Preserve tint.Err attribute key (#66)

* Preserve the 'err' Attribute Name

We do not always have control over how attribute names are defined.
However, it is beneficial to highlight errors in development logs for better visibility.

* make testcase consistent with the test suite

---------

Co-authored-by: lmittmann <[email protected]>

handler.go

@@ -345,7 +345,7 @@ func (h *handler) appendAttr(buf *buffer, attr slog.Attr, groupsPrefix string, g
 		}
 	} else if err, ok := attr.Value.Any().(tintError); ok {
 		// append tintError
-		h.appendTintError(buf, err, groupsPrefix)
+		h.appendTintError(buf, err, attr.Key, groupsPrefix)
 		buf.WriteByte(' ')
 	} else {
 		h.appendKey(buf, attr.Key, groupsPrefix)
@@ -395,9 +395,9 @@ func (h *handler) appendValue(buf *buffer, v slog.Value, quote bool) {
 	}
 }
 
-func (h *handler) appendTintError(buf *buffer, err error, groupsPrefix string) {
+func (h *handler) appendTintError(buf *buffer, err error, attrKey, groupsPrefix string) {
 	buf.WriteStringIf(!h.noColor, ansiBrightRedFaint)
-	appendString(buf, groupsPrefix+errKey, true)
+	appendString(buf, groupsPrefix+attrKey, true)
 	buf.WriteByte('=')
 	buf.WriteStringIf(!h.noColor, ansiResetFaint)
 	appendString(buf, err.Error(), true)

handler_test.go

@@ -344,6 +344,14 @@ func TestHandler(t *testing.T) {
 			},
 			Want: `Nov 10 23:00:00.000 INF test key="{A:123 B:<nil>}"`,
 		},
+		{ // https://github.com/lmittmann/tint/pull/66
+			F: func(l *slog.Logger) {
+				errAttr := tint.Err(errors.New("fail"))
+				errAttr.Key = "error"
+				l.Error("test", errAttr)
+			},
+			Want: `Nov 10 23:00:00.000 ERR test error=fail`,
+		},
 	}
 
 	for i, test := range tests {