song/{Tag,Uri}SongFilter: use libfmt in ToExpression()

Author: MaxKellermann

src/song/TagSongFilter.cxx

@@ -8,15 +8,20 @@
 #include "tag/Tag.hxx"
 #include "tag/Fallback.hxx"
 
+#include <fmt/core.h>
+
+using std::string_view_literals::operator""sv;
+
 std::string
 TagSongFilter::ToExpression() const noexcept
 {
 	const char *name = type == TAG_NUM_OF_ITEM_TYPES
 		? "any"
 		: tag_item_names[type];
 
-	return std::string("(") + name + " " + filter.GetOperator()
-		+ " \"" + EscapeFilterString(filter.GetValue()) + "\")";
+	return fmt::format("({} {} \"{}\")"sv,
+			   name, filter.GetOperator(),
+			   EscapeFilterString(filter.GetValue()));
 }
 
 bool

src/song/UriSongFilter.cxx

@@ -5,11 +5,15 @@
 #include "Escape.hxx"
 #include "LightSong.hxx"
 
+#include <fmt/core.h>
+
+using std::string_view_literals::operator""sv;
+
 std::string
 UriSongFilter::ToExpression() const noexcept
 {
-	return std::string("(file ") + filter.GetOperator()
-		+ " \"" + EscapeFilterString(filter.GetValue()) + "\")";
+	return fmt::format("(file {} \"{}\")"sv,
+			   filter.GetOperator(), EscapeFilterString(filter.GetValue()));
 }
 
 bool