Setting SMALL CAPS font protrusion in microtype for Charis SIL?

Question

I am trying to enable font protrusion for Charis SIL in microtype when typesetting with the XeTeX engine. The mycrotype manual indicates that it has protrusion settings for Charis SIL, but only in the EU1 and EU2 encodings. Per the answer from Alan Munn, fontenc automatically uses the EU1 encoding, so I don't need to do anything special to get characters in Charis SIL to protrude.

Does microtype have protrusion settings for Charis SIL small caps as well? I am getting a string of warnings when I typeset to the effect of "Unknown slot number of character ____ in font encoding EU1 in protrusion list Charis-sc." This error only shows up when my source has small capitals. Here is a minimal example that produces the errors:

\documentclass[12pt,draft]{memoir}
\usepackage{fontspec}
\setmainfont{Charis SIL}[StylisticSet=1]
\usepackage[final, expansion = false]{microtype}
\begin{document}
\section{Introduction}

Now a sentence, with, many; types --- of punctuation!  Can, you believe it?  I am making: words.  And more... And more.

But if I decide to \textsc{nasa} me around, I disagree.

\end{document}

Answer 1

The small caps protrusion settings for Charis SIL are specified by referring to the glyph names, which is usually the most stable way. At some point, however, the developers of Charis SIL have renamed all small caps glyphs from, say, a.SC to a.sc:

(left: version 4.106, right: version 5.000)

EDIT: As of version 2.7, microtype will automatically choose the right names both with luatex and xetex, so it is no longer necessary to fix the settings manually (in mt-CharisSIL.cfg).

Answer 2

The EU1 encoding is the standard encoding used by fontspec with XeTeX. As of 2016 there is an experimental TU encoding which will be used, but at time of writing, this encoding is enabled only by a package option [tuenc], although should become the default. The microtype package already supports this encoding too. So, you should not need to do anything special:

\documentclass{article}
\usepackage{fontspec}
\usepackage[protrusion=true]{microtype}
\setmainfont{Charis SIL}
\makeatletter
\newcommand*{\showenc}{The encoding is \f@encoding.\par}
\makeatother
\begin{document}
\showenc
\end{document}

There are some protrusion settings in the mt-CharisSIL.cfg file for small caps, and your document produces no warnings or errors using microtype 2016/05/14 v2.6a (when I remove the [StylisticSet=1] from the \setmainfont command.)