callback interface Foo { void passStringSequence(sequence<DOMString> arg); }; The code produced by this does not compile: /Users/amccreight/mz/cent/obj-dbg/dom/bindings/JSImplBinding.cpp:49:16: error: no matching function for call to 'NonVoidStringToJsval' if (!xpc::NonVoidStringToJsval(cx, arg[i], &tmp)) { ^~~~~~~~~~~~~~~~~~~~~~~~~ /Users/amccreight/mz/cent/js/xpconnect/src/xpcpublic.h:283:6: note: candidate function not viable: no known conversion from 'const elem_type' (aka 'const nsString') to 'nsAString_internal &' for 2nd argument; bool NonVoidStringToJsval(JSContext *cx, nsAString &str, JS::Value *rval); ^ /Users/amccreight/mz/cent/js/xpconnect/src/xpcpublic.h:298:6: note: candidate function not viable: no known conversion from 'const elem_type' (aka 'const nsString') to 'mozilla::dom::DOMString &' for 2nd argument; bool NonVoidStringToJsval(JSContext* cx, mozilla::dom::DOMString& str, ^ 1 error generated. On the other hand, the output from this method does compile: void passCastableObjectSequence(sequence<Foo> arg);

From CallbackMember.getArgConversion: if arg.type.isString(): # XPConnect string-to-JS conversion wants to mutate the string. So # let's give it a string it can mutate # XXXbz if we try to do a sequence of strings, this will kinda fail.

So one interesting option is to just add non-mutating version of NonVoidStringToJsval and whatnot that take a const string and make sure to deal accordingly...

of non-const strings, but I think that's OK now that we're mostly autogenerating this stuff

Pushed https://hg.mozilla.org/integration/mozilla-inbound/rev/1d8af801d850

Unfortunately, I had to backout because something in this push caused a spike in mochitest-a11y assertions. https://hg.mozilla.org/integration/mozilla-inbound/rev/fb572a342efa https://tbpl.mozilla.org/php/getParsedLog.php?id=20189591&tree=Mozilla-Inbound A quick skim of the log shows them to be of the type: ###!!! ASSERTION: bad size recorded: 'aInstanceSize == 0 || entry->GetClassSize() == aInstanceSize', file ../../../xpcom/base/nsTraceRefcntImpl.cpp, line 441

That wasn't due to this patch. Repushed: https://hg.mozilla.org/integration/mozilla-inbound/rev/38578014b8a8