fix: Err() should record error when ErrorStackMarshaler returns nil

[Yanhu007]

Fixes #762

Problem

Since commit f6fbd33, when ErrorStackMarshaler returns nil (e.g. for errors that don't have stack traces), Err() returns immediately without calling AnErr(ErrorFieldName, err). This silently drops the error message from log output.

Before the regression:

{"level":"error","error":"something failed","message":"..."}

After the regression:

{"level":"error","message":"..."}

(error field is missing!)

Fix

Change case nil: return e to an empty case nil: so control falls through to the AnErr() call that records the error field. The stack field is correctly skipped when the marshaler returns nil.

[Copilot]

Pull request overview

Fixes a regression where Event.Err() could drop the error field when ErrorStackMarshaler returns nil (e.g., errors without stack traces), while still allowing stack output when available.

Changes:

• Adjust Event.Err() stack-marshaling switch so a nil stack-marshaler result skips only the stack field (and does not short-circuit the error field recording).

Comments suppressed due to low confidence (1)

event.go:483

• This change fixes Event.Err() when ErrorStackMarshaler returns nil, but the same early-return behavior still exists in Context.Err() (context.go:210-226) and in stack handling inside appendFieldList (fields.go:82-99). If the intent of this PR/issue is that Err() should always still record the error field when the stack marshaler returns nil, those call sites should be updated too; otherwise the behavior will remain inconsistent depending on whether the error is added via an Event, via Logger.With().Err(...), or via Fields(...). Also note there are existing tests that currently assert the old behavior (e.g. TestEvent_ErrWithStackMarshalerNil, TestContext_ErrWithNilStackMarshaler) and will need to be updated to match the new contract.

	if e.stack && ErrorStackMarshaler != nil {
		switch m := ErrorStackMarshaler(err).(type) {
		case nil:
			// Stack marshaler returned nil; skip the stack field
			// but still record the error below.
		case LogObjectMarshaler:
			e = e.Object(ErrorStackFieldName, m)
		case error:
			e = e.Str(ErrorStackFieldName, m.Error())
		case string:
			e = e.Str(ErrorStackFieldName, m)
		default:
			e = e.Interface(ErrorStackFieldName, m)
		}
	}

	return e.AnErr(ErrorFieldName, err)
}

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[Yanhu007]

Good catch on Context.Err() having the same issue! I've pushed an additional commit that fixes the identical case nil: return c early return in context.go as well.

Both Event.Err() and Context.Err() now correctly skip only the stack field when the marshaler returns nil, while still recording the error itself.

[IDisposable]

This one doesn't update the unit tests, probably should prefer #758

[IDisposable]

#763 was merged so this should be closed.