[clang-tools-extra] [clangd] Make the --skip-preamble-build option take an enum (PR #229515)

[Tomáš Slanina via cfe-commits]

https://github.com/itzexpoexpo created https://github.com/llvm/llvm-project/pull/229515

Add a new value `never` allowing the preamble skipping to be force-disabled, in addition to `modules` and `always`.

The default is `modules`, equivalent to the old behavior.

Closes #212403

>From 731b9f48c354ec901455e55412b4496a921504c7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tom=C3=A1=C5=A1=20Slanina?= <itzexpoexpo at gmail.com>
Date: Tue, 6 Oct 2026 20:37:37 +0200
Subject: [PATCH] [clangd] Make the skip-preamble-build option take an enum

Add a new value "never" allowing the preamble skipping
to be force-disabled, in addition to "modules" and "always".
The default is "modules" equivalent to the old behavior.

Closes #212403
---
 clang-tools-extra/clangd/ClangdServer.cpp    | 11 ++++++++++-
 clang-tools-extra/clangd/ClangdServer.h      |  9 ++++++---
 clang-tools-extra/clangd/tool/ClangdMain.cpp | 12 +++++++++---
 3 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/clang-tools-extra/clangd/ClangdServer.cpp b/clang-tools-extra/clangd/ClangdServer.cpp
index d4d313b40d76b7d..3d3a851ef80e814 100644
--- a/clang-tools-extra/clangd/ClangdServer.cpp
+++ b/clang-tools-extra/clangd/ClangdServer.cpp
@@ -1220,7 +1220,16 @@ void ClangdServer::adjustParseInputs(ParseInputs &Inputs, PathRef File) const {
                                     : Context::current().clone());
     return ModulesManager->hasRequiredModules(File);
   };
-  Inputs.Opts.SkipPreambleBuild = SkipPreambleBuild || HasRequiredModules();
+
+  Inputs.Opts.SkipPreambleBuild = [&](){
+    switch(SkipPreambleBuild){
+      using enum SkipPreambleBuildPolicy;
+
+      case Never: return false;
+      case Modules: return HasRequiredModules();
+      case Always: return true;
+    }
+  }();
 }
 
 } // namespace clangd
diff --git a/clang-tools-extra/clangd/ClangdServer.h b/clang-tools-extra/clangd/ClangdServer.h
index 264ab7437c248c3..f519e04cab5880b 100644
--- a/clang-tools-extra/clangd/ClangdServer.h
+++ b/clang-tools-extra/clangd/ClangdServer.h
@@ -98,6 +98,8 @@ class ClangdServer {
   createConfiguredContextProvider(const config::Provider *Provider,
                                   ClangdServer::Callbacks *);
 
+  enum class SkipPreambleBuildPolicy { Never, Modules, Always };
+
   struct Options {
     /// To process requests asynchronously, ClangdServer spawns worker threads.
     /// If this is zero, no threads are spawned. All work is done on the calling
@@ -191,8 +193,9 @@ class ClangdServer {
     // If true, parse emplace-like functions in the preamble.
     bool PreambleParseForwardingFunctions = true;
 
-    // If true, skip preamble build.
-    bool SkipPreambleBuild = false;
+    // If Always, discard preamble during update
+    // If Modules, only discard when modules are required
+    SkipPreambleBuildPolicy SkipPreambleBuild = {};
 
     /// Whether include fixer insertions for Objective-C code should use #import
     /// instead of #include.
@@ -513,7 +516,7 @@ class ClangdServer {
 
   bool PreambleParseForwardingFunctions = true;
 
-  bool SkipPreambleBuild = false;
+  SkipPreambleBuildPolicy SkipPreambleBuild = {};
 
   bool ImportInsertions = false;
 
diff --git a/clang-tools-extra/clangd/tool/ClangdMain.cpp b/clang-tools-extra/clangd/tool/ClangdMain.cpp
index 9dffa3bef8caa2b..9437f8a86bc2a7b 100644
--- a/clang-tools-extra/clangd/tool/ClangdMain.cpp
+++ b/clang-tools-extra/clangd/tool/ClangdMain.cpp
@@ -518,12 +518,18 @@ opt<bool> PreambleParseForwardingFunctions{
     init(ParseOptions().PreambleParseForwardingFunctions),
 };
 
-opt<bool> SkipPreambleBuild{
+opt<ClangdServer::SkipPreambleBuildPolicy> SkipPreambleBuild{
     "skip-preamble-build",
     cat(Misc),
-    desc("If ture, skip preamble build"),
+    desc("Skip building the preamble"),
     Hidden,
-    init(ParseOptions().SkipPreambleBuild),
+    values(clEnumValN(ClangdServer::SkipPreambleBuildPolicy::Never, "never",
+                      "never skip"),
+           clEnumValN(ClangdServer::SkipPreambleBuildPolicy::Modules, "modules",
+                      "only if module use is encountered (default)"),
+           clEnumValN(ClangdServer::SkipPreambleBuildPolicy::Always, "always",
+                      "always skip")),
+    init(ClangdServer::SkipPreambleBuildPolicy::Modules),
 };
 
 #if defined(__GLIBC__) && CLANGD_MALLOC_TRIM