This PR is for phase 2 of the nix-error-proposal project.

Overall the idea is to enhance the format of error messages, and their content. This phase is just for the formatting part, the content of error messages is largely left alone.

To use the new format there are some new macros in logging.hh:

    logError
    logWarning

These require an ErrorInfo struct as an argument instead of a format string. In the default logger, the ErrorInfo is used to print errors to cerr. This late conversion of ErrorInfo to string should help with future language server support, or other similar efforts.

These macros prevent creating unnecessary ErrorInfo structs, similar to the printInfo and printError macros. That is, filtering takes place before the struct is created, not after.

ErrorInfo uses a special fmt - 'hintfmt' - that highlights templated values in yellow. For consistency, I changed the printError/warn/printInfo macros to use the same format style. That also required removing the remaining calls to boost::format.

ErrorInfo usage fits into two big categories: BaseError descendants, and converted printError messages.

Exceptions

BaseError descendants - Error, SysError, and so forth, all use the new format now. The MakeError macro uses the class name as the error name, so SysError will print:

  error: --- SysError ------------------------------------------ error-demo
  statting file : Bad file descriptor

Pos goes into ErrorInfo as a special field, and gets printed at a standardized place in the message. It got removed from the string templates so as not to duplicate information.

printError

For printError messages, there were more decisions to make. The ErrorInfo format is pretty heavy and not always appropriate. I categorized printError calls as follows:

1. Problems. reporting a problem that may require user intervention.
   - converted to ErrorInfo format.
2. Status. printing status messages that inform the user of progress.
   - converted to printInfo.
3. Warnings. printing warnings with error verbosity, as in printError("warning: ...
   - converted to logWarning()
4. Other. technical uses, like forwarding remote build messages to cerr.
   - left alone.

warn() messages.
these I left alone for now; reasoning that the printError("warning..." ones are more important, so those are distiguished by logWarning calls. warn() calls are intact other than converting from boost::format.

What's missing

This is already a huge PR, so I'm leaving some things out that belong in phase 2.

• show-trace is not included in ErrorInfo. It still works as it has in the past, but its printed to cerr and doesn't make it into the logger except as strings.

• lines of code. This is a big part of the new format! But I think there's enough to look at in this PR. Its next on my list.

Moving code around.

BaseError and descendants got moved out of types.hh (and util.cc) and into error.hh.

ErrorInfo now uses Verbosity instead of its own ErrLevel type. Consequently, Verbosity was moved from logging.hh to error.hh.

Theres a new header, fmt.hh, where fmt, FormatOrString, hintfmt, and so forth are defined.

Misc

In eval.cc and eval-inline are some functions like throwEvalError, that used pos in a string template. With pos being passed to ErrorInfo instead, I found it confusing to leave pos as the last argument in the function:

// old version: pos used as a value in the string template
throwTypeError("value is %1% while a list was expected, at %2%", v, pos);

// new version: pos is the first argument, indicating that its not used 
// in the template.
throwTypeError(pos, "value is %1% while a list was expected", v);

It seems like cpu affinity was meant to be printed in affinity.cc, but wasn't. I added an ostream operator<< for affinity, so I could print it:

{
        std::ostringstream oss;
        oss << savedAffinity;
        printError("failed to restore CPU affinity %1%", oss.str());
}