Handlers are supposed to avoid logging empty attributes.

• "- If an Attr's key and value are both the zero value, ignore the Attr."

Handlers should expand groups named "" (the empty string) into the enclosing log record.

• "- If a group's key is empty, inline the group's Attrs."

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

The slog.Record.PC field can be loaded with a program counter (PC). This is normally done by the slog.Logger code. If the PC is non-zero and the slog.HandlerOptions.AddSource flag is set the source field will contain a slog.Source record containing the function name, file name, and file line at which the log record was generated. If the PC is zero then this field and its associated group should not be logged.

• "- If r.PC is zero, ignore it."

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.

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.