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, 24 insertions, 0 deletions

diff --git a/src/libexpr/print-ambiguous.hh b/src/libexpr/print-ambiguous.hh
new file mode 100644
index 000000000..50c260a9b
--- /dev/null
+++ b/src/libexpr/print-ambiguous.hh
@@ -0,0 +1,24 @@
+#pragma once
+
+#include "value.hh"
+
+namespace nix {
+
+/**
+ * Print a value in the deprecated format used by `nix-instantiate --eval` and
+ * `nix-env` (for manifests).
+ *
+ * This output can't be changed because it's part of the `nix-instantiate` API,
+ * but it produces ambiguous output; unevaluated thunks and lambdas (and a few
+ * other types) are printed as Nix path syntax like `<CODE>`.
+ *
+ * See: https://github.com/NixOS/nix/issues/9730
+ */
+void printAmbiguous(
+    Value &v,
+    const SymbolTable &symbols,
+    std::ostream &str,
+    std::set<const void *> *seen,
+    int depth);
+
+}