Add basic import std support for GCC.

2 files changed, 55 insertions, 1 deletions

diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index 00db7a206..fb7ac5b01 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -477,6 +477,12 @@ class RustCrate:
         return ret
 
 
+@dataclass
+class ImportStdInfo:
+    gen_target: NinjaBuildElement
+    gen_module_file: str
+    gen_objects: T.List[str]
+
 class NinjaBackend(backends.Backend):
 
     def __init__(self, build: T.Optional[build.Build]):
@@ -500,6 +506,7 @@ class NinjaBackend(backends.Backend):
         # - https://github.com/mesonbuild/meson/pull/9453
         # - https://github.com/mesonbuild/meson/issues/9479#issuecomment-953485040
         self.allow_thin_archives = PerMachine[bool](True, True)
+        self.import_std: T.Optional[ImportStdInfo] = None
 
     def create_phony_target(self, dummy_outfile: str, rulename: str, phony_infilename: str) -> NinjaBuildElement:
         '''
@@ -3241,6 +3248,11 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
                                                     rel_obj)
                         self.add_build(depelem)
             commands += compiler.get_module_outdir_args(self.get_target_private_dir(target))
+
+        # C++ import std is complicated enough to get its own method.
+        istd_args, istd_dep = self.handle_cpp_import_std(target, compiler)
+        commands.extend(istd_args)
+        header_deps += istd_dep
         if extra_args is not None:
             commands.extend(extra_args)
 
@@ -3290,6 +3302,41 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         assert isinstance(rel_src, str)
         return (rel_obj, rel_src.replace('\\', '/'))
 
+    def target_uses_import_std(self, target: build.BuildTarget) -> bool:
+        if 'cpp' not in target.compilers:
+            return False
+        if 'cpp_importstd' not in self.environment.coredata.optstore:
+            return False
+        if self.environment.coredata.get_option_for_target(target, 'cpp_importstd') == 'false':
+            return False
+        return True
+
+    def handle_cpp_import_std(self, target: build.BuildTarget, compiler):
+        istd_args = []
+        istd_dep = []
+        if not self.target_uses_import_std(target):
+            return istd_args, istd_dep
+        mlog.warning('Import std support is experimental and might break compatibility in the future.')
+        # At the time of writing, all three major compilers work
+        # wildly differently. Keep this isolated here until things
+        # consolidate.
+        if compiler.id == 'gcc':
+            if self.import_std is None:
+                mod_file = 'gcm.cache/std.gcm'
+                mod_obj_file = 'std.o'
+                elem = NinjaBuildElement(self.all_outputs, [mod_file, mod_obj_file], 'CUSTOM_COMMAND', [])
+                compile_args = compiler.get_option_compile_args(target, self.environment)
+                compile_args += compiler.get_option_std_args(target, self.environment)
+                compile_args += ['-c', '-fmodules', '-fsearch-include-path', 'bits/std.cc']
+                elem.add_item('COMMAND', compiler.exelist + compile_args)
+                self.add_build(elem)
+                self.import_std = ImportStdInfo(elem, mod_file, [mod_obj_file])
+            istd_args = ['-fmodules']
+            istd_dep = [File(True, '', self.import_std.gen_module_file)]
+            return istd_args, istd_dep
+        else:
+            raise MesonException(f'Import std not supported on compiler {compiler.id} yet.')
+
     def add_dependency_scanner_entries_to_element(self, target: build.BuildTarget, compiler, element, src) -> None:
         if not self.should_use_dyndeps_for_target(target):
             return
@@ -3763,6 +3810,8 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         dep_targets = []
         dep_targets.extend(self.guess_external_link_dependencies(linker, target, commands, internal))
 
+        obj_list += self.get_import_std_object(target)
+
         # Add libraries generated by custom targets
         custom_target_libraries = self.get_custom_target_provided_libraries(target)
         commands += extra_args
@@ -3791,6 +3840,11 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         self.create_target_linker_introspection(target, linker, commands)
         return elem
 
+    def get_import_std_object(self, target: build.BuildTarget) -> T.List[File]:
+        if not self.target_uses_import_std(target):
+            return []
+        return self.import_std.gen_objects
+
     def get_dependency_filename(self, t) -> str:
         if isinstance(t, build.SharedLibrary):
             if t.uses_rust() and t.rust_crate_type == 'proc-macro':
diff --git a/test cases/common/286 importstd/useistd.cpp b/test cases/common/286 importstd/useistd.cpp
index d8d707a8b..70f62ec3b 100644
--- a/test cases/common/286 importstd/useistd.cpp
+++ b/test cases/common/286 importstd/useistd.cpp
@@ -1,4 +1,4 @@
-#include<print>
+import std;
 
 int main(int, char**) {
     std::print("Import STD is working.\n");