hash/maphash: mark call into runtime hash function as not escaping

This allows maphash.Hash to be allocated on the stack for typical uses.

Fixes #35636

Change-Id: I8366507d26ea717f47a9fb46d3bd69ba799845ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/207444
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/hash/maphash/maphash.go

@@ -193,6 +193,7 @@ func rthash(b []byte, seed uint64) uint64 {
 }
 
 //go:linkname runtime_memhash runtime.memhash
+//go:noescape
 func runtime_memhash(p unsafe.Pointer, seed, s uintptr) uintptr
 
 // Sum appends the hash's current 64-bit value to b.

test/escape_hash_maphash.go

@@ -0,0 +1,19 @@
+// errorcheck -0 -m -l
+
+// Copyright 2019 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.
+
+// Test escape analysis for hash/maphash.
+
+package escape
+
+import (
+	"hash/maphash"
+)
+
+func f() {
+	var x maphash.Hash // should be stack allocatable
+	x.WriteString("foo")
+	x.Sum64()
+}