devlog: For Christmas: Self-Referential Global Values

Author: mlugg

content/en-US/devlog/2024/index.smd

@@ -27,6 +27,44 @@
 // an example of a correct date string.
 ---
 
+# [For Christmas: Self-Referential Global Values]($section.id('2024-12-25'))
+
+Author: Matthew Lugg
+
+I recently took it upon myself to tackle [issue #131](https://github.com/ziglang/zig/issues/131). This
+has been a long-standing bug in the compiler, dating way back to the earliest days of the C++ bootstrap
+compiler: it was the 9th oldest open issue in the Zig respository! Essentially, fixing it allows you to
+write code like this:
+
+```zig
+const S = struct { ptr: *const S, x: u32 };
+
+// `foo` contains a pointer to itself
+const foo: S = .{ .ptr = &foo, .x = 123 };
+
+// Or:
+
+// `a` contains a pointer to `b`, and...
+// `b` contains a pointer to `a`
+const a: S = .{ .ptr = &b, .x = 123 };
+const b: S = .{ .ptr = &a, .x = 456 };
+```
+
+Previously, both of the examples above would have failed with a "dependency loop detected" error; now,
+they compile just as you expect (and the same works if they're declared `var`, of course).
+
+This issue had been open for so long because it actually requires a non-trivial extension to the compiler:
+separating semantic analysis for the *type* and *value* of container-level declarations. This is complicated
+further by the fact that some global declarations might not have a type annotation, and that some (such as
+`extern` declarations) have a *type* but no *value*. It also interacts with the work-in-progress incremental
+compilation feature in non-trivial ways, so we had to make sure we got that right. All in all, this resulted
+in a [pretty bulky diff](https://github.com/ziglang/zig/pull/22303) to the compiler.
+
+Regardless, the point is, this now works on Zig `master`! If you have a use case for it, feel free to give it
+a try -- and please do open an issue if you encounter any problems along the way.
+
+Merry Christmas to those who celebrate!
+
 # [Zig Bootstrap Porting Progress]($section.id('2024-11-19'))
 
 Author: Alex Rønne Petersen