Fix enqueue for sure

Author: g41797

README.md

@@ -226,12 +226,13 @@ Example:
         .value = 1,
     };
 
-    try mbox.send(&msg.node);
+    _ = try mbox.send(&msg.node);
 
     const node: *Node = try mbox.receive(1000);
     const rcvdMsg: *Msg = @fieldParentPtr("node", node);
     var shouldBeOne: usize = rcvdMsg.*.value;
 ```
+_TypeErasedMailbox_ has exactly the same functionality as former _MailBox_.
 
 ## Eat your own dog food  
 

build.zig.zon

@@ -3,7 +3,7 @@
     .name = .mailbox,
     .version = "0.0.13",
 
-    .fingerprint = 0xa69fe20b9bc5eb6b,
+    .fingerprint = 0xa69fe20bd24b00f0,
 
     .minimum_zig_version = "0.15.2",
 

src/mailbox.zig

@@ -139,6 +139,12 @@ pub fn MailBox(comptime Letter: type) type {
             new_Envelope.prev = null;
             new_Envelope.next = null;
 
+            if (fifo.len == 0) {
+                std.debug.assert(fifo.first == null);
+                std.debug.assert(fifo.last == null);
+                fifo.first = new_Envelope;
+            }
+
             if (fifo.last) |last| {
                 last.next = new_Envelope;
                 new_Envelope.prev = last;
@@ -312,6 +318,12 @@ pub fn MailBoxIntrusive(comptime Envelope: type) type {
             new_Envelope.prev = null;
             new_Envelope.next = null;
 
+            if (fifo.len == 0) {
+                std.debug.assert(fifo.first == null);
+                std.debug.assert(fifo.last == null);
+                fifo.first = new_Envelope;
+            }
+
             if (fifo.last) |last| {
                 last.next = new_Envelope;
                 new_Envelope.prev = last;