<y>> becomes tuple<...> even if tuple<foo<...>, foo<...>> could fit under the limit). Omitting is also fine initially, even though it's gonna feel weird (as if clangd couldn't find any hints) it is still better than extending the line to infinity!

[nridge]

In D108972#2981893, @kadircet wrote:

A stepping stone would be to just truncate all the template arguments (e.g. tuple<foo<x>, foo<y>> becomes tuple<...> even if tuple<foo<...>, foo<...>> could fit under the limit).

I was thinking the same thing :) I will write a patch to do this as a follow-up.

clang-tools-extra/clangd/InlayHints.cpp

Results.push_back(InlayHint{
         Kind, Label.str()});
   }
 
   void addTypeHint(SourceRange R, QualType T, llvm::StringRef Prefix) {
     // Do not print useless "NULL TYPE" hint.
     if (!T.getTypePtrOrNull())
       return;
 
-    addInlayHint(R, InlayHintKind::TypeHint,
-                 std::string(Prefix) + T.getAsString(TypeHintPolicy));
+    std::string TypeName = T.getAsString(TypeHintPolicy);
+    if (TypeName.length() < TypeNameLimit)
+      addInlayHint(R, InlayHintKind::TypeHint, std::string(Prefix) + TypeName);
   }
 
   std::vector<InlayHint> &Results;
   ASTContext &AST;
   FileID MainFileID;
   StringRef MainFileBuf;
   const HeuristicResolver *Resolver;
   PrintingPolicy TypeHintPolicy;
+
+  static const size_t TypeNameLimit = 32;
 };
 
 std::vector<InlayHint> inlayHints(ParsedAST &AST) {
   std::vector<InlayHint> Results;
   InlayHintVisitor Visitor(Results, AST);
   Visitor.TraverseAST(AST.getASTContext());
   return Results;
 }
 
 } // namespace clangd
 } // namespace clang

clang-tools-extra/clangd/unittests/InlayHintTests.cpp

void foo(T arg) {
       // The hint would just be "auto" and we can't do any better.
       auto var1 = arg.method();
       // FIXME: It would be nice to show "T" as the hint.
       auto $var2[[var2]] = arg;
     }
   )cpp");
 }
 
+TEST(TypeHints, LongTypeName) {
+  assertTypeHints(R"cpp(
+    template <typename, typename, typename>
+    struct A {};
+    struct MultipleWords {};
+    A<MultipleWords, MultipleWords, MultipleWords> foo();
+    // Omit type hint past a certain length (currently 32)
+    auto var = foo();
+  )cpp");
+}
+
 // FIXME: Low-hanging fruit where we could omit a type hint:
 //  - auto x = TypeName(...);
 //  - auto x = (TypeName) (...);
 //  - auto x = static_cast<TypeName>(...);  // and other built-in casts
 
 // Annoyances for which a heuristic is not obvious:
 //  - auto x = llvm::dyn_cast<LongTypeName>(y);  // and similar
 //  - stdlib algos return unwieldy __normal_iterator<X*, ...> type
 //    (For this one, perhaps we should omit type hints that start
 //     with a double underscore.)
 
 } // namespace
 } // namespace clangd
 } // namespace clang
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