Fix 480937a103708a651/lyxgit, second attempt.

Prevent encoding changes whenever the TeX engine is XeTeX or LuaTeX,
as XeTeX/LuaTeX use only one encoding per document:

* with useNonTeXFonts: "utf8plain",
* with XeTeX and TeX fonts: "ascii" (inputenc fails),
* with LuaTeX and TeX fonts: only one encoding accepted by luainputenc.

+1 no needless encoding switches
+1 runparams.encoding matches the correct encoding at any time
+1 less complicated code.

-1 there may still be problems with CJK (possibly impossible to
   solve for Xe/LuaTeX with TeX fonts).

For LuaTeX & TeX fonts, the complete document uses the encoding
of the global document language.

See also #9740.

src/BufferParams.cpp

@@ -2941,15 +2941,13 @@ docstring BufferParams::getGraphicsDriver(string const & package) const
 void BufferParams::writeEncodingPreamble(otexstream & os,
 					 LaTeXFeatures & features) const
 {
-	// "inputenc" package not required with non-TeX fonts.
-	if (useNonTeXFonts)
+	// XeTeX/LuaTeX: (see also #9740)
+	// With Unicode fonts we use utf8-plain without encoding package.
+	// With TeX fonts, we cannot use utf8-plain, but "inputenc" fails.
+	// XeTeX must use ASCII encoding, for LuaTeX, we load
+	// "luainputenc" (see below).
+	if (useNonTeXFonts || features.runparams().flavor == OutputParams::XETEX)
 		return;
-	// "inputenc"  fails with XeTeX (even in 8-bit compatiblitly mode) and with TeX fonts,
-	// (this is a bug in the "inputenc" package see #9740).
-	if (features.runparams().flavor == OutputParams::XETEX)
-		return;
-	// For LuaTeX with TeX fonts, we can load
-	// the "luainputenc" package with the specified encoding(s) (see below).
 
 	if (inputenc == "auto") {
 		string const doc_encoding =
@@ -2957,10 +2955,12 @@ void BufferParams::writeEncodingPreamble(otexstream & os,
 		Encoding::Package const package =
 			language->encoding()->package();
 
-		// Create a list with all the input encodings used
-		// in the document
-		set<string> encodings =
-			features.getEncodingSet(doc_encoding);
+		// Create list of inputenc options:
+		set<string> encodings;
+		// luainputenc fails with more than one encoding
+		if (!features.runparams().isFullUnicode()) // if we reach this point, this means LuaTeX with TeX fonts
+			// list all input encodings used in the document
+			encodings = features.getEncodingSet(doc_encoding);
 
 		// If the "japanese" package (i.e. pLaTeX) is used,
 		// inputenc must be omitted.

src/Font.cpp

@@ -327,6 +327,7 @@ int Font::latexWriteStartChanges(odocstream & os, BufferParams const & bparams,
 		}
 	}
 
+	// FIXME: skip this for XeTeX/LuaTeX? With nonTeXfonts or always?
 	if (language()->encoding()->package() == Encoding::CJK) {
 		pair<bool, int> const c = switchEncoding(os, bparams,
 				runparams, *(language()->encoding()));

src/LaTeXFeatures.cpp

@@ -711,8 +711,9 @@ set<string> LaTeXFeatures::getEncodingSet(string const & doc_encoding) const
 
 void LaTeXFeatures::getFontEncodings(vector<string> & encodings) const
 {
-	// these must be loaded if glyphs of this script
-	// are used (notwithstanding the language)
+	// these must be loaded if glyphs of this script are used
+	// unless a language providing them is used in the document
+	// FIXME: currently the option is written twice in this case
 	if (mustProvide("textgreek"))
 		encodings.insert(encodings.begin(), "LGR");
 	if (mustProvide("textcyr"))

src/Paragraph.cpp

@@ -2567,8 +2567,7 @@ void Paragraph::latex(BufferParams const & bparams,
 	if (allowcust && d->endTeXParParams(bparams, os, runparams)
 	    && runparams.encoding != prev_encoding) {
 		runparams.encoding = prev_encoding;
-		if (runparams.flavor != OutputParams::XETEX) // see BufferParams::encoding
-			os << setEncoding(prev_encoding->iconvName());
+		os << setEncoding(prev_encoding->iconvName());
 	}
 
 	LYXERR(Debug::LATEX, "Paragraph::latex... done " << this);

src/output_latex.cpp

@@ -249,8 +249,7 @@ static void finishEnvironment(otexstream & os, OutputParams const & runparams,
 		state->prev_env_language_ = data.par_language;
 		if (runparams.encoding != data.prev_encoding) {
 			runparams.encoding = data.prev_encoding;
-			if (runparams.flavor != OutputParams::XETEX) // see BufferParams::encoding
-				os << setEncoding(data.prev_encoding->iconvName());
+			os << setEncoding(data.prev_encoding->iconvName());
 		}
 	}
 
@@ -259,8 +258,7 @@ static void finishEnvironment(otexstream & os, OutputParams const & runparams,
 		state->prev_env_language_ = data.par_language;
 		if (runparams.encoding != data.prev_encoding) {
 			runparams.encoding = data.prev_encoding;
-			if (runparams.flavor != OutputParams::XETEX) // see BufferParams::encoding
-				os << setEncoding(data.prev_encoding->iconvName());
+			os << setEncoding(data.prev_encoding->iconvName());
 		}
 	}
 
@@ -772,6 +770,7 @@ void TeXOnePar(Buffer const & buf,
 	// encoding, since this only affects the position of the outputted
 	// \inputencoding command; the encoding switch will occur when necessary
 	if (bparams.inputenc == "auto"
+		&& !runparams.isFullUnicode() // Xe/LuaTeX use one document-wide encoding  (see also switchEncoding())
 		&& runparams.encoding->package() != Encoding::none) {
 		// Look ahead for future encoding changes.
 		// We try to output them at the beginning of the paragraph,
@@ -884,8 +883,7 @@ void TeXOnePar(Buffer const & buf,
 			latexArgInsets(par, os, runparams, style.postcommandargs(), "post:");
 		if (runparams.encoding != prev_encoding) {
 			runparams.encoding = prev_encoding;
-			if (runparams.flavor != OutputParams::XETEX) // see BufferParams::encoding
-				os << setEncoding(prev_encoding->iconvName());
+			os << setEncoding(prev_encoding->iconvName());
 		}
 	}
 
@@ -1043,13 +1041,13 @@ void TeXOnePar(Buffer const & buf,
 	}
 
 	// If this is the last paragraph, and a local_font was set upon entering
-	// the inset, and we're using "auto" or "default" encoding, the encoding
+	// the inset, and we're using "auto" or "default" encoding, and not
+	// compiling with XeTeX or LuaTeX, the encoding
 	// should be set back to that local_font's encoding.
-	// However, do not change the encoding when non-TeX fonts are used.
 	if (runparams.isLastPar && runparams_in.local_font != 0
 	    && runparams_in.encoding != runparams_in.local_font->language()->encoding()
 	    && (bparams.inputenc == "auto" || bparams.inputenc == "default")
-		&& runparams.flavor != OutputParams::XETEX // see BufferParams::encoding
+		&& !runparams.isFullUnicode()
 	   ) {
 		runparams_in.encoding = runparams_in.local_font->language()->encoding();
 		os << setEncoding(runparams_in.encoding->iconvName());
@@ -1288,6 +1286,13 @@ pair<bool, int> switchEncoding(odocstream & os, BufferParams const & bparams,
 		   OutputParams const & runparams, Encoding const & newEnc,
 		   bool force)
 {
+	// XeTeX/LuaTeX use only one encoding per document:
+	// * with useNonTeXFonts: "utf8plain",
+	// * with XeTeX and TeX fonts: "ascii" (inputenc fails),
+	// * with LuaTeX and TeX fonts: only one encoding accepted by luainputenc.
+	if (runparams.isFullUnicode())
+		return make_pair(false, 0);
+
 	Encoding const & oldEnc = *runparams.encoding;
 	bool moving_arg = runparams.moving_arg;
 	// If we switch from/to CJK, we need to switch anyway, despite custom inputenc