weak: prevent unsafe conversions using weak pointers

Prevent conversions between Pointer types,
like we do for sync/atomic.Pointer.

Fixes #71583

Change-Id: I20e83106d8a27996f221e6cd9d52637b0442cea4
Reviewed-on: https://go-review.googlesource.com/c/go/+/647195
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/weak/pointer.go

@@ -56,6 +56,9 @@ import (
 // referenced object. Typically, this batching only happens for tiny
 // (on the order of 16 bytes or less) and pointer-free objects.
 type Pointer[T any] struct {
+	// Mention T in the type definition to prevent conversions
+	// between Pointer types, like we do for sync/atomic.Pointer.
+	_ [0]*T
 	u unsafe.Pointer
 }
 
@@ -69,7 +72,7 @@ func Make[T any](ptr *T) Pointer[T] {
 		u = runtime_registerWeakPointer(unsafe.Pointer(ptr))
 	}
 	runtime.KeepAlive(ptr)
-	return Pointer[T]{u}
+	return Pointer[T]{u: u}
 }
 
 // Value returns the original pointer used to create the weak pointer.

src/weak/pointer_test.go

@@ -6,10 +6,12 @@ package weak_test
 
 import (
 	"context"
+	"internal/goarch"
 	"runtime"
 	"sync"
 	"testing"
 	"time"
+	"unsafe"
 	"weak"
 )
 
@@ -155,6 +157,14 @@ func TestPointerFinalizer(t *testing.T) {
 	}
 }
 
+func TestPointerSize(t *testing.T) {
+	var p weak.Pointer[T]
+	size := unsafe.Sizeof(p)
+	if size != goarch.PtrSize {
+		t.Errorf("weak.Pointer[T] size = %d, want %d", size, goarch.PtrSize)
+	}
+}
+
 // Regression test for issue 69210.
 //
 // Weak-to-strong conversions must shade the new strong pointer, otherwise

test/weak.go

@@ -0,0 +1,24 @@
+// errorcheck
+
+// Copyright 2025 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 weak pointers.
+
+package p
+
+import (
+	"runtime"
+	"weak"
+)
+
+// Adapted from example in https://github.com/golang/go/issues/67552#issuecomment-2639661220
+func conversion() {
+	p := "hello"
+	a := weak.Make(&p)
+	b := (weak.Pointer[*byte])(a) // ERROR "cannot convert a \(variable of struct type weak\.Pointer\[string\]\) to type weak.Pointer\[\*byte\]"
+	c := b.Value()
+	println(**c)
+	runtime.KeepAlive(p)
+}