Disallow statics initializing themselves

src/test/ui/consts/recursive-zst-static.rs

@@ -1,6 +1,10 @@
-// build-pass
+// This test ensures that we do not allow ZST statics to initialize themselves without ever
+// actually creating a value of that type. This is important, as the ZST may have private fields
+// that users can reasonably expect to only get initialized by their own code. Thus unsafe code
+// can depend on this fact and will thus do unsound things when it is violated.
+// See https://github.com/rust-lang/rust/issues/71078 for more details.
 
-static FOO: () = FOO;
+static FOO: () = FOO; //~ cycle detected when const-evaluating `FOO`
 
 fn main() {
     FOO

src/test/ui/consts/recursive-zst-static.stderr

@@ -0,0 +1,21 @@
+error[E0391]: cycle detected when const-evaluating `FOO`
+  --> $DIR/recursive-zst-static.rs:7:18
+   |
+LL | static FOO: () = FOO;
+   |                  ^^^
+   |
+note: ...which requires const-evaluating `FOO`...
+  --> $DIR/recursive-zst-static.rs:7:1
+   |
+LL | static FOO: () = FOO;
+   | ^^^^^^^^^^^^^^^^^^^^^
+   = note: ...which again requires const-evaluating `FOO`, completing the cycle
+note: cycle used when const-evaluating + checking `FOO`
+  --> $DIR/recursive-zst-static.rs:7:1
+   |
+LL | static FOO: () = FOO;
+   | ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.