Print top-level errors normally in `nix repl`

Previously, errors while printing values in `nix repl` would be printed
in `«error: ...»` brackets rather than displayed normally:

```
nix-repl> legacyPackages.aarch64-darwin.pythonPackages.APScheduler
«error: Package ‘python-2.7.18.7’ in /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:335 is marked as insecure, refusing to evaluate.»
```

Now, errors will be displayed normally if they're emitted at the
top-level of an expression:

```
nix-repl> legacyPackages.aarch64-darwin.pythonPackages.APScheduler
error:
       … in the condition of the assert statement
         at /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/lib/customisation.nix:268:17:
          267|     in commonAttrs // {
          268|       drvPath = assert condition; drv.drvPath;
             |                 ^
          269|       outPath = assert condition; drv.outPath;

       … in the left operand of the OR (||) operator
         at /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/passthrufun.nix:28:45:
           27|         if lib.isDerivation value then
           28|           lib.extendDerivation (valid value || throw "${name} should use `buildPythonPackage` or `toPythonModule` if it is to be part of the Python packages set.") {} value
             |                                             ^
           29|         else

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: Package ‘python-2.7.18.7’ in /nix/store/6s0m1qc31zw3l3kq0q4wd5cp3lqpkq0q-source/pkgs/development/interpreters/python/cpython/2.7/default.nix:335 is marked as insecure, refusing to evaluate.
```

Errors emitted in nested structures (like e.g. when printing `nixpkgs`)
will still be printed in brackets.

Change-Id: I25aeddf08c017582718cb9772a677bf51b9fc2ad

2 files changed, 28 insertions, 0 deletions

diff --git a/tests/functional/repl_characterization/data/errors.test b/tests/functional/repl_characterization/data/errors.test
new file mode 100644
index 000000000..47d7e7e13
--- /dev/null
+++ b/tests/functional/repl_characterization/data/errors.test
@@ -0,0 +1,27 @@
+Errors at the top of an expression are printed normally:
+
+  nix-repl> builtins.throw "Evil puppy detected!!!"
+  error:
+         … while calling the 'throw' builtin
+           at «string»:1:1:
+              1| builtins.throw "Evil puppy detected!!!"
+               | ^
+
+         error: Evil puppy detected!!!
+
+Errors in attribute values are printed inline, to make it easier to explore
+values like nixpkgs where some parts of the value fail to evaluate:
+
+  nix-repl> { puppy = builtins.throw "This puppy is EVIL!!!"; puppy2 = "This puppy is GOOD :)"; }
+  {
+    puppy = «error: This puppy is EVIL!!!»;
+    puppy2 = "This puppy is GOOD :)";
+  }
+
+Same for list values:
+
+  nix-repl> [ (builtins.throw "This puppy is EVIL!!!") ("This puppy is GOOD :)") ]
+  [
+    «error: This puppy is EVIL!!!»
+    "This puppy is GOOD :)"
+  ]
diff --git a/tests/functional/repl_characterization/repl_characterization.cc b/tests/functional/repl_characterization/repl_characterization.cc
index f84bbdaae..0d3e5352d 100644
--- a/tests/functional/repl_characterization/repl_characterization.cc
+++ b/tests/functional/repl_characterization/repl_characterization.cc
@@ -186,5 +186,6 @@ REPL_TEST(repl_overlays_destructure_without_formals_ok);
 REPL_TEST(repl_overlays_error);
 REPL_TEST(repl_printing);
 REPL_TEST(stack_vars);
+REPL_TEST(errors);
 
 }; // namespace nix