[wasm] Add support for __builtin_va_list type mapping (#84029)

Cover all va_list family of types just aliased by __builtin_va_list, including __isoc_va_list from wasi-libc. This enables proper C interop for variable argument functions on WASI targets.

Close https://github.com/swiftlang/swift/issues/72398

lib/ClangImporter/MappedTypes.def

@@ -129,6 +129,9 @@ MAP_STDLIB_TYPE("u_int64_t", UnsignedInt, 64, "UInt64", false, DoNothing)
 MAP_STDLIB_TYPE("va_list", VaList, 0, "CVaListPointer", false, DoNothing)
 MAP_STDLIB_TYPE("__gnuc_va_list", VaList, 0, "CVaListPointer", false, DoNothing)
 MAP_STDLIB_TYPE("__va_list", VaList, 0, "CVaListPointer", false, DoNothing)
+// Cover all va_list family of types just aliased by __builtin_va_list, including
+// __isoc_va_list from wasi-libc.
+MAP_STDLIB_TYPE("__builtin_va_list", VaList, 0, "CVaListPointer", false, DoNothing)
 
 // libkern/OSTypes.h types.
 MAP_STDLIB_TYPE("UInt", UnsignedInt, 32, "CUnsignedInt", false, DoNothing)

test/ClangImporter/Inputs/custom-modules/CVarArgs.h

@@ -0,0 +1,7 @@
+#include <stdarg.h>
+extern void takeVaList(va_list args);
+
+extern void takeVaList2(__builtin_va_list args);
+
+typedef __builtin_va_list my_builtin_va_list_alias;
+extern void takeVaList3(my_builtin_va_list_alias args);

test/ClangImporter/Inputs/custom-modules/module.modulemap

@@ -283,3 +283,8 @@ module Aliases {
 module RetroactiveVersioning {
   header "versioning.h"
 }
+
+module CVarArgs {
+  header "CVarArgs.h"
+  export *
+}

test/ClangImporter/ctypes_valist_wasm.swift

@@ -0,0 +1,18 @@
+// RUN: %swift-frontend -target wasm32-unknown-wasip1 -parse-stdlib -module-name Swift -I %S/Inputs/custom-modules -typecheck %s
+
+// REQUIRES: CODEGENERATOR=WebAssembly
+
+import CVarArgs
+
+/// ===== Minimal stdlib definitions =====
+typealias Void = ()
+struct CVaListPointer {}
+// Optional definition is required for isOptional check in ClangImporter
+enum Optional<T> {}
+/// ======================================
+
+func testVaList() {
+  let _: (CVaListPointer) -> Void = CVarArgs.takeVaList
+  let _: (CVaListPointer) -> Void = CVarArgs.takeVaList2
+  let _: (CVaListPointer) -> Void = CVarArgs.takeVaList3
+}

test/stdlib/VarArgs.swift

@@ -193,6 +193,25 @@ func test_varArgs7() {
 }
 test_varArgs7()
 
+func test_varArgs8() {
+  // Check with vsnprintf as well just in case to make sure it works with C APIs
+  // imported from actual libc headers. Why checks with vprintf above are not enough?
+  // ClangImporter respects the vprintf interface re-defined in LibcShims.h but it could
+  // be slightly different than the one in platform libc on some platforms, including WASI.
+  // (e.g. wasi-libc defines `int vprintf(const char *__restrict, __isoc_va_list);` where
+  // `__isoc_va_list` is `typedef __builtin_va_list __isoc_va_list;`, but LibcShims.h aliases
+  // __builtin_va_list as `va_list`.)
+  // https://github.com/swiftlang/swift/issues/72398
+  let string = withUnsafeTemporaryAllocation(of: CChar.self, capacity: 512) { buffer in
+    withVaList([CInt(123)]) { args in
+      _ = vsnprintf(buffer.baseAddress!, buffer.count, "%d", args)
+    }
+    return String(cString: buffer.baseAddress!)
+  }
+  print(string)
+  // CHECK: 123
+}
+test_varArgs8()
 
 // CHECK: done.
 my_printf("done.")