feat!: make Error and Fatal compatible with testing.T

Change Error and Fatal to accept args ...any instead of err error,
matching testing.T and allowing shared test helpers between f1
scenarios and standard go tests.

- Error, Errorf, Fatal, Fatalf log at ERROR level (not via Log/Logf)
- Log/Logf use fmt.Sprintln for formatting (aligned with testing.T)
- Log/Logf add iteration and vuid context to all output
- Add commonTInterface for compile-time signature verification
- Add tests for exact logging format (level, msg, iteration, vuid)

Based on #286 by @sirockin

BREAKING CHANGE: Error(err error) and Fatal(err error) are now
Error(args ...any) and Fatal(args ...any). Single-error call sites
remain valid: Error(err) and Fatal(err) still compile and behave
as before.

Closes #278
Closes #286

Author: nvloff-f3

internal/run/run_cmd_test.go

@@ -831,9 +831,11 @@ func TestOutput_JSONLogging(t *testing.T) {
 
 	scenarioOnlyLogs := []logFieldMatchers{
 		{
-			"message":  "setup",
-			"level":    "info",
-			"scenario": "scenario_where_each_iteration_takes_200ms",
+			"message":   "setup",
+			"level":     "info",
+			"scenario":  "scenario_where_each_iteration_takes_200ms",
+			"iteration": "setup",
+			"vuid":      float64(-1),
 		},
 		{
 			"message":  "slog - setup",
@@ -843,9 +845,11 @@ func TestOutput_JSONLogging(t *testing.T) {
 		},
 
 		{
-			"message":  "first iteration",
-			"level":    "info",
-			"scenario": "scenario_where_each_iteration_takes_200ms",
+			"message":   "first iteration",
+			"level":     "info",
+			"scenario":  "scenario_where_each_iteration_takes_200ms",
+			"iteration": "setup",
+			"vuid":      float64(-1),
 		},
 		{
 			"message":  "slog - first iteration",
@@ -862,9 +866,11 @@ func TestOutput_JSONLogging(t *testing.T) {
 			"scenario": "scenario_where_each_iteration_takes_200ms",
 		},
 		{
-			"message":  "setup",
-			"level":    "info",
-			"scenario": "scenario_where_each_iteration_takes_200ms",
+			"message":   "setup",
+			"level":     "info",
+			"scenario":  "scenario_where_each_iteration_takes_200ms",
+			"iteration": "setup",
+			"vuid":      float64(-1),
 		},
 		{
 			"message":  "slog - setup",
@@ -874,9 +880,11 @@ func TestOutput_JSONLogging(t *testing.T) {
 		},
 
 		{
-			"message":  "first iteration",
-			"level":    "info",
-			"scenario": "scenario_where_each_iteration_takes_200ms",
+			"message":   "first iteration",
+			"level":     "info",
+			"scenario":  "scenario_where_each_iteration_takes_200ms",
+			"iteration": "setup",
+			"vuid":      float64(-1),
 		},
 		{
 			"message":  "slog - first iteration",

internal/run/run_stage_test.go

@@ -64,7 +64,7 @@ type parsedLogLine struct {
 }
 
 type (
-	logFieldMatchers map[string]string
+	logFieldMatchers map[string]any
 )
 
 type RunTestStage struct {
@@ -739,7 +739,7 @@ func (s *RunTestStage) assertJSONLogMatches(t *testing.T, output string, expecte
 		matchers := expectedLogLines[lineIndex]
 		for key, value := range matchers {
 			if value != anyValue {
-				s.assert.Equal(value, parsedLine.parsed[key])
+				s.assert.Equalf(value, parsedLine.parsed[key], "field %q: expected %v, got %v in %s", key, value, parsedLine.parsed[key], parsedLine.raw)
 			}
 		}
 

pkg/f1/f1testing/t.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"log/slog"
 	"runtime/debug"
+	"strings"
 	"sync/atomic"
 
 	"github.com/stretchr/testify/require"
@@ -117,40 +118,45 @@ func (t *T) Fail() {
 	}
 }
 
-// Errorf is equivalent to Logf followed by Fail.
+// Errorf is equivalent to Logf followed by Fail. Logs at Error level.
 func (t *T) Errorf(format string, args ...any) {
-	t.logger.Error(fmt.Sprintf(format, args...))
+	t.logger.With(log.IterationAttr(t.Iteration), log.VUIDAttr(t.VUID)).Error(fmt.Sprintf(format, args...))
 	t.Fail()
 }
 
-// Error is equivalent to Log followed by Fail.
-func (t *T) Error(err error) {
-	t.logger.Error("iteration failed", log.IterationAttr(t.Iteration), log.VUIDAttr(t.VUID), log.ErrorAttr(err))
+// Error is equivalent to Log followed by Fail. Logs at Error level.
+func (t *T) Error(args ...any) {
+	msg := strings.TrimSuffix(fmt.Sprintln(args...), "\n")
+	t.logger.With(log.IterationAttr(t.Iteration), log.VUIDAttr(t.VUID)).Error(msg)
 	t.Fail()
 }
 
-// Fatalf is equivalent to Logf followed by FailNow.
+// Fatalf is equivalent to Logf followed by FailNow. Logs at Error level.
 func (t *T) Fatalf(format string, args ...any) {
-	t.logger.Error(fmt.Sprintf(format, args...))
+	t.logger.With(log.IterationAttr(t.Iteration), log.VUIDAttr(t.VUID)).Error(fmt.Sprintf(format, args...))
 	t.FailNow()
 }
 
-// Fatal is equivalent to Log followed by FailNow.
-func (t *T) Fatal(err error) {
-	t.logger.Error("iteration failed", log.IterationAttr(t.Iteration), log.VUIDAttr(t.VUID), log.ErrorAttr(err))
+// Fatal is equivalent to Log followed by FailNow. Logs at Error level.
+func (t *T) Fatal(args ...any) {
+	msg := strings.TrimSuffix(fmt.Sprintln(args...), "\n")
+	t.logger.With(log.IterationAttr(t.Iteration), log.VUIDAttr(t.VUID)).Error(msg)
 	t.FailNow()
 }
 
 // Log formats its arguments using default formatting, analogous to Println, and records the text in the error log.
 // The text will be printed only if f1 is running in verbose mode.
+// Aligns with testing.T: uses fmt.Sprintln for space-separated args (trailing newline trimmed for structured logs).
 func (t *T) Log(args ...any) {
-	t.logger.Info(fmt.Sprint(args...))
+	msg := strings.TrimSuffix(fmt.Sprintln(args...), "\n")
+	t.logger.With(log.IterationAttr(t.Iteration), log.VUIDAttr(t.VUID)).Info(msg)
 }
 
 // Logf formats its arguments according to the format, analogous to Printf, and records the text in the error log.
 // A final newline is added if not provided. The text will be printed only if f1 is running in verbose mode.
+// Aligns with testing.T: uses fmt.Sprintf.
 func (t *T) Logf(format string, args ...any) {
-	t.logger.Info(fmt.Sprintf(format, args...))
+	t.logger.With(log.IterationAttr(t.Iteration), log.VUIDAttr(t.VUID)).Info(fmt.Sprintf(format, args...))
 }
 
 // Failed reports whether the function has failed.