Re-land [clangd] Add support for missing includes analysis.

This reverts commit 2eb5ac9.

clang-tools-extra/clangd/Config.h

@@ -88,11 +88,12 @@ struct Config {
     bool StandardLibrary = true;
   } Index;
 
-  enum class UnusedIncludesPolicy {
-    /// Diagnose unused includes.
+  enum class IncludesPolicy {
+    /// Diagnose missing and unused includes.
     Strict,
     None,
-    /// The same as Strict, but using the include-cleaner library.
+    /// The same as Strict, but using the include-cleaner library for
+    /// unused includes.
     Experiment,
   };
   /// Controls warnings and errors when parsing code.
@@ -107,11 +108,12 @@ struct Config {
       llvm::StringMap<std::string> CheckOptions;
     } ClangTidy;
 
-    UnusedIncludesPolicy UnusedIncludes = UnusedIncludesPolicy::None;
-
     /// Enable emitting diagnostics using stale preambles.
     bool AllowStalePreamble = false;
 
+    IncludesPolicy UnusedIncludes = IncludesPolicy::None;
+    IncludesPolicy MissingIncludes = IncludesPolicy::None;
+
     /// IncludeCleaner will not diagnose usages of these headers matched by
     /// these regexes.
     struct {

clang-tools-extra/clangd/ConfigCompile.cpp

@@ -431,23 +431,33 @@ struct FragmentCompiler {
           });
 
     if (F.UnusedIncludes)
-      if (auto Val =
-              compileEnum<Config::UnusedIncludesPolicy>("UnusedIncludes",
-                                                        **F.UnusedIncludes)
-                  .map("Strict", Config::UnusedIncludesPolicy::Strict)
-                  .map("Experiment", Config::UnusedIncludesPolicy::Experiment)
-                  .map("None", Config::UnusedIncludesPolicy::None)
-                  .value())
+      if (auto Val = compileEnum<Config::IncludesPolicy>("UnusedIncludes",
+                                                         **F.UnusedIncludes)
+                         .map("Strict", Config::IncludesPolicy::Strict)
+                         .map("Experiment", Config::IncludesPolicy::Experiment)
+                         .map("None", Config::IncludesPolicy::None)
+                         .value())
         Out.Apply.push_back([Val](const Params &, Config &C) {
           C.Diagnostics.UnusedIncludes = *Val;
         });
+
     if (F.AllowStalePreamble) {
       if (auto Val = F.AllowStalePreamble)
         Out.Apply.push_back([Val](const Params &, Config &C) {
           C.Diagnostics.AllowStalePreamble = **Val;
         });
     }
 
+    if (F.MissingIncludes)
+      if (auto Val = compileEnum<Config::IncludesPolicy>("MissingIncludes",
+                                                         **F.MissingIncludes)
+                         .map("Strict", Config::IncludesPolicy::Strict)
+                         .map("None", Config::IncludesPolicy::None)
+                         .value())
+        Out.Apply.push_back([Val](const Params &, Config &C) {
+          C.Diagnostics.MissingIncludes = *Val;
+        });
+
     compile(std::move(F.Includes));
     compile(std::move(F.ClangTidy));
   }

clang-tools-extra/clangd/ConfigFragment.h

@@ -221,7 +221,7 @@ struct Fragment {
     /// This often has other advantages, such as skipping some analysis.
     std::vector<Located<std::string>> Suppress;
 
-    /// Controls how clangd will correct "unnecessary #include directives.
+    /// Controls how clangd will correct "unnecessary" #include directives.
     /// clangd can warn if a header is `#include`d but not used, and suggest
     /// removing it.
     //
@@ -236,9 +236,23 @@ struct Fragment {
     /// - None
     std::optional<Located<std::string>> UnusedIncludes;
 
+
     /// Enable emitting diagnostics using stale preambles.
     std::optional<Located<bool>> AllowStalePreamble;
 
+    /// Controls if clangd should analyze missing #include directives.
+    /// clangd will warn if no header providing a symbol is `#include`d
+    /// (missing) directly, and suggest adding it.
+    ///
+    /// Strict means a header providing a symbol is missing if it is not
+    /// *directly #include'd. The file might still compile if the header is
+    /// included transitively.
+    ///
+    /// Valid values are:
+    /// - Strict
+    /// - None
+    std::optional<Located<std::string>> MissingIncludes;
+
     /// Controls IncludeCleaner diagnostics.
     struct IncludesBlock {
       /// Regexes that will be used to avoid diagnosing certain includes as

clang-tools-extra/clangd/ConfigYAML.cpp

@@ -128,6 +128,9 @@ class Parser {
     Dict.handle("UnusedIncludes", [&](Node &N) {
       F.UnusedIncludes = scalarValue(N, "UnusedIncludes");
     });
+    Dict.handle("MissingIncludes", [&](Node &N) {
+      F.MissingIncludes = scalarValue(N, "MissingIncludes");
+    });
     Dict.handle("Includes", [&](Node &N) { parse(F.Includes, N); });
     Dict.handle("ClangTidy", [&](Node &N) { parse(F.ClangTidy, N); });
     Dict.handle("AllowStalePreamble", [&](Node &N) {