[clang] [libclang] Document CXUnsavedFile::Length width and fix test narrowing (PR #228405)

[via cfe-commits]

https://github.com/UebelAndre updated https://github.com/llvm/llvm-project/pull/228405

>From acbc805c31adf21dad9d1255962362fd7651c09d Mon Sep 17 00:00:00 2001
From: UebelAndre <github at uebelandre.com>
Date: Fri, 2 Oct 2026 05:06:50 -0700
Subject: [PATCH] [libclang] Document CXUnsavedFile::Length width and fix test
 narrowing

`CXUnsavedFile::Length` is `unsigned long`, which is 64 bits on LP64
targets but 32 bits on 64-bit Windows and on 32-bit targets.

The libclang unit test fixture list-initializes it from
`std::string::size()`, which is a narrowing conversion on 64-bit
Windows and therefore ill-formed. MSVC rejects it with error C2397 and
clang-cl with -Wc++11-narrowing, a default error. No CMake bot sees
this because clang/unittests/CMakeLists.txt skips the libclang unit
tests on Windows; the Bazel libclang_tests target builds them there
and fails to compile under --config=msvc. Cast explicitly.

Bindings that describe the struct layout by hand have also assumed a
fixed width for this field (see openjdk/jextract#289). Changing the
type would break the stable libclang ABI, so note the platform-
dependent width in the header instead.

Fixes #160729
---
 clang/include/clang-c/Index.h        | 3 +++
 clang/unittests/libclang/TestUtils.h | 7 ++++---
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/clang/include/clang-c/Index.h b/clang/include/clang-c/Index.h
index f83db6a428ccf2..66979097dc8bd9 100644
--- a/clang/include/clang-c/Index.h
+++ b/clang/include/clang-c/Index.h
@@ -118,6 +118,9 @@ struct CXUnsavedFile {
 
   /**
    * The length of the unsaved contents of this buffer.
+   *
+   * Note that this is an \c unsigned \c long, which has a platform-dependent
+   * width. Do not assume it is a 64-bit integer.
    */
   unsigned long Length;
 };
diff --git a/clang/unittests/libclang/TestUtils.h b/clang/unittests/libclang/TestUtils.h
index 013aad17358211..9a7e8b34fefc53 100644
--- a/clang/unittests/libclang/TestUtils.h
+++ b/clang/unittests/libclang/TestUtils.h
@@ -82,9 +82,10 @@ class LibclangParseTest : public ::testing::Test {
         fixed_addr_string(new std::string(Filename)),
         fixed_addr_string(new std::string(Contents))));
     UnsavedFiles.push_back({
-        it.first->first->c_str(),   // filename
-        it.first->second->c_str(),  // contents
-        it.first->second->size()    // length
+        it.first->first->c_str(),  // filename
+        it.first->second->c_str(), // contents
+        // Length is unsigned long: 32 bits on 64-bit Windows, so this narrows.
+        static_cast<unsigned long>(it.first->second->size()) // length
     });
   }
   template <typename F>