Fix building with --feature js_backtrace

[KallynGowdy]

This PR fixes an build error that I ran into when using the --features js_backtrace flag.

In particular, the error I ran into was this:

error[E0061]: this function takes 2 arguments but 1 argument was supplied
   --> components\script\dom\bindings\error.rs:113:45
    |
113 |         let js_stack = stack.and_then(|s| s.as_string(None));
    |                                             ^^^^^^^^^ ---- supplied 1 argument
    |                                             |
    |                                             expected 2 arguments

It seems that the mozjs crate updated the as_string() method on CapturedJSStack to take two parameters. I've chosen to use StackFormat::Default because that will presumably preserve the original intended behavior.

I additionally had to make the block wrapping the feature unsafe as I ran into a "must be marked as unsafe to call unsafe code" build error after fixing the above issue. Seems that this commit (0703a1a) forgot that part.

Other Notes

After these changes, I was able to build but ran into an access violation error during runtime. When a JS error hit throw_dom_exception() and the JS stack was pulled, some sort of issue was hit inside mozjs and MOZ_NoReturn() was called, which will obviously cause an issue.

It is worth noting that this only happened on my test platform (Hololens 2) and not on MacOS or in the Hololens 2 emulator. Additionally, the debug logs were not particularly helpful at identifying the culprit. Maybe it is worth making a full issue for?

Here's the full details:

Error message:

Exception thrown at 0x00007FFBCDD16784 (simpleservo.dll) in ServoApp.exe: 0xC0000005: Access violation writing location 0x0000000000000000.

Stack Trace:

simpleservo.dll!MOZ_NoReturn(int aLine) Line 236
	at E:\Projects\Yeti\servo\target\aarch64-uwp-windows-msvc\debug\build\mozjs_sys-aa48eb6c20c205d4\out\build\dist\include\mozilla\Assertions.h(236)
simpleservo.dll!js::SavedStacks::insertFrames(JSContext * cx, JS::MutableHandle<js::SavedFrame *> frame, mozilla::Variant<JS::AllFrames,JS::MaxFrames,JS::FirstSubsumedFrame> && capture) Line 0
	at E:\.cargo\git\checkouts\mozjs-fa11ffc7d4f1cc2d\9a6d8fc\mozjs\js\src\vm\SavedStacks.cpp(0)
simpleservo.dll!js::SavedStacks::saveCurrentStack(JSContext * cx, JS::MutableHandle<js::SavedFrame *> frame, mozilla::Variant<JS::AllFrames,JS::MaxFrames,JS::FirstSubsumedFrame> && capture) Line 1292
	at E:\.cargo\git\checkouts\mozjs-fa11ffc7d4f1cc2d\9a6d8fc\mozjs\js\src\vm\SavedStacks.cpp(1292)
simpleservo.dll!JS::CaptureCurrentStack(JSContext * cx, JS::MutableHandle<JSObject *> stackp, mozilla::Variant<JS::AllFrames,JS::MaxFrames,JS::FirstSubsumedFrame> && capture) Line 5926
	at E:\.cargo\git\checkouts\mozjs-fa11ffc7d4f1cc2d\9a6d8fc\mozjs\js\src\jsapi.cpp(5926)
simpleservo.dll!mozjs::rust::CapturedJSStack::new(mozjs::rust::RootedGuard<mut mozjs_sys::generated::root::JSObject*> cx, core::option::Option<u32> guard) Line 1337
	at E:\.cargo\git\checkouts\rust-mozjs-8611526964119dd6\28248e1\src\rust.rs(1337)
simpleservo.dll!script::dom::bindings::error::throw_dom_exception(script::script_runtime::JSContext cx, script::dom::globalscope::GlobalScope * global, script::dom::bindings::error::Error result) Line 114
	at E:\Projects\Yeti\servo\components\script\dom\bindings\error.rs(114)

Repro:

1. Make a build on Windows for the Hololens 2.
   • This is the command I used: C:\Python27\python.exe mach build -d --uwp --win-arm64 --features js_backtrace
2. Open the project in Visual Studio.
3. Change the default URL in DefaultUrl.h to "http://yeticgi.casualos.com/?story=test1&pagePortal=home".
   • Note the lack of HTTPS. There's a separate issue that causes a websocket error. Was trying to debug when I ran into this issue.
   • The same issue occurs if you load from a build of this this repository. Follow the instructions in DEVELOPERS.md if you do.
4. Run on the Hololens 2 via Visual Studio.
   • If the default URL doesn't take effect, you may have to uninstall the app from the Hololens 2 and reinstall.
5. Wait for it to load and stop at a breakpoint for SavedStacks.cpp.
   • It will say the source cannot be found but its actually because the breakpoint doesn't have a related line in the source code.
   • If you continue then it will run into the access violation exception.

---

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors

• There are tests for these changes OR
• These changes do not require tests because they fix a build error.

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @asajeffrey (or someone else) soon.

[highfive]

Heads up! This PR modifies the following files:

• @asajeffrey: components/script/dom/bindings/error.rs
• @KiChjang: components/script/dom/bindings/error.rs

[highfive]

Warning

• These commits modify unsafe code. Please review it carefully!
• These commits modify script code, but no tests are modified. Please consider adding a test!

[jdm]

Please file an issue about the crash! That's very interesting.

[jdm]

@bors-servo r+
Thanks for fixing this! I keep doing it locally while investigating other things and forgetting to make a pull request.

[bors-servo]

📌 Commit 6a5a77a has been approved by jdm

[bors-servo]

⌛ Testing commit 6a5a77a with merge 0916ae3...

[bors-servo]

☀️ Test successful - status-taskcluster
Approved by: jdm
Pushing 0916ae3 to master...