Fix variadic template union layout recursion and generic field codegen

Early return None in CompInfo::layout for uninstantiated C++ template
unions (self.kind == CompKind::Union && !self.template_params.is_empty()),
avoiding infinite layout recursion on variadic template unions while
preserving opaque type alias codegen across LLVM 18 and LLVM 19+.

In CompInfo::is_rust_union, check ctx.uses_any_template_parameters
during codegen, emitting struct + __BindgenUnionField<T> wrappers for
unconstrained generic template union fields to prevent Rust compiler
E0740 errors.

This also adds LLVM-21 tests, since clang generates different code than
LLVM-20.

Test: cargo test -p bindgen-tests

TAG=agy
CONV=619bdf72-4d2d-494f-8cb0-f61a6db9674c

Author: erickt

.github/workflows/bindgen.yml

@@ -111,7 +111,7 @@ jobs:
             libtinfo: libtinfo5_6.3+20220423-2ubuntu0.1_amd64.deb
           - os: ubuntu-24.04-arm
             libtinfo: libtinfo5_6.3+20220423-2ubuntu0.1_arm64.deb
-        llvm_version: ["16.0", "20.1"]
+        llvm_version: ["16.0", "20.1", "21.1"]
         release_build: [0, 1]
         no_default_features: [0, 1]
         # FIXME: There are no pre-built static libclang libraries, so the
@@ -125,15 +125,15 @@ jobs:
           - platform:
               os: ubuntu-latest
               libtinfo: libtinfo5_6.3+20220423-2ubuntu0.1_amd64.deb
-            llvm_version: "20.1"
+            llvm_version: "21.1"
             release_build: 0
             no_default_features: 0
             feature_extra_asserts: 1
 
           # Ensure stuff works on macos too
           - platform:
               os: macos-latest
-            llvm_version: "20.1"
+            llvm_version: "21.1"
             release_build: 0
             no_default_features: 0
             feature_extra_asserts: 0
@@ -160,7 +160,7 @@ jobs:
           BINDGEN_FEATURE_RUNTIME: ${{matrix.feature_runtime}}
           BINDGEN_FEATURE_EXTRA_ASSERTS: ${{matrix.feature_extra_asserts}}
           BINDGEN_NO_DEFAULT_FEATURES: ${{matrix.no_default_features}}
-          BINDGEN_RUST_FOR_LINUX_TEST: ${{startsWith(matrix.platform.os, 'ubuntu') && matrix.llvm_version == '20.1' && matrix.feature_extra_asserts == 0 && 1 || 0}}
+          BINDGEN_RUST_FOR_LINUX_TEST: ${{startsWith(matrix.platform.os, 'ubuntu') && matrix.llvm_version == '21.1' && matrix.feature_extra_asserts == 0 && 1 || 0}}
         run: ./ci/test.sh
 
   test-book:

bindgen-tests/Cargo.toml

@@ -26,3 +26,4 @@ runtime = ["bindgen/runtime"]
 __testing_only_extra_assertions = ["bindgen/__testing_only_extra_assertions"]
 __testing_only_libclang_16 = ["bindgen/__testing_only_libclang_16"]
 __testing_only_libclang_20 = ["bindgen/__testing_only_libclang_20"]
+__testing_only_libclang_21 = []

bindgen-tests/tests/expectations/tests/libclang-21/issue-544-stylo-creduce-2.rs

@@ -0,0 +1,9 @@
+// bindgen-flags: -- -std=c++14
+
+namespace test {
+template <class _Arg0, class... _Args>
+union __union {
+  _Arg0 __arg;
+  __union<_Args...> __u;
+};
+}

bindgen-tests/tests/expectations/tests/libclang-21/nsBaseHashtable.rs

@@ -0,0 +1,33 @@
+// bindgen-flags: --raw-line '#[test] fn test_mixed_union_pointer_size() { assert_eq!(::std::mem::size_of::<test_MixedCharPointer>(), 8); }' --opaque-type '.*MixedUnionValue.*' -- -std=c++14
+
+namespace test {
+
+template <class T, class... Args>
+union MixedUnionPointer {
+    T val;
+    MixedUnionPointer<Args...>* ptr;
+};
+
+template <class T>
+union MixedUnionPointer<T> {
+    T val;
+};
+
+template <class T, class... Args>
+union MixedUnionValue {
+    T val;
+    MixedUnionValue<Args...> next;
+};
+
+template <class T>
+union MixedUnionValue<T> {
+    T val;
+};
+
+using MixedCharPointer = MixedUnionPointer<char, int>;
+using MixedCharDouble = MixedUnionValue<char, double>;
+
+static_assert(sizeof(MixedCharPointer) == 8, "Expected sizeof(MixedCharPointer) == 8 in C++");
+static_assert(sizeof(MixedCharDouble) == 8, "Expected sizeof(MixedCharDouble) == 8 in C++");
+
+}

bindgen-tests/tests/expectations/tests/transform-op.rs

@@ -141,16 +141,20 @@ fn compare_generated_header(
     {
         let mut expectation = expectation.clone();
 
-        if cfg!(feature = "__testing_only_libclang_20") {
+        if cfg!(feature = "__testing_only_libclang_21") {
+            expectation.push("libclang-21");
+        } else if cfg!(feature = "__testing_only_libclang_20") {
             expectation.push("libclang-20");
         } else if cfg!(feature = "__testing_only_libclang_16") {
             expectation.push("libclang-16");
         } else {
             match clang_version().parsed {
-                None => expectation.push("libclang-20"),
+                None => expectation.push("libclang-21"),
                 Some(version) => {
                     let (maj, min) = version;
-                    let version_str = if maj >= 20 {
+                    let version_str = if maj >= 21 {
+                        "21".to_owned()
+                    } else if maj >= 20 {
                         "20".to_owned()
                     } else if maj >= 16 {
                         "16".to_owned()