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, 56 insertions, 115 deletions

diff --git a/src/libutil/error.cc b/src/libutil/error.cc
index e7dc3f1d3..bc5f9e440 100644
--- a/src/libutil/error.cc
+++ b/src/libutil/error.cc
@@ -204,168 +204,109 @@ void printAtPos(const string & prefix, const ErrPos & pos, std::ostream & out)
     }
 }
 
-std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool showTrace)
+static std::string indent(std::string_view indentFirst, std::string_view indentRest, std::string_view s)
 {
-    auto errwidth = std::max<size_t>(getWindowSize().second, 20);
-    string prefix = "";
+    std::string res;
+    bool first = true;
+
+    while (!s.empty()) {
+        auto end = s.find('\n');
+        if (!first) res += "\n";
+        res += first ? indentFirst : indentRest;
+        res += s.substr(0, end);
+        first = false;
+        if (end == s.npos) break;
+        s = s.substr(end + 1);
+    }
+
+    return res;
+}
 
-    string levelString;
+std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool showTrace)
+{
+    std::string prefix;
     switch (einfo.level) {
         case Verbosity::lvlError: {
-            levelString = ANSI_RED;
-            levelString += "error:";
-            levelString += ANSI_NORMAL;
+            prefix = ANSI_RED "error";
+            break;
+        }
+        case Verbosity::lvlNotice: {
+            prefix = ANSI_RED "note";
             break;
         }
         case Verbosity::lvlWarn: {
-            levelString = ANSI_YELLOW;
-            levelString += "warning:";
-            levelString += ANSI_NORMAL;
+            prefix = ANSI_YELLOW "warning";
             break;
         }
         case Verbosity::lvlInfo: {
-            levelString = ANSI_GREEN;
-            levelString += "info:";
-            levelString += ANSI_NORMAL;
+            prefix = ANSI_GREEN "info";
             break;
         }
         case Verbosity::lvlTalkative: {
-            levelString = ANSI_GREEN;
-            levelString += "talk:";
-            levelString += ANSI_NORMAL;
+            prefix = ANSI_GREEN "talk";
             break;
         }
         case Verbosity::lvlChatty: {
-            levelString = ANSI_GREEN;
-            levelString += "chat:";
-            levelString += ANSI_NORMAL;
+            prefix = ANSI_GREEN "chat";
             break;
         }
         case Verbosity::lvlVomit: {
-            levelString = ANSI_GREEN;
-            levelString += "vomit:";
-            levelString += ANSI_NORMAL;
+            prefix = ANSI_GREEN "vomit";
             break;
         }
         case Verbosity::lvlDebug: {
-            levelString = ANSI_YELLOW;
-            levelString += "debug:";
-            levelString += ANSI_NORMAL;
-            break;
-        }
-        default: {
-            levelString = fmt("invalid error level: %1%", einfo.level);
+            prefix = ANSI_YELLOW "debug";
             break;
         }
+        default:
+            assert(false);
     }
 
-    auto ndl = prefix.length()
-        + filterANSIEscapes(levelString, true).length()
-        + 7
-        + einfo.name.length()
-        + einfo.programName.value_or("").length();
-    auto dashwidth = std::max<int>(errwidth - ndl, 3);
-
-    std::string dashes(dashwidth, '-');
-
-    // divider.
-    if (einfo.name != "")
-        out << fmt("%1%%2%" ANSI_BLUE " --- %3% %4% %5%" ANSI_NORMAL,
-            prefix,
-            levelString,
-            einfo.name,
-            dashes,
-            einfo.programName.value_or(""));
+    // FIXME: show the program name as part of the trace?
+    if (einfo.programName && einfo.programName != ErrorInfo::programName)
+        prefix += fmt(" [%s]:" ANSI_NORMAL " ", einfo.programName.value_or(""));
     else
-        out << fmt("%1%%2%" ANSI_BLUE " -----%3% %4%" ANSI_NORMAL,
-            prefix,
-            levelString,
-            dashes,
-            einfo.programName.value_or(""));
-
-    bool nl = false;  // intersperse newline between sections.
-    if (einfo.errPos.has_value() && (*einfo.errPos)) {
-        out << prefix << std::endl;
-        printAtPos(prefix, *einfo.errPos, out);
-        nl = true;
-    }
+        prefix += ":" ANSI_NORMAL " ";
 
-    // description
-    if (einfo.description != "") {
-        if (nl)
-            out << std::endl << prefix;
-        out << std::endl << prefix << einfo.description;
-        nl = true;
-    }
+    std::ostringstream oss;
+    oss << einfo.msg << "\n";
+
+    if (einfo.errPos.has_value() && *einfo.errPos) {
+        oss << "\n";
+        printAtPos("", *einfo.errPos, oss);
 
-    if (einfo.errPos.has_value() && (*einfo.errPos)) {
         auto loc = getCodeLines(*einfo.errPos);
 
         // lines of code.
         if (loc.has_value()) {
-            if (nl)
-                out << std::endl << prefix;
-            printCodeLines(out, prefix, *einfo.errPos, *loc);
-            nl = true;
+            oss << "\n";
+            printCodeLines(oss, "", *einfo.errPos, *loc);
+            oss << "\n";
         }
     }
 
-    // hint
-    if (einfo.hint.has_value()) {
-        if (nl)
-            out << std::endl << prefix;
-        out << std::endl << prefix << *einfo.hint;
-        nl = true;
-    }
-
     // traces
-    if (showTrace && !einfo.traces.empty())
-    {
-        const string tracetitle(" show-trace ");
-
-        int fill = errwidth - tracetitle.length();
-        int lw = 0;
-        int rw = 0;
-        const int min_dashes = 3;
-        if (fill > min_dashes * 2) {
-            if (fill % 2 != 0) {
-                lw = fill / 2;
-                rw = lw + 1;
-            }
-            else
-            {
-                lw = rw = fill / 2;
-            }
-        }
-        else
-            lw = rw = min_dashes;
-
-        if (nl)
-            out << std::endl << prefix;
-
-        out << ANSI_BLUE << std::string(lw, '-') << tracetitle << std::string(rw, '-') << ANSI_NORMAL;
-
-        for (auto iter = einfo.traces.rbegin(); iter != einfo.traces.rend(); ++iter)
-        {
-            out << std::endl << prefix;
-            out << ANSI_BLUE << "trace: " << ANSI_NORMAL << iter->hint.str();
+    if (showTrace && !einfo.traces.empty()) {
+        for (auto iter = einfo.traces.rbegin(); iter != einfo.traces.rend(); ++iter) {
+            oss << "\n" << "… " << iter->hint.str() << "\n";
 
             if (iter->pos.has_value() && (*iter->pos)) {
                 auto pos = iter->pos.value();
-                out << std::endl << prefix;
-                printAtPos(prefix, pos, out);
+                oss << "\n";
+                printAtPos("", pos, oss);
 
                 auto loc = getCodeLines(pos);
-                if (loc.has_value())
-                {
-                    out << std::endl << prefix;
-                    printCodeLines(out, prefix, pos, *loc);
-                    out << std::endl << prefix;
+                if (loc.has_value()) {
+                    oss << "\n";
+                    printCodeLines(oss, "", pos, *loc);
+                    oss << "\n";
                 }
             }
         }
     }
 
+    out << indent(prefix, std::string(filterANSIEscapes(prefix, true).size(), ' '), chomp(oss.str()));
+
     return out;
 }
 }