v.help: document the -is_o option (in v help build-c) and add a test for it (#25052)

Author: spytheman

vlib/v/builder/cc.v

@@ -592,7 +592,7 @@ fn (mut v Builder) thirdparty_object_args(ccoptions CcompilerOptions, middle []s
 
 fn (mut v Builder) setup_output_name() {
 	if !v.pref.is_shared && v.pref.build_mode != .build_module && v.pref.os == .windows
-		&& !v.pref.out_name.ends_with('.exe') {
+		&& !v.pref.is_o && !v.pref.out_name.ends_with('.exe') {
 		v.pref.out_name += '.exe'
 	}
 	// Output executable name

vlib/v/help/build/build-c.txt

@@ -176,6 +176,15 @@ see also `v help build`.
       Tell V to compile a shared object instead of an executable.
       The resulting file extension will be `.dll` on Windows and `.so` on Unix systems
 
+   -is_o
+      Tell V to compile an .o object file instead of an executable.
+      This is useful if you plan to create a module, that will be later linked to a
+      larger program, potentially written in another language.
+      Note: this option currently works with gcc/clang/tcc, but not msvc.
+      Note: use either -no-skip-unused or tag the functions you expect to be publicly
+      usable in the .o file with `@[markused]` . Adding `-gc none` may be also needed,
+      unless you plan to link with -lgc later.
+
 # Memory management
    -autofree
       Free memory used in functions automatically.

vlib/v/slow_tests/run_project_folders/compile_obj_file_and_link_separately/abc/module_in_v.v

@@ -0,0 +1,6 @@
+module abc
+
+@[markused]
+pub fn addition(x int, y int) int {
+	return x + y
+}

vlib/v/slow_tests/run_project_folders/compile_obj_file_and_link_separately/main.v

@@ -0,0 +1,31 @@
+module main
+
+import os
+
+const tfolder = os.join_path(os.vtmp_dir(), 'vobjfile_${os.getuid()}')
+const vexe = os.quoted_path(@VEXE)
+const gcc = os.quoted_path(os.find_abs_path_of_executable('gcc') or {
+	println('This program needs `gcc` to be present.')
+	println('OK')
+	exit(0)
+})
+
+fn lexec(cmd string) os.Result {
+	println('>>> lexec cmd: ${cmd}')
+	res := os.execute_or_exit(cmd)
+	return res
+}
+
+fn main() {
+	os.chdir(@DIR)!
+	os.rmdir_all(tfolder) or {}
+	os.mkdir_all(tfolder)!
+	lexec('${vexe} -is_o -o "${tfolder}/abc.o"        -gc none -cc ${gcc} abc/')
+	lexec('${gcc}        -o "${tfolder}/main_in_c.o"  -c main_in_c.c')
+	lexec('${gcc}        -o "${tfolder}/program.exe"  "${tfolder}/abc.o"   "${tfolder}/main_in_c.o"')
+	res := lexec('${tfolder}/program.exe')
+	print(res.output)
+	assert res.output.starts_with('Result of addition: 30')
+	os.rmdir_all(tfolder) or {}
+	println('OK')
+}

vlib/v/slow_tests/run_project_folders/compile_obj_file_and_link_separately/main_in_c.c

@@ -0,0 +1,7 @@
+#include <stdio.h>
+extern int abc__addition(int x, int y);
+int main() {
+	int res = abc__addition(10,20);
+	printf("Result of addition: %d\n", res);
+	return 0;
+}

vlib/v/slow_tests/run_project_folders_test.v

@@ -30,7 +30,7 @@ fn test_v_profile_works() {
 		res := os.execute('${os.quoted_path(vexe)} run ${os.quoted_path(folder_path)}')
 		delta := time.ticks() - t
 		// eprintln('res: $res')
-		assert res.exit_code == 0
+		assert res.exit_code == 0, 'failing res: ${res}'
 		assert res.output.len > 0
 		assert res.output.contains('OK')
 		term.clear_previous_line()