feat(completion): mark deprecated symbols with CompletionItemTag

Check CXAvailability_Deprecated on CodeCompletionResult and set
CompletionItemTag::Deprecated on the CompletionItem. Editors render
this as a strikethrough on the completion label.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 16bit-ykiko

src/feature/code_completion.cpp

@@ -240,7 +240,8 @@ class CodeCompletionCollector final : public clang::CodeCompleteConsumer {
                            llvm::StringRef insert_text,
                            llvm::StringRef overload_key,
                            llvm::StringRef signature = {},
-                           llvm::StringRef return_type = {}) {
+                           llvm::StringRef return_type = {},
+                           bool is_deprecated = false) {
             if(label.empty()) {
                 return;
             }
@@ -271,6 +272,9 @@ class CodeCompletionCollector final : public clang::CodeCompleteConsumer {
                         }
                         item.label_details = std::move(details);
                     }
+                    if(is_deprecated) {
+                        item.tags = std::vector{protocol::CompletionItemTag::Deprecated};
+                    }
                     overloads.push_back({
                         .item = std::move(item),
                         .score = *score,
@@ -299,6 +303,9 @@ class CodeCompletionCollector final : public clang::CodeCompleteConsumer {
                 }
                 item.label_details = std::move(details);
             }
+            if(is_deprecated) {
+                item.tags = std::vector{protocol::CompletionItemTag::Deprecated};
+            }
             collected.push_back(std::move(item));
         };
 
@@ -355,7 +362,14 @@ class CodeCompletionCollector final : public clang::CodeCompleteConsumer {
                         return_type = extract_return_type(*ccs);
                     }
 
-                    try_add(label, kind, label, qualified_name.str(), signature, return_type);
+                    bool deprecated = candidate.Availability == CXAvailability_Deprecated;
+                    try_add(label,
+                            kind,
+                            label,
+                            qualified_name.str(),
+                            signature,
+                            return_type,
+                            deprecated);
                     break;
                 }
             }

tests/unit/feature/code_completion_tests.cpp

@@ -192,6 +192,33 @@ void bar() {
     ASSERT_EQ(*it->kind, protocol::CompletionItemKind::Class);
 }
 
+TEST_CASE(DeprecatedTag) {
+    code_complete(R"cpp(
+[[deprecated]] int foooo(int x);
+int z = fo$(pos)
+)cpp");
+
+    auto it = find_item("foooo");
+    ASSERT_TRUE(it != items.end());
+    ASSERT_TRUE(it->tags.has_value());
+    auto& tags = *it->tags;
+    ASSERT_TRUE(std::ranges::find(tags, protocol::CompletionItemTag::Deprecated) != tags.end());
+}
+
+TEST_CASE(NotDeprecated) {
+    code_complete(R"cpp(
+int foooo(int x);
+int z = fo$(pos)
+)cpp");
+
+    auto it = find_item("foooo");
+    ASSERT_TRUE(it != items.end());
+    // Non-deprecated should have no Deprecated tag.
+    ASSERT_TRUE(!it->tags.has_value() ||
+                std::ranges::find(*it->tags, protocol::CompletionItemTag::Deprecated) ==
+                    it->tags->end());
+}
+
 TEST_CASE(NoBundleOverloads) {
     feature::CodeCompletionOptions opts;
     opts.bundle_overloads = false;