Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libexpr/print.hh

@@ -0,0 +1,91 @@
+#pragma once
+/**
+ * @file
+ * @brief Common printing functions for the Nix language
+ *
+ * While most types come with their own methods for printing, they share some
+ * functions that are placed here.
+ */
+
+#include <iostream>
+
+#include "fmt.hh"
+#include "print-options.hh"
+
+namespace nix {
+
+class EvalState;
+struct Value;
+
+/**
+ * Print a string as a Nix string literal.
+ *
+ * Quotes and fairly minimal escaping are added.
+ *
+ * @param o The output stream to print to
+ * @param s The logical string
+ */
+std::ostream & printLiteralString(std::ostream & o, std::string_view s);
+inline std::ostream & printLiteralString(std::ostream & o, const char * s) {
+    return printLiteralString(o, std::string_view(s));
+}
+inline std::ostream & printLiteralString(std::ostream & o, const std::string & s) {
+    return printLiteralString(o, std::string_view(s));
+}
+
+/** Print `true` or `false`. */
+std::ostream & printLiteralBool(std::ostream & o, bool b);
+
+/**
+ * Print a string as an attribute name in the Nix expression language syntax.
+ *
+ * Prints a quoted string if necessary.
+ */
+std::ostream & printAttributeName(std::ostream & o, std::string_view s);
+
+/**
+ * Returns `true' is a string is a reserved keyword which requires quotation
+ * when printing attribute set field names.
+ */
+bool isReservedKeyword(const std::string_view str);
+
+/**
+ * Print a string as an identifier in the Nix expression language syntax.
+ *
+ * FIXME: "identifier" is ambiguous. Identifiers do not have a single
+ *        textual representation. They can be used in variable references,
+ *        let bindings, left-hand sides or attribute names in a select
+ *        expression, or something else entirely, like JSON. Use one of the
+ *        `print*` functions instead.
+ */
+std::ostream & printIdentifier(std::ostream & o, std::string_view s);
+
+void printValue(EvalState & state, std::ostream & str, Value & v, PrintOptions options = PrintOptions {});
+
+/**
+ * A partially-applied form of `printValue` which can be formatted using `<<`
+ * without allocating an intermediate string.
+ */
+class ValuePrinter {
+    friend std::ostream & operator << (std::ostream & output, const ValuePrinter & printer);
+private:
+    EvalState & state;
+    Value & value;
+    PrintOptions options;
+
+public:
+    ValuePrinter(EvalState & state, Value & value, PrintOptions options = PrintOptions {})
+        : state(state), value(value), options(options) { }
+};
+
+std::ostream & operator<<(std::ostream & output, const ValuePrinter & printer);
+
+
+/**
+ * `ValuePrinter` does its own ANSI formatting, so we don't color it
+ * magenta.
+ */
+template<>
+HintFmt & HintFmt::operator%(const ValuePrinter & value);
+
+}