cmd: Exit with status 0 when -h is used (#87)

This is a pretty standard CLI behavior:
`cmd -h` should print the help message and exit with status 0.
This is the default behavior in the flag package under ExitOnError mode.
We neglected to replicate that in ContinueOnError mode.

CHANGELOG.md

@@ -5,6 +5,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+### Fixed
+- cmd/errtrace: Don't exit with a non-zero status when `-h` is used.
+
 ## v0.3.0 - 2023-12-22
 
 This release adds support to the CLI for using Go package patterns like `./...`

cmd/errtrace/main.go

@@ -32,6 +32,7 @@ package main
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"flag"
 	"fmt"
 	"go/ast"
@@ -184,6 +185,9 @@ func (cmd *mainCmd) Run(args []string) (exitCode int) {
 
 	var p mainParams
 	if err := p.Parse(cmd.Stderr, args); err != nil {
+		if errors.Is(err, flag.ErrHelp) {
+			return 0
+		}
 		cmd.log.Printf("errtrace: %+v", err)
 		return 1
 	}

cmd/errtrace/main_test.go

@@ -22,6 +22,17 @@ import (
 	"braces.dev/errtrace/internal/diff"
 )
 
+func TestErrHelp(t *testing.T) {
+	exitCode := (&mainCmd{
+		Stdin:  strings.NewReader(""),
+		Stdout: testWriter{t},
+		Stderr: testWriter{t},
+	}).Run([]string{"-h"})
+	if want := 0; exitCode != want {
+		t.Errorf("exit code = %d, want %d", exitCode, want)
+	}
+}
+
 // TestGolden runs errtrace on all .go files inside testdata/golden,
 // and compares the output to the corresponding .golden file.
 // Files must match exactly.