strings: prevent copyCheck from forcing Builder to escape and allocate

All credit and blame goes to Ian for this suggestion, copied from the
runtime.

Fixes #23382
Updates #7921

Change-Id: I3d5a9ee4ab730c87e0f3feff3e7fceff9bcf9e18
Reviewed-on: https://go-review.googlesource.com/86976
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/strings/builder.go

@@ -17,9 +17,26 @@ type Builder struct {
 	buf  []byte
 }
 
+// noescape hides a pointer from escape analysis.  noescape is
+// the identity function but escape analysis doesn't think the
+// output depends on the input. noescape is inlined and currently
+// compiles down to zero instructions.
+// USE CAREFULLY!
+// This was copied from the runtime; see issues 23382 and 7921.
+//go:nosplit
+func noescape(p unsafe.Pointer) unsafe.Pointer {
+	x := uintptr(p)
+	return unsafe.Pointer(x ^ 0)
+}
+
 func (b *Builder) copyCheck() {
 	if b.addr == nil {
-		b.addr = b
+		// This hack works around a failing of Go's escape analysis
+		// that was causing b to escape and be heap allocated.
+		// See issue 23382.
+		// TODO: once issue 7921 is fixed, this should be reverted to
+		// just "b.addr = b".
+		b.addr = (*Builder)(noescape(unsafe.Pointer(b)))
 	} else if b.addr != b {
 		panic("strings: illegal use of non-zero Builder copied by value")
 	}