Improve error formatting

Changes:

* The divider lines are gone. These were in practice a bit confusing,
  in particular with --show-trace or --keep-going, since then there
  were multiple lines, suggesting a start/end which wasn't the case.

* Instead, multi-line error messages are now indented to align with
  the prefix (e.g. "error: ").

* The 'description' field is gone since we weren't really using it.

* 'hint' is renamed to 'msg' since it really wasn't a hint.

* The error is now printed *before* the location info.

* The 'name' field is no longer printed since most of the time it
  wasn't very useful since it was just the name of the exception (like
  EvalError). Ideally in the future this would be a unique, easily
  googleable error ID (like rustc).

* "trace:" is now just "…". This assumes error contexts start with
  something like "while doing X".

Example before:

  error: --- AssertionError ---------------------------------------------------------------------------------------- nix
  at: (7:7) in file: /home/eelco/Dev/nixpkgs/pkgs/applications/misc/hello/default.nix

       6|
       7|   x = assert false; 1;
        |       ^
       8|

  assertion 'false' failed
  ----------------------------------------------------- show-trace -----------------------------------------------------
  trace: while evaluating the attribute 'x' of the derivation 'hello-2.10'
  at: (192:11) in file: /home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/make-derivation.nix

     191|         // (lib.optionalAttrs (!(attrs ? name) && attrs ? pname && attrs ? version)) {
     192|           name = "${attrs.pname}-${attrs.version}";
        |           ^
     193|         } // (lib.optionalAttrs (stdenv.hostPlatform != stdenv.buildPlatform && !dontAddHostSuffix && (attrs ? name || (attrs ? pname && attrs ? version)))) {

Example after:

  error: assertion 'false' failed

         at: (7:7) in file: /home/eelco/Dev/nixpkgs/pkgs/applications/misc/hello/default.nix

              6|
              7|   x = assert false; 1;
               |       ^
              8|

         … while evaluating the attribute 'x' of the derivation 'hello-2.10'

         at: (192:11) in file: /home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/make-derivation.nix

            191|         // (lib.optionalAttrs (!(attrs ? name) && attrs ? pname && attrs ? version)) {
            192|           name = "${attrs.pname}-${attrs.version}";
               |           ^
            193|         } // (lib.optionalAttrs (stdenv.hostPlatform != stdenv.buildPlatform && !dontAddHostSuffix && (attrs ? name || (attrs ? pname && attrs ? version)))) {

1 files changed, 5 insertions, 15 deletions

diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 9963f05d9..d6a16999f 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -124,10 +124,7 @@ static void getAllExprs(EvalState & state,
             if (hasSuffix(attrName, ".nix"))
                 attrName = string(attrName, 0, attrName.size() - 4);
             if (!attrs.insert(attrName).second) {
-                logError({
-                    .name = "Name collision",
-                    .hint = hintfmt("warning: name collision in input Nix expressions, skipping '%1%'", path2)
-                });
+                printError("warning: name collision in input Nix expressions, skipping '%1%'", path2);
                 continue;
             }
             /* Load the expression on demand. */
@@ -876,11 +873,7 @@ static void queryJSON(Globals & globals, vector<DrvInfo> & elems)
             auto placeholder = metaObj.placeholder(j);
             Value * v = i.queryMeta(j);
             if (!v) {
-                logError({
-                    .name = "Invalid meta attribute",
-                    .hint = hintfmt("derivation '%s' has invalid meta attribute '%s'",
-                        i.queryName(), j)
-                });
+                printError("derivation '%s' has invalid meta attribute '%s'", i.queryName(), j);
                 placeholder.write(nullptr);
             } else {
                 PathSet context;
@@ -1131,12 +1124,9 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
                             attrs2["name"] = j;
                             Value * v = i.queryMeta(j);
                             if (!v)
-                                logError({
-                                    .name = "Invalid meta attribute",
-                                    .hint = hintfmt(
-                                        "derivation '%s' has invalid meta attribute '%s'",
-                                        i.queryName(), j)
-                                });
+                                printError(
+                                    "derivation '%s' has invalid meta attribute '%s'",
+                                    i.queryName(), j);
                             else {
                                 if (v->type() == nString) {
                                     attrs2["type"] = "string";