errors: optimize *joinError's Error method for less allocation and faster execution

Handle the case of one error at the beginning.
Use unsafe.String to avoid memory allocation when converting byte slice to string.

Change-Id: Ib23576f72b1d87489e6f17762be483f62ca4998a
GitHub-Last-Rev: ed8003bfbc
GitHub-Pull-Request: golang/go#60026
Reviewed-on: https://go-review.googlesource.com/c/go/+/493237
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

src/errors/join.go

@@ -4,6 +4,10 @@
 
 package errors
 
+import (
+	"unsafe"
+)
+
 // Join returns an error that wraps the given errors.
 // Any nil error values are discarded.
 // Join returns nil if every value in errs is nil.
@@ -38,14 +42,19 @@ type joinError struct {
 }
 
 func (e *joinError) Error() string {
-	var b []byte
-	for i, err := range e.errs {
-		if i > 0 {
-			b = append(b, '\n')
-		}
+	// Since Join returns nil if every value in errs is nil,
+	// e.errs cannot be empty.
+	if len(e.errs) == 1 {
+		return e.errs[0].Error()
+	}
+
+	b := []byte(e.errs[0].Error())
+	for _, err := range e.errs[1:] {
+		b = append(b, '\n')
 		b = append(b, err.Error()...)
 	}
-	return string(b)
+	// At this point, b has at least one byte '\n'.
+	return unsafe.String(&b[0], len(b))
 }
 
 func (e *joinError) Unwrap() []error {