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, 59 insertions, 21 deletions

diff --git a/doc/manual/src/command-ref/nix-instantiate.md b/doc/manual/src/command-ref/nix-instantiate.md
index e1b4a3e80..483150aa8 100644
--- a/doc/manual/src/command-ref/nix-instantiate.md
+++ b/doc/manual/src/command-ref/nix-instantiate.md
@@ -35,13 +35,50 @@ standard input.
 
   - `--parse`\
     Just parse the input files, and print their abstract syntax trees on
-    standard output in ATerm format.
+    standard output as a Nix expression.
 
   - `--eval`\
     Just parse and evaluate the input files, and print the resulting
     values on standard output. No instantiation of store derivations
     takes place.
 
+    > **Warning**
+    >
+    > This option produces ambiguous output which is not suitable for machine
+    > consumption. For example, these two Nix expressions print the same result
+    > despite having different types:
+    >
+    > ```console
+    > $ nix-instantiate --eval --expr '{ a = {}; }'
+    > { a = <CODE>; }
+    > $ nix-instantiate --eval --expr '{ a = <CODE>; }'
+    > { a = <CODE>; }
+    > ```
+    >
+    > For human-readable output, `nix eval` (experimental) is more informative:
+    >
+    > ```console
+    > $ nix-instantiate --eval --expr 'a: a'
+    > <LAMBDA>
+    > $ nix eval --expr 'a: a'
+    > «lambda @ «string»:1:1»
+    > ```
+    >
+    > For machine-readable output, the `--xml` option produces unambiguous
+    > output:
+    >
+    > ```console
+    > $ nix-instantiate --eval --xml --expr '{ foo = <CODE>; }'
+    > <?xml version='1.0' encoding='utf-8'?>
+    > <expr>
+    >   <attrs>
+    >     <attr column="3" line="1" name="foo">
+    >       <unevaluated />
+    >     </attr>
+    >   </attrs>
+    > </expr>
+    > ```
+
   - `--find-file`\
     Look up the given files in Nix’s search path (as specified by the
     `NIX_PATH` environment variable). If found, print the corresponding
@@ -61,11 +98,11 @@ standard input.
 
   - `--json`\
     When used with `--eval`, print the resulting value as an JSON
-    representation of the abstract syntax tree rather than as an ATerm.
+    representation of the abstract syntax tree rather than as a Nix expression.
 
   - `--xml`\
     When used with `--eval`, print the resulting value as an XML
-    representation of the abstract syntax tree rather than as an ATerm.
+    representation of the abstract syntax tree rather than as a Nix expression.
     The schema is the same as that used by the [`toXML`
     built-in](../language/builtins.md).
 
@@ -133,28 +170,29 @@ $ nix-instantiate --eval --xml --expr '1 + 2'
 The difference between non-strict and strict evaluation:
 
 ```console
-$ nix-instantiate --eval --xml --expr 'rec { x = "foo"; y = x; }'
-...
-  <attr name="x">
-    <string value="foo" />
-  </attr>
-  <attr name="y">
-    <unevaluated />
-  </attr>
-...
+$ nix-instantiate --eval --xml --expr '{ x = {}; }'
+<?xml version='1.0' encoding='utf-8'?>
+<expr>
+  <attrs>
+    <attr column="3" line="1" name="x">
+      <unevaluated />
+    </attr>
+  </attrs>
+</expr>
 ```
 
 Note that `y` is left unevaluated (the XML representation doesn’t
 attempt to show non-normal forms).
 
 ```console
-$ nix-instantiate --eval --xml --strict --expr 'rec { x = "foo"; y = x; }'
-...
-  <attr name="x">
-    <string value="foo" />
-  </attr>
-  <attr name="y">
-    <string value="foo" />
-  </attr>
-...
+$ nix-instantiate --eval --xml --strict --expr '{ x = {}; }'
+<?xml version='1.0' encoding='utf-8'?>
+<expr>
+  <attrs>
+    <attr column="3" line="1" name="x">
+      <attrs>
+      </attrs>
+    </attr>
+  </attrs>
+</expr>
 ```