Handlers should resolve all objects implementing the LogValuer or Stringer interfaces. This is a powerful feature which can customize logging of objects and speed up logging by delaying argument resolution until logging time.

• "- Attr's values should be resolved."

Documentation on building a handler suggests testing using slog/slogtest, part of the Go release since 1.21. While the verification suite includes tests patterned after those in slogtest, an additional single test invokes the slogtest testing sequence (involving multiple tests). Since this is a separate package, all error messages are returned at once. This is the only warning that affects TestSlogTest and all of its error messages.

• "To verify that your handler follows these rules and generally produces proper output, use the testing/slogtest package."
• slog/slogtest

Handlers should not log the basic time field if it is zero.

• "- If r.Time is the zero time, ignore the time."

A new slog.Handler should default to slog.LevelInfo.

• "First, we wanted the default level to be Info, Since Levels are ints, Info is the default value for int, zero."

The field name of the "message" key should be msg.

• Constant values are defined for slog/log
  
• Field values are defined for the JSONHandler.Handle() implementation

Some handlers (e.g. phsym/zeroslog) support HandlerOptions.ReplaceAttr except for the four main fields time, level, msg, and source. When that is the case it is better to use this (WarnNoReplAttrBasic) warning.

The slog.JSONHandler uses nanoseconds for time.Duration but some other handlers use seconds.

• Go issue 59345: Nanoseconds is a recent change with Go 1.21

Some handlers that change the way time.Duration objects are logged (see warnings DurationMillis and DurationSeconds) only manage to make the change at the top level of the logged record, duration objects in groups are still in nanoseconds.

Each JSON log record contains the logging level of the log statement as a string. Different handlers provide that string in uppercase or lowercase. Documentation for slog.Level says that its String() and MarshalJSON() methods will return uppercase but UnmarshalJSON() will parse in a case-insensitive manner.

Until documented otherwise, an attribute with an empty field name ("") and a non-nil value should be logged.

• Empty field names are logged by the JSONHandler.Handle() implementation

The slog.JSONHandler uses nanoseconds for time.Time but some other handlers use milliseconds. This does not apply to the basic time field, only attribute fields. I can't find any supporting documentation or bug on this but Go issue 59345 (see previous warning) may have fixed this as well in Go 1.21.

If a warning is specified but the condition is not actually present one of these warnings will be issued with the specified warning. These are intended to help clean out unnecessary WarnOnly settings from a test suite as issues are fixed in the tested handler.