Add set_flags_from_command_line_with_usage (#536)

The motivation for this is to enable embedders of rusty_v8 to
pass their own usage string and have their usage printed before V8's
options.

src/V8.rs

@@ -11,7 +11,11 @@ use crate::support::UniqueRef;
 use crate::support::UnitType;
 
 extern "C" {
-  fn v8__V8__SetFlagsFromCommandLine(argc: *mut c_int, argv: *mut *mut c_char);
+  fn v8__V8__SetFlagsFromCommandLine(
+    argc: *mut c_int,
+    argv: *mut *mut c_char,
+    usage: *const c_char,
+  );
   fn v8__V8__SetFlagsFromString(flags: *const u8, length: usize);
   fn v8__V8__SetEntropySource(callback: EntropySource);
   fn v8__V8__GetVersion() -> *const c_char;
@@ -83,6 +87,12 @@ pub fn assert_initialized() {
 /// Returns a vector of command line arguments that V8 did not understand.
 /// TODO: Check whether this is safe to do after globally initializing v8.
 pub fn set_flags_from_command_line(args: Vec<String>) -> Vec<String> {
+  set_flags_from_command_line_with_usage(args, None)
+}
+pub fn set_flags_from_command_line_with_usage(
+  args: Vec<String>,
+  usage: Option<&str>,
+) -> Vec<String> {
   // deno_set_v8_flags(int* argc, char** argv) mutates argc and argv to remove
   // flags that v8 understands.
 
@@ -102,8 +112,16 @@ pub fn set_flags_from_command_line(args: Vec<String>) -> Vec<String> {
   // updates its value.
   let mut c_argv_len = c_argv.len() as c_int;
   // Let v8 parse the arguments it recognizes and remove them from c_argv.
+  let c_usage = match usage {
+    Some(str) => CString::new(str).unwrap().into_raw() as *const c_char,
+    None => std::ptr::null(),
+  };
   unsafe {
-    v8__V8__SetFlagsFromCommandLine(&mut c_argv_len, c_argv.as_mut_ptr())
+    v8__V8__SetFlagsFromCommandLine(
+      &mut c_argv_len,
+      c_argv.as_mut_ptr(),
+      c_usage,
+    );
   };
   // If c_argv_len was updated we have to change the length of c_argv to match.
   c_argv.truncate(c_argv_len as usize);

src/binding.cc

@@ -10,6 +10,7 @@
 #include "v8/include/v8.h"
 #include "v8/src/execution/isolate-utils-inl.h"
 #include "v8/src/execution/isolate-utils.h"
+#include "v8/src/flags/flags.h"
 #include "v8/src/objects/objects-inl.h"
 #include "v8/src/objects/objects.h"
 #include "v8/src/objects/smi.h"
@@ -89,8 +90,12 @@ v8::MaybeLocal<v8::Promise> HostImportModuleDynamicallyCallback(
 }
 
 extern "C" {
-void v8__V8__SetFlagsFromCommandLine(int* argc, char** argv) {
+void v8__V8__SetFlagsFromCommandLine(int* argc, char** argv,
-  v8::V8::SetFlagsFromCommandLine(argc, argv, true);
+                                     const char* usage) {
+  namespace i = v8::internal;
+  using HelpOptions = i::FlagList::HelpOptions;
+  HelpOptions help_options = HelpOptions(HelpOptions::kExit, usage);
+  i::FlagList::SetFlagsFromCommandLine(argc, argv, true, help_options);
 }
 
 void v8__V8__SetFlagsFromString(const char* flags, size_t length) {

tests/test_api.rs

@@ -999,6 +999,15 @@ fn set_flags_from_command_line() {
   );
 }
 
+#[test]
+fn set_flags_from_command_line_with_usage() {
+  let r = v8::V8::set_flags_from_command_line_with_usage(
+    vec!["binaryname".to_string(), "--help".to_string()],
+    Some("Usage: binaryname --startup-src=file\n\n"),
+  );
+  assert_eq!(r, vec!["binaryname".to_string()]);
+}
+
 #[test]
 fn inspector_string_view() {
   let chars = b"Hello world!";