[prelude] cxx 20 module support

.gitignore

@@ -4,6 +4,7 @@ buck-out
 /.direnv
 .buckconfig.local
 .envrc.local
+__pycache__/
 
 # symlinks
 /examples/with_prelude/prelude

prelude/cxx/attr_selection.bzl

@@ -33,7 +33,7 @@ def cxx_by_language_ext(x: dict[typing.Any, typing.Any], ext: str) -> list[typin
         # And you can see them in java code, but somehow it works with
         # this one, which is seem across the repo. Find out what's happening.
         key_compiler = "c_cpp_output"
-    elif ext in (".cpp", ".cc", ".cl", ".cxx", ".c++", ".bc"):
+    elif ext in (".cpp", ".cc", ".cl", ".cxx", ".c++", ".bc", ".cppm", ".ixx"):
         key_pp = "cxx"
         key_compiler = "cxx_cpp_output"
     elif ext == ".m":

prelude/cxx/compile.bzl

@@ -1114,7 +1114,7 @@ def _validate_target_headers(label: Label, preprocessor: list[CPreprocessor]):
 
 def _get_compiler_info(toolchain: CxxToolchainInfo, ext: CxxExtension) -> typing.Any:
     compiler_info = None
-    if ext.value in (".cpp", ".cc", ".cl", ".cxx", ".c++", ".h", ".hpp", ".hh", ".h++", ".hxx", ".bc"):
+    if ext.value in (".cpp", ".cc", ".cl", ".cxx", ".c++", ".h", ".hpp", ".hh", ".h++", ".hxx", ".bc", ".cppm", ".ixx"):
         compiler_info = toolchain.cxx_compiler_info
     elif ext.value == ".c":
         compiler_info = toolchain.c_compiler_info
@@ -1140,7 +1140,7 @@ def _get_compiler_info(toolchain: CxxToolchainInfo, ext: CxxExtension) -> typing
     return compiler_info
 
 def _get_category(ext: CxxExtension) -> str:
-    if ext.value in (".cpp", ".cc", ".cl", ".cxx", ".c++", ".h", ".hpp", ".hh", ".h++", ".hxx"):
+    if ext.value in (".cpp", ".cc", ".cl", ".cxx", ".c++", ".h", ".hpp", ".hh", ".h++", ".hxx", ".cppm", ".ixx"):
         return "cxx_compile"
     if ext.value == ".c":
         return "c_compile"
@@ -1186,7 +1186,7 @@ def _dep_file_type(ext: CxxExtension) -> [DepFileType, None]:
         return None
 
     # Return the file type as well
-    if ext.value in (".cpp", ".cc", ".cl", ".mm", ".cxx", ".c++", ".h", ".hpp", ".hh", ".h++", ".hxx"):
+    if ext.value in (".cpp", ".cc", ".cl", ".mm", ".cxx", ".c++", ".h", ".hpp", ".hh", ".h++", ".hxx", ".cppm", ".ixx"):
         return DepFileType("cpp")
     elif ext.value in (".c", ".m"):
         return DepFileType("c")

prelude/cxx/compile_types.bzl

@@ -37,6 +37,8 @@ CxxExtension = enum(
     ".h++",
     ".hxx",
     ".bc",
+    ".cppm",
+    ".ixx",
     *AsmExtensions.values()
 )
 

prelude/cxx/cxx.bzl

@@ -245,6 +245,7 @@ def cxx_library_generate(ctx: AnalysisContext, rule_type: str) -> list[Provider]
         # @oss-disable[end= ]: extra_linker_outputs_factory = get_extra_linker_outputs,
         # @oss-disable[end= ]: extra_linker_outputs_flags_factory = get_extra_linker_output_flags,
         supports_stripping = ctx.attrs.supports_stripping,
+        cxx_module_scanning = ctx.attrs.module_scanning,
     )
     output = cxx_library_parameterized(ctx, params)
     return output.providers
@@ -318,6 +319,7 @@ def cxx_binary_impl(ctx: AnalysisContext) -> list[Provider]:
         coverage_instrumentation_compiler_flags = ctx.attrs.coverage_instrumentation_compiler_flags,
         separate_debug_info = ctx.attrs.separate_debug_info,
         cuda_compile_style = CudaCompileStyle(ctx.attrs.cuda_compile_style),
+        cxx_module_scanning = ctx.attrs.module_scanning,
     )
     output = cxx_executable(ctx, params)
 
@@ -837,6 +839,7 @@ def cxx_test_impl(ctx: AnalysisContext) -> list[Provider]:
         coverage_instrumentation_compiler_flags = ctx.attrs.coverage_instrumentation_compiler_flags,
         separate_debug_info = ctx.attrs.separate_debug_info,
         cuda_compile_style = CudaCompileStyle(ctx.attrs.cuda_compile_style),
+        cxx_module_scanning = ctx.attrs.module_scanning,
     )
     output = cxx_executable(ctx, params, is_cxx_test = True)
 

prelude/cxx/cxx_toolchain_types.bzl

@@ -229,6 +229,7 @@ CxxToolchainInfo = provider(
         "clang_llvm_statistics": provider_field(typing.Any, default = None),
         "clang_remarks": provider_field(typing.Any, default = None),
         "clang_trace": provider_field(typing.Any, default = None),
+        "clang_scan_deps": provider_field(typing.Any, default = None),
         "compiler_flavor_flags": provider_field(typing.Any, default = {}),
         "cpp_dep_tracking_mode": provider_field(typing.Any, default = None),
         "cuda_compiler_info": provider_field(typing.Any, default = None),

prelude/cxx/cxx_types.bzl

@@ -293,4 +293,8 @@ CxxRuleConstructorParams = record(
     # are using symlink trees.
     skip_exported_headers = field(bool, False),
     supports_stripping = field(bool, True),
+    # If set, the source files must be scanned for module dependencies
+    # prior to compilation, so we can know the correct order to compile them
+    # as well as additional compiler flags to pass to the compiler.
+    cxx_module_scanning = field(bool, False),
 )

prelude/decls/cxx_common.bzl

@@ -340,6 +340,14 @@ def _use_content_based_paths_arg():
         "use_content_based_paths": attrs.bool(default = True),
     }
 
+def _module_scanning_arg():
+    return {
+        "module_scanning": attrs.bool(default = False, doc = """
+    If set, the source files must be scanned for module dependencies
+    prior to compilation.
+"""),
+    }
+
 cxx_common = struct(
     srcs_arg = _srcs_arg,
     deps_arg = _deps_arg,
@@ -375,4 +383,5 @@ cxx_common = struct(
     use_fbcc_rust_wrapper_arg = _use_fbcc_rust_wrapper_arg,
     use_content_based_paths_arg = _use_content_based_paths_arg,
     supports_stripping = _supports_stripping,
+    module_scanning_arg = _module_scanning_arg,
 )

prelude/decls/cxx_rules.bzl

@@ -145,6 +145,7 @@ cxx_binary = prelude_rule(
         cxx_common.runtime_dependency_handling_arg() |
         cxx_common.use_fbcc_rust_wrapper_arg() |
         cxx_common.use_content_based_paths_arg() |
+        cxx_common.module_scanning_arg() |
         {
             "cxx_runtime_type": attrs.option(attrs.enum(CxxRuntimeType), default = None),
             "default_host_platform": attrs.option(attrs.configuration_label(), default = None),
@@ -484,6 +485,7 @@ library_attrs = (
     cxx_common.version_arg() |
     cxx_common.use_fbcc_rust_wrapper_arg() |
     cxx_common.use_content_based_paths_arg() |
+    cxx_common.module_scanning_arg() |
     validation_common.attrs_validators_arg() |
     {
         "archive_allow_cache_upload": attrs.bool(default = False),
@@ -845,6 +847,7 @@ cxx_test = prelude_rule(
         cxx_common.headers_arg() |
         cxx_common.preprocessor_flags_arg() |
         cxx_common.compiler_flags_arg() |
+        cxx_common.module_scanning_arg() |
         cxx_common.linker_flags_arg() |
         cxx_common.precompiled_header_arg() |
         buck.deps_query_arg() |
@@ -1061,6 +1064,7 @@ cxx_toolchain = prelude_rule(
             "strip_debug_flags": attrs.option(attrs.list(attrs.arg()), default = None),
             "strip_non_global_flags": attrs.option(attrs.list(attrs.arg()), default = None),
             "use_header_map": attrs.bool(default = False),
+            "clang_scan_deps": attrs.option(attrs.source(), default = None),
         } |
         buck.licenses_arg() |
         buck.labels_arg() |

prelude/toolchains/cxx.bzl

@@ -42,6 +42,7 @@ CxxToolsInfo = provider(
         "linker": provider_field(typing.Any, default = None),
         "linker_type": LinkerType,
         "rc_compiler": provider_field(typing.Any, default = None),
+        "clang_scan_deps": provider_field(typing.Any, default = None),
     },
 )
 
@@ -58,6 +59,7 @@ def _legacy_equivalent_cxx_tools_info_windows(ctx: AnalysisContext, default_tool
         archiver_type = default_toolchain.archiver_type,
         linker = default_toolchain.linker if ctx.attrs.linker == None or ctx.attrs.linker == "link.exe" else ctx.attrs.linker,
         linker_type = default_toolchain.linker_type,
+        clang_scan_deps = default_toolchain.clang_scan_deps if ctx.attrs.clang_scan_deps == None else ctx.attrs.clang_scan_deps,
     )
 
 def _legacy_equivalent_cxx_tools_info_non_windows(ctx: AnalysisContext, default_toolchain: CxxToolsInfo) -> CxxToolsInfo:
@@ -73,6 +75,7 @@ def _legacy_equivalent_cxx_tools_info_non_windows(ctx: AnalysisContext, default_
         archiver_type = default_toolchain.archiver_type,
         linker = default_toolchain.linker if ctx.attrs.linker == None else ctx.attrs.linker,
         linker_type = default_toolchain.linker_type,
+        clang_scan_deps = default_toolchain.clang_scan_deps if ctx.attrs.clang_scan_deps == None else ctx.attrs.clang_scan_deps,
     )
 
 def _system_cxx_toolchain_impl(ctx: AnalysisContext):
@@ -224,6 +227,7 @@ def _cxx_toolchain_from_cxx_tools_info(ctx: AnalysisContext, cxx_tools_info: Cxx
             pic_behavior = pic_behavior,
             llvm_link = llvm_link,
             use_dep_files = True,
+            clang_scan_deps = _run_info(cxx_tools_info.clang_scan_deps),
         ),
         CxxPlatformInfo(name = target_name),
     ]
@@ -239,6 +243,7 @@ system_cxx_toolchain = rule(
         "compiler": attrs.option(attrs.string(), default = None),
         "compiler_type": attrs.option(attrs.string(), default = None),  # one of CxxToolProviderType
         "cpp_dep_tracking_mode": attrs.string(default = "makefile"),
+        "clang_scan_deps": attrs.option(attrs.string(), default = None),
         "cvtres_compiler": attrs.option(attrs.string(), default = None),
         "cvtres_flags": attrs.list(attrs.arg(), default = []),
         "cxx_compiler": attrs.option(attrs.string(), default = None),

prelude/toolchains/cxx/clang/tools.bzl

@@ -24,6 +24,7 @@ def _path_clang_tools_impl(_ctx) -> list[Provider]:
             archiver_type = "gnu",
             linker = "clang++",
             linker_type = LinkerType("gnu"),
+            clang_scan_deps = "clang-scan-deps",
         ),
     ]
 

prelude/toolchains/msvc/tools.bzl

@@ -137,6 +137,8 @@ def _find_msvc_tools_impl(ctx: AnalysisContext) -> list[Provider]:
             archiver_type = "windows",
             linker = _windows_linker_wrapper(ctx, link_exe_script),
             linker_type = LinkerType("windows"),
+            # MSVC can do module scanning without needing a separate tool.
+            clang_scan_deps = None,
         ),
     ]
 

rust-toolchain

@@ -9,6 +9,7 @@
 #   * NOTE: You may have to change this ocaml file in a follow up commit as ocamlrep
 #     has a dependency on buck2 git trunk.
 # * Update `../monarch/rust-toolchain` (one instance)
+# * Update flake.lock via `nix flake update rust-overlay`
 
 # @rustc_version: rustc 1.91.0-nightly (02c7b1a7a 2025-09-13)
 channel = "nightly-2025-09-14"