internal/strs: use go1.20 unsafe interface for strings

This is slightly more efficient and removes explicit dependence
on string layout and alignment.  Build tags control use.

Because of what is arguably a bug in go 1.20's treatment of build tags,
is build-tagged to go1.21, not go1.20.  (1.20 sees that it matches the
build tag and tries to compile the file, but then demands that it only use
language features from the version mentioned in go.mod. Go 1.21 and 1.22
behave more sensibly.)

Change-Id: I0cfc3b2e5651595edd3a82d9071d918700f961cd
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/545195
Reviewed-by: Cassondra Foesch <cfoesch@gmail.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/strs/strings_unsafe.go → internal/strs/strings_unsafe_go120.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !purego && !appengine
-// +build !purego,!appengine
+//go:build !purego && !appengine && !go1.21
+// +build !purego,!appengine,!go1.21
 
 package strs
 

internal/strs/strings_unsafe_go121.go

@@ -0,0 +1,74 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !purego && !appengine && go1.21
+// +build !purego,!appengine,go1.21
+
+package strs
+
+import (
+	"unsafe"
+
+	"google.golang.org/protobuf/reflect/protoreflect"
+)
+
+// UnsafeString returns an unsafe string reference of b.
+// The caller must treat the input slice as immutable.
+//
+// WARNING: Use carefully. The returned result must not leak to the end user
+// unless the input slice is provably immutable.
+func UnsafeString(b []byte) string {
+	return unsafe.String(unsafe.SliceData(b), len(b))
+}
+
+// UnsafeBytes returns an unsafe bytes slice reference of s.
+// The caller must treat returned slice as immutable.
+//
+// WARNING: Use carefully. The returned result must not leak to the end user.
+func UnsafeBytes(s string) []byte {
+	return unsafe.Slice(unsafe.StringData(s), len(s))
+}
+
+// Builder builds a set of strings with shared lifetime.
+// This differs from strings.Builder, which is for building a single string.
+type Builder struct {
+	buf []byte
+}
+
+// AppendFullName is equivalent to protoreflect.FullName.Append,
+// but optimized for large batches where each name has a shared lifetime.
+func (sb *Builder) AppendFullName(prefix protoreflect.FullName, name protoreflect.Name) protoreflect.FullName {
+	n := len(prefix) + len(".") + len(name)
+	if len(prefix) == 0 {
+		n -= len(".")
+	}
+	sb.grow(n)
+	sb.buf = append(sb.buf, prefix...)
+	sb.buf = append(sb.buf, '.')
+	sb.buf = append(sb.buf, name...)
+	return protoreflect.FullName(sb.last(n))
+}
+
+// MakeString is equivalent to string(b), but optimized for large batches
+// with a shared lifetime.
+func (sb *Builder) MakeString(b []byte) string {
+	sb.grow(len(b))
+	sb.buf = append(sb.buf, b...)
+	return sb.last(len(b))
+}
+
+func (sb *Builder) grow(n int) {
+	if cap(sb.buf)-len(sb.buf) >= n {
+		return
+	}
+
+	// Unlike strings.Builder, we do not need to copy over the contents
+	// of the old buffer since our builder provides no API for
+	// retrieving previously created strings.
+	sb.buf = make([]byte, 0, 2*(cap(sb.buf)+n))
+}
+
+func (sb *Builder) last(n int) string {
+	return UnsafeString(sb.buf[len(sb.buf)-n:])
+}