Zero freed memory in trace mode.

src/heap.zig

@@ -26,6 +26,7 @@ var gpa: std.heap.GeneralPurposeAllocator(.{
     .stack_trace_frames = if (builtin.mode == .Debug) 12 else 0,
 }) = .{};
 var miAlloc: mi.Allocator = undefined;
+var trace_allocator: TraceAllocator = undefined;
 var initedAllocator = false;
 
 fn initAllocator() void {
@@ -57,7 +58,12 @@ pub fn getAllocator() std.mem.Allocator {
         },
         .zig => {
             if (builtin.is_test) {
-                return t.alloc;
+                if (cy.Trace) {
+                    trace_allocator.alloc = t.alloc;
+                    return trace_allocator.allocator();
+                } else {
+                    return t.alloc;
+                }
             }
             if (cy.isWasm) {
                 return std.heap.wasm_allocator;
@@ -88,6 +94,40 @@ pub fn deinitAllocator() void {
     }
 }
 
+// Uses a backing allocator and zeros the freed memory to surface UB more consistently.
+pub const TraceAllocator = struct {
+    alloc: std.mem.Allocator,
+
+    const vtable = std.mem.Allocator.VTable{
+        .alloc = alloc,
+        .resize = resize,
+        .free = free,
+    };
+
+    pub fn allocator(self: *TraceAllocator) std.mem.Allocator {
+        return std.mem.Allocator{
+            .ptr = self,
+            .vtable = &vtable,
+        };
+    }
+
+    fn alloc(ptr: *anyopaque, len: usize, log2_align: u8, ret_addr: usize) ?[*]u8 {
+        const self: *TraceAllocator = @ptrCast(@alignCast(ptr));
+        return self.alloc.rawAlloc(len, log2_align, ret_addr);
+    }
+
+    fn resize(ptr: *anyopaque, buf: []u8, log2_align: u8, new_len: usize, ret_addr: usize) bool {
+        const self: *TraceAllocator = @ptrCast(@alignCast(ptr));
+        return self.alloc.rawResize(buf, log2_align, new_len, ret_addr);
+    }
+
+    fn free(ptr: *anyopaque, buf: []u8, log2_align: u8, ret_addr: usize) void {
+        const self: *TraceAllocator = @ptrCast(@alignCast(ptr));
+        @memset(buf, 0);
+        return self.alloc.rawFree(buf, log2_align, ret_addr);
+    }
+};
+
 // Keep it just under 4kb page.
 pub const HeapPage = struct {
     objects: [102]HeapObject,