From 0d555de37dfdaed7d5b2f938359bbcdc86906b49 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20K=C3=A5re=20Alsaker?= <john.kare.alsaker@gmail.com>
Date: Sat, 25 Apr 2026 02:35:46 +0200
Subject: [PATCH] Allow building LLVM DLL with MSVC

---
 compiler/rustc_llvm/build.rs               | 10 ++++++++
 src/bootstrap/src/core/build_steps/llvm.rs | 27 ++++++++++++++++++----
 2 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/compiler/rustc_llvm/build.rs b/compiler/rustc_llvm/build.rs
index ef262a5b19b08..696a9e3d000e9 100644
--- a/compiler/rustc_llvm/build.rs
+++ b/compiler/rustc_llvm/build.rs
@@ -287,6 +287,16 @@ fn main() {
         cfg.flag(&*flag);
     }
 
+    // When linking a shared (dylib) LLVM on MSVC, the LLVM build itself is compiled with
+    // `/Zc:dllexportInlines-` (see llvm-project's AddLLVM.cmake) so that inline class members
+    // are *not* dllexport'ed (to stay under the 64k export-symbol limit). The consumer must
+    // match that flag, otherwise it emits `__imp_` references for those inline members (e.g.
+    // `Module::setTargetTriple`, `Function::getAttributes`) that the DLL intentionally does not
+    // export, breaking the link. With the flag, the in-header inline bodies are used directly.
+    if target.contains("msvc") && tracked_env_var_os("LLVM_LINK_SHARED").is_some() {
+        cfg.flag("/Zc:dllexportInlines-");
+    }
+
     // Remap ci-llvm include paths in debug info for reproducible builds.
     if let Some(maps) = tracked_env_var_os("RUSTC_DEBUGINFO_MAP")
         && let Some(maps_str) = maps.to_str()
diff --git a/src/bootstrap/src/core/build_steps/llvm.rs b/src/bootstrap/src/core/build_steps/llvm.rs
index ff96f341966b7..5388832f9ed44 100644
--- a/src/bootstrap/src/core/build_steps/llvm.rs
+++ b/src/bootstrap/src/core/build_steps/llvm.rs
@@ -24,7 +24,7 @@ use crate::utils::exec::command;
 use crate::utils::helpers::{
     self, exe, get_clang_cl_resource_dir, libdir, t, unhashed_basename, up_to_date,
 };
-use crate::{CLang, GitRepo, Kind, exit, trace};
+use crate::{CLang, FileType, GitRepo, Kind, exit, trace};
 
 #[derive(Clone)]
 pub struct LlvmResult {
@@ -308,7 +308,10 @@ impl CommandLineStep for Llvm {
             LlvmBuildStatus::ShouldBuild(m) => m,
         };
 
-        if builder.llvm_link_shared() && target.is_windows() && !target.is_windows_gnullvm() {
+        if builder.llvm_link_shared()
+            && target.is_windows()
+            && !(target.is_windows_gnullvm() || target.is_msvc())
+        {
             panic!("shared linking to LLVM is not currently supported on {}", target.triple);
         }
 
@@ -409,6 +412,8 @@ impl CommandLineStep for Llvm {
         // for the tools. We don't do this on every platform as it doesn't work
         // equally well everywhere.
         if builder.llvm_link_shared() {
+            cfg.define("LLVM_BUILD_LLVM_DYLIB_VIS", "ON");
+            cfg.define("LLVM_BUILD_LLVM_DYLIB", "ON");
             cfg.define("LLVM_LINK_LLVM_DYLIB", "ON");
         }
 
@@ -557,12 +562,13 @@ impl CommandLineStep for Llvm {
 
         cfg.build();
 
-        // Helper to find the name of LLVM's shared library on darwin and linux.
+        // Helper to find the name of LLVM's shared library.
         let find_llvm_lib_name = |extension| {
             let major = get_llvm_version_major(builder, &res.host_llvm_config);
+            let prefix = if target.is_msvc() { "" } else { "lib" };
             match &llvm_version_suffix {
-                Some(version_suffix) => format!("libLLVM-{major}{version_suffix}.{extension}"),
-                None => format!("libLLVM-{major}.{extension}"),
+                Some(version_suffix) => format!("{prefix}LLVM-{major}{version_suffix}.{extension}"),
+                None => format!("{prefix}LLVM-{major}.{extension}"),
             }
         };
 
@@ -578,6 +584,17 @@ impl CommandLineStep for Llvm {
             }
         }
 
+        // Create the .dll.lib import file which llvm-config incorrectly points to
+        if builder.llvm_link_shared() && target.is_msvc() {
+            let lib_name = find_llvm_lib_name("");
+            let lib_path = out_dir.join("lib");
+            let wanted = lib_path.clone().join(lib_name.clone() + "dll.lib");
+            let wrong = lib_path.join(lib_name + "lib");
+            if wrong.exists() && !wanted.exists() {
+                builder.copy_link(&wrong, &wanted, FileType::Regular);
+            }
+        }
+
         // When building LLVM as a shared library on linux, it can contain unexpected debuginfo:
         // some can come from the C++ standard library. Unless we're explicitly requesting LLVM to
         // be built with debuginfo, strip it away after the fact, to make dist artifacts smaller.