Do not use undefined for the ptr field of Allocator/Random instances

The use of undefined here meant that it was always unsafe/illegal to compare the ptr fields of Allocator/Random. However, this restriction is not mentioned anywhere and the ptr field *is* being compared in real code, e.g. `std.process.Child.collectOutput`, which can lead to undefined behavior in release modes (see ziglang#21756).

There are two ways to address this:
1. Codify that comparing `ptr` is always illegal/unsafe, and remove all existing comparisons.
2. Set `ptr` to a legal dummy value, similar to the value returned by Allocator.alloc when a zero-length slice is requested.

The first option seems like footgun central (at least until usage of `undefined` is fully safety-checked in safe modes), so I went with the second option.

Closes ziglang#21756

Author: squeek502

lib/std/crypto/tlcsprng.zig

@@ -12,7 +12,7 @@ const posix = std.posix;
 /// We use this as a layer of indirection because global const pointers cannot
 /// point to thread-local variables.
 pub const interface: std.Random = .{
-    .ptr = undefined,
+    .ptr = mem.dummyPointer(*anyopaque),
     .fillFn = tlsCsprngFill,
 };
 

lib/std/heap.zig

@@ -150,7 +150,7 @@ const CAllocator = struct {
 /// `malloc_usable_size` if available. For an allocator that directly calls
 /// `malloc`/`free`, see `raw_c_allocator`.
 pub const c_allocator = Allocator{
-    .ptr = undefined,
+    .ptr = mem.dummyPointer(*anyopaque),
     .vtable = &c_allocator_vtable,
 };
 const c_allocator_vtable = Allocator.VTable{
@@ -165,7 +165,7 @@ const c_allocator_vtable = Allocator.VTable{
 /// `ArenaAllocator` for example and is more optimal in such a case
 /// than `c_allocator`.
 pub const raw_c_allocator = Allocator{
-    .ptr = undefined,
+    .ptr = mem.dummyPointer(*anyopaque),
     .vtable = &raw_c_allocator_vtable,
 };
 const raw_c_allocator_vtable = Allocator.VTable{
@@ -231,17 +231,17 @@ pub const page_allocator = if (@hasDecl(root, "os") and
     root.os.heap.page_allocator
 else if (builtin.target.isWasm())
     Allocator{
-        .ptr = undefined,
+        .ptr = mem.dummyPointer(*anyopaque),
         .vtable = &WasmPageAllocator.vtable,
     }
 else if (builtin.target.os.tag == .plan9)
     Allocator{
-        .ptr = undefined,
+        .ptr = mem.dummyPointer(*anyopaque),
         .vtable = &SbrkAllocator(std.os.plan9.sbrk).vtable,
     }
 else
     Allocator{
-        .ptr = undefined,
+        .ptr = mem.dummyPointer(*anyopaque),
         .vtable = &PageAllocator.vtable,
     };
 
@@ -251,7 +251,7 @@ else
 /// and wasm64 architectures.
 /// Until then, it is available here to play with.
 pub const wasm_allocator = Allocator{
-    .ptr = undefined,
+    .ptr = mem.dummyPointer(*anyopaque),
     .vtable = &std.heap.WasmAllocator.vtable,
 };
 

lib/std/heap/WasmAllocator.zig

@@ -158,7 +158,7 @@ fn allocBigPages(n: usize) usize {
 }
 
 const test_ally = Allocator{
-    .ptr = undefined,
+    .ptr = mem.dummyPointer(*anyopaque),
     .vtable = &vtable,
 };
 

lib/std/mem.zig

@@ -134,7 +134,7 @@ pub fn alignAllocLen(full_len: usize, alloc_len: usize, len_align: u29) usize {
 }
 
 const fail_allocator = Allocator{
-    .ptr = undefined,
+    .ptr = dummyPointer(*anyopaque),
     .vtable = &failAllocator_vtable,
 };
 

lib/std/os/uefi/pool_allocator.zig

@@ -69,7 +69,7 @@ const UefiPoolAllocator = struct {
 /// Supports the full Allocator interface, including alignment.
 /// For a direct call of `allocatePool`, see `raw_pool_allocator`.
 pub const pool_allocator = Allocator{
-    .ptr = undefined,
+    .ptr = mem.dummyPointer(*anyopaque),
     .vtable = &pool_allocator_vtable,
 };
 
@@ -81,7 +81,7 @@ const pool_allocator_vtable = Allocator.VTable{
 
 /// Asserts allocations are 8 byte aligned and calls `boot_services.allocatePool`.
 pub const raw_pool_allocator = Allocator{
-    .ptr = undefined,
+    .ptr = mem.dummyPointer(*anyopaque),
     .vtable = &raw_pool_allocator_table,
 };