[XeTeX] Active dash or interchartoks?

[Joel C. Salomon]

I just came across this thread from about 15 months back, entitled
“Conflict between xunicode and fontspec?”, where Will Robertson wrote:
> On a final note, Arthur wrote:
> 
>> Of course, active characters have always had a
>> rather bad reputation in TeX
> 
> Only when they're not expected :) For example, no-one has a problem with
> the active character ~.
<snip>
> For example, it would be nice to make ` active so you can write `quoted
> text` and have the quotes render correctly and check their nesting (e.g.
> see the csquotes package). But then you'd better be really careful about
> writing code like \catcode`\x !
> 
> In short, characters can be made active to enforce their unicode
> meaning. I even make — (emdash) active so it inserts the right about of
> (breaking and no-breaking) space around it; although interchartoks will
> soon make this the wrong way of doing that sort of thing.

I’m intrigued.  I’ve had this code in my preamble for a while (don’t
recall who gave it to me):
	\DeclareRobustCommand\dash{%
		\ifvmode\leavevmode\else\unskip\nobreak\thinspace\fi
		\textemdash\thinspace\ignorespaces
	}
	\catcode`\—=\active
	\let—=\dash
What are interchartoks, and how would I do this with them?

—Joel Salomon