runtime: change heapObjectsCanMove to a func

A var is problematic because the zero value is already false,
so if it goes away, it will appear to be false.
I'm also not sure about go:linkname on vars,
so switch to func for both reasons.

Also add a test.

Change-Id: I2318a5390d98577aec025152e65543491489defb
Reviewed-on: https://go-review.googlesource.com/c/go/+/498261
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/heap_test.go

@@ -0,0 +1,21 @@
+// Copyright 2023 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.
+
+package runtime_test
+
+import (
+	"testing"
+	_ "unsafe"
+)
+
+//go:linkname heapObjectsCanMove runtime.heapObjectsCanMove
+func heapObjectsCanMove() bool
+
+func TestHeapObjectsCanMove(t *testing.T) {
+	if heapObjectsCanMove() {
+		// If this happens (or this test stops building),
+		// it will break go4.org/unsafe/assume-no-moving-gc.
+		t.Fatalf("heap objects can move!")
+	}
+}

src/runtime/mgc.go

@@ -149,7 +149,7 @@ const (
 	sweepMinHeapDistance = 1024 * 1024
 )
 
-// heapObjectsCanMove is always false in the current garbage collector.
+// heapObjectsCanMove always returns false in the current garbage collector.
 // It exists for go4.org/unsafe/assume-no-moving-gc, which is an
 // unfortunate idea that had an even more unfortunate implementation.
 // Every time a new Go release happened, the package stopped building,
@@ -165,7 +165,11 @@ const (
 //
 // If the Go garbage collector ever does move heap objects, we can set
 // this to true to break all the programs using assume-no-moving-gc.
-var heapObjectsCanMove = false
+//
+//go:linkname heapObjectsCanMove
+func heapObjectsCanMove() bool {
+	return false
+}
 
 func gcinit() {
 	if unsafe.Sizeof(workbuf{}) != _WorkbufSize {