Panic within syntax extension triggers ICE

[aochagavia]

This code ICEs when the function in the crate x, marked with #[plugin_registrar] panics:

/// Crate x
#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    panic!()
}

/// Another crate
#[phase(plugin)] extern crate x;

This code ICEs when the macro expansion fails

/// Crate x
#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_macro("wrong", expand);
}

fn expand(cx: &mut ExtCtxt, sp: codemap::Span, tts: &[ast::TokenTree]) -> Box<MacResult + 'static> {
    panic!()
}

/// Another crate
wrong!()

It would be nice to tell the user that the plugin registrar has panicked or that the macro expansion has panicked, respectively.

If there is a simple way to "catch" a panic! I would like to hear about it and fix this.

[steveklabnik]

If there is a simple way to "catch" a fail! I would like to hear about it and fix this.

There is no way to catch a panic!, by design. It ends the current Thread. So, the only way to fix this kind of thing would be to run each plugin in its own thread...

[kmcallister]

Which would require major changes throughout rustc, as I understand it.

We could instead have a global flag that tells the existing panic handler what context rustc was in at the time of panic, e.g. built-in code or the name of a plugin. We'd print this in the message and maybe use it to be more equivocal about whether to report the bug (kind of like Linux's taint bits).

I'm going to close this as a subissue of #15509. Please yell or reopen if you disagree.