Move /src to /subprojects

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

subprojects/libexpr/print-options.hh

@@ -0,0 +1,120 @@
+#pragma once
+/**
+ * @file
+ * @brief Options for printing Nix values.
+ */
+
+#include <limits>
+
+namespace nix {
+
+/**
+ * How errors should be handled when printing values.
+ */
+enum class ErrorPrintBehavior {
+    /**
+     * Print the first line of the error in brackets: `«error: oh no!»`
+     */
+    Print,
+    /**
+     * Throw the error to the code that attempted to print the value, instead
+     * of suppressing it it.
+     */
+    Throw,
+    /**
+     * Only throw the error if encountered at the top level of the expression.
+     *
+     * This will cause expressions like `builtins.throw "uh oh!"` to throw
+     * errors, but will print attribute sets and other nested structures
+     * containing values that error (like `nixpkgs`) normally.
+     */
+    ThrowTopLevel,
+};
+
+/**
+ * Options for printing Nix values.
+ */
+struct PrintOptions
+{
+    /**
+     * If true, output ANSI color sequences.
+     */
+    bool ansiColors = false;
+
+    /**
+     * If true, force values.
+     */
+    bool force = false;
+
+    /**
+     * If true and `force` is set, print derivations as
+     * `«derivation /nix/store/...»` instead of as attribute sets.
+     */
+    bool derivationPaths = false;
+
+    /**
+     * If true, track which values have been printed and skip them on
+     * subsequent encounters. Useful for self-referential values.
+     */
+    bool trackRepeated = true;
+
+    /**
+     * Maximum depth to evaluate to.
+     */
+    size_t maxDepth = std::numeric_limits<size_t>::max();
+
+    /**
+     * Maximum number of attributes in attribute sets to print.
+     *
+     * Note that this is a limit for the entire print invocation, not for each
+     * attribute set encountered.
+     */
+    size_t maxAttrs = std::numeric_limits<size_t>::max();
+
+    /**
+     * Maximum number of list items to print.
+     *
+     * Note that this is a limit for the entire print invocation, not for each
+     * list encountered.
+     */
+    size_t maxListItems = std::numeric_limits<size_t>::max();
+
+    /**
+     * Maximum string length to print.
+     */
+    size_t maxStringLength = std::numeric_limits<size_t>::max();
+
+    /**
+     * Indentation width for pretty-printing.
+     *
+     * If set to 0 (the default), values are not pretty-printed.
+     */
+    size_t prettyIndent = 0;
+
+    /**
+     * How to handle errors encountered while printing values.
+     */
+    ErrorPrintBehavior errors = ErrorPrintBehavior::Print;
+
+    /**
+     * True if pretty-printing is enabled.
+     */
+    inline bool shouldPrettyPrint()
+    {
+        return prettyIndent > 0;
+    }
+};
+
+/**
+ * `PrintOptions` for unknown and therefore potentially large values in error messages,
+ * to avoid printing "too much" output.
+ */
+static PrintOptions errorPrintOptions = PrintOptions {
+    .ansiColors = true,
+    .maxDepth = 10,
+    .maxAttrs = 10,
+    .maxListItems = 10,
+    .maxStringLength = 1024,
+};
+
+}