loopvec: label -d diffs with the file's path

main.go

@@ -15,7 +15,6 @@
 package main
 
 import (
-	"bytes"
 	"encoding/json"
 	"flag"
 	"fmt"
@@ -321,7 +320,9 @@ func addBuildTag(src []byte, tag string) []byte {
 }
 
 // printDiff prints a unified diff between orig and rewritten for path,
-// using the system diff command.
+// using the system diff command. Both sides are labeled with path, so the
+// header does not depend on the temporary files or on which diff is installed
+// (GNU and BSD diff print their timestamps differently).
 func printDiff(path string, orig, rewritten []byte) error {
 	origFile, err := os.CreateTemp("", "loopvec-orig-*.go")
 	if err != nil {
@@ -345,9 +346,7 @@ func printDiff(path string, orig, rewritten []byte) error {
 	}
 	_ = newFile.Close()
 
-	out, _ := exec.Command("diff", "-u", origFile.Name(), newFile.Name()).Output()
-	out = bytes.ReplaceAll(out, []byte(origFile.Name()), []byte(path))
-	out = bytes.ReplaceAll(out, []byte(newFile.Name()), []byte(path))
+	out, _ := exec.Command("diff", "-u", "-L", path, "-L", path, origFile.Name(), newFile.Name()).Output()
 	_, err = os.Stdout.Write(out)
 	return err
 }

testdata/flag_diff.txt

@@ -1,11 +1,11 @@
-# -d prints a unified diff (paths in the headers, timestamps aside) and leaves
-# the source untouched.
+# -d prints a unified diff, labeled with the file's path on both sides so the
+# headers are the same with GNU and BSD diff, and leaves the source untouched.
 
 cp ops.go ops.go.orig
 loopvec -d .
 cmpenv stderr stderr.want
-stdout -count=1 '^--- \S+/ops\.go\t\S+ \S+ [+-][0-9]{4}$'
-stdout -count=1 '^\+\+\+ \S+/ops\.go\t\S+ \S+ [+-][0-9]{4}$'
+stdout -count=1 '^--- \S+/ops\.go$'
+stdout -count=1 '^\+\+\+ \S+/ops\.go$'
 stdout -count=1 '^@@ -1,7 \+1,43 @@$'
 stdout -count=1 '^\+//go:build goexperiment.simd$'
 stdout -count=1 '^\+import \($'

testdata/readme_go_tool.txt

@@ -6,7 +6,7 @@ go work use $LOOPVEC_SRC
 
 exec go tool loopvec -d ./...
 cmpenv stderr stderr.want
-stdout -count=1 '^\+\+\+ \S+/mypkg/ops\.go\t'
+stdout -count=1 '^\+\+\+ \S+/mypkg/ops\.go$'
 
 exec go tool loopvec -split ./mypkg/...
 ! stdout .

testdata/readme_patterns.txt

@@ -10,10 +10,10 @@ cmp b/ops.go b/ops.go.orig
 
 # unified diff, once per changed file
 loopvec -d ./...
-stdout -count=1 '^--- \S+/a/ops\.go\t'
-stdout -count=1 '^\+\+\+ \S+/a/ops\.go\t'
-stdout -count=1 '^--- \S+/b/ops\.go\t'
-stdout -count=1 '^\+\+\+ \S+/b/ops\.go\t'
+stdout -count=1 '^--- \S+/a/ops\.go$'
+stdout -count=1 '^\+\+\+ \S+/a/ops\.go$'
+stdout -count=1 '^--- \S+/b/ops\.go$'
+stdout -count=1 '^\+\+\+ \S+/b/ops\.go$'
 stdout -count=2 '^@@ -1,7 \+1,43 @@$'
 cmpenv stderr both.stderr
 cmp a/ops.go a/ops.go.orig

testdata/readme_workflow.txt

@@ -4,7 +4,7 @@
 # 1. Preview changes
 loopvec -d ./mypkg/...
 cmpenv stderr stderr.want
-stdout -count=1 '^\+\+\+ \S+/mypkg/ops\.go\t'
+stdout -count=1 '^\+\+\+ \S+/mypkg/ops\.go$'
 ! exists mypkg/ops_simd.go
 
 # 2. Apply: creates ops_simd.go and guards ops.go