Unify and refactor value printing

Previously, there were two mostly-identical value printers -- one in
`libexpr/eval.cc` (which didn't force values) and one in
`libcmd/repl.cc` (which did force values and also printed ANSI color
codes).

This PR unifies both of these printers into `print.cc` and provides a
`PrintOptions` struct for controlling the output, which allows for
toggling whether values are forced, whether repeated values are tracked,
and whether ANSI color codes are displayed.

Additionally, `PrintOptions` allows tuning the maximum number of
attributes, list items, and bytes in a string that will be displayed;
this makes it ideal for contexts where printing too much output (e.g.
all of Nixpkgs) is distracting. (As requested by @roberth in
https://github.com/NixOS/nix/pull/9554#issuecomment-1845095735)

Please read the tests for example output.

Future work:
- It would be nice to provide this function as a builtin, perhaps
  `builtins.toStringDebug` -- a printing function that never fails would
  be useful when debugging Nix code.
- It would be nice to support customizing `PrintOptions` members on the
  command line, e.g. `--option to-string-max-attrs 1000`.

(cherry picked from commit 0fa08b451682fb3311fe58112ff05c4fe5bee3a4, )

===

Restore ambiguous value printer for `nix-instantiate`

The Nix team has requested that this output format remain unchanged.
I've added a warning to the man page explaining that `nix-instantiate
--eval` output will not parse correctly in many situations.

(cherry picked from commit df84dd4d8dd3fd6381ac2ca3064432ab31a16b79)

Change-Id: I7cca6b4b53cd0642f2d49af657d5676a8554c9f8

1 files changed, 6 insertions, 0 deletions

diff --git a/src/libexpr/print.hh b/src/libexpr/print.hh
index abf830864..40207d777 100644
--- a/src/libexpr/print.hh
+++ b/src/libexpr/print.hh
@@ -9,6 +9,9 @@
 
 #include <iostream>
 
+#include "eval.hh"
+#include "print-options.hh"
+
 namespace nix {
 
 /**
@@ -16,6 +19,7 @@ namespace nix {
  *
  * 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);
@@ -53,4 +57,6 @@ bool isReservedKeyword(const std::string_view str);
  */
 std::ostream & printIdentifier(std::ostream & o, std::string_view s);
 
+void printValue(EvalState & state, std::ostream & str, Value & v, PrintOptions options = PrintOptions {});
+
 }