[llvm] [Bazel] Prefer forward slashes for MinGW builds (PR #218181)

[Corentin Kerisit via llvm-commits]

https://github.com/cerisier updated https://github.com/llvm/llvm-project/pull/218181

>From 2c4490ea6be5a0cf614ae4885ced77daaf329e72 Mon Sep 17 00:00:00 2001
From: Corentin Kerisit <corentin.kerisit at gmail.com>
Date: Sun, 23 Aug 2026 01:35:12 +0900
Subject: [PATCH] [Bazel] Prefer forward slashes with MinGW

---
 utils/bazel/llvm-project-overlay/llvm/config.bzl           | 7 ++++++-
 .../llvm-project-overlay/llvm/include/llvm/Config/config.h | 2 +-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/utils/bazel/llvm-project-overlay/llvm/config.bzl b/utils/bazel/llvm-project-overlay/llvm/config.bzl
index 49f70f95d2c79..5cd0ecb3a4621 100644
--- a/utils/bazel/llvm-project-overlay/llvm/config.bzl
+++ b/utils/bazel/llvm-project-overlay/llvm/config.bzl
@@ -131,8 +131,13 @@ builtin_thread_pointer = select({
     "//conditions:default": [],
 })
 
+windows_prefer_forward_slash = select({
+    Label("//llvm:is_windows_clang_mingw"): ["LLVM_WINDOWS_PREFER_FORWARD_SLASH=1"],
+    "//conditions:default": ["LLVM_WINDOWS_PREFER_FORWARD_SLASH=0"],
+})
+
 # TODO: We should split out host vs. target here.
-llvm_config_defines = os_defines + builtin_thread_pointer + select({
+llvm_config_defines = os_defines + builtin_thread_pointer + windows_prefer_forward_slash + select({
     Label("//llvm:is_aarch64_windows_clang_mingw"): native_arch_defines("AArch64", "aarch64-w64-windows-gnu"),
     Label("//llvm:is_aarch64_windows_clang_cl"): native_arch_defines("AArch64", "aarch64-pc-windows-msvc"),
     Label("//llvm:is_aarch64_windows_msvc"): native_arch_defines("AArch64", "aarch64-pc-windows-msvc"),
diff --git a/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h b/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h
index bb69f3c472998..fdfd6a8488d64 100644
--- a/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h
+++ b/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h
@@ -37,7 +37,7 @@
 
 /* Define to 1 to prefer forward slashes on Windows, and to 0 prefer
    backslashes. */
-#define LLVM_WINDOWS_PREFER_FORWARD_SLASH 0
+/* LLVM_WINDOWS_PREFER_FORWARD_SLASH defined in Bazel */
 
 /* Define to 1 if you have the `backtrace' function. */
 /* HAVE_BACKTRACE defined in Bazel */