Add lowering and interpreter support for :latestworld (#56523)

Split out from #56509 to facilitate adjusting downstream packages.

base/boot.jl

@@ -271,6 +271,8 @@ macro nospecialize(x)
 end
 Expr(@nospecialize args...) = _expr(args...)
 
+macro latestworld() Expr(:latestworld) end
+
 _is_internal(__module__) = __module__ === Core
 # can be used in place of `@assume_effects :total` (supposed to be used for bootstrapping)
 macro _total_meta()

src/ast.c

@@ -119,6 +119,7 @@ JL_DLLEXPORT jl_sym_t *jl_release_sym;
 JL_DLLEXPORT jl_sym_t *jl_acquire_release_sym;
 JL_DLLEXPORT jl_sym_t *jl_sequentially_consistent_sym;
 JL_DLLEXPORT jl_sym_t *jl_uninferred_sym;
+JL_DLLEXPORT jl_sym_t *jl_latestworld_sym;
 
 static const uint8_t flisp_system_image[] = {
 #include <julia_flisp.boot.inc>
@@ -461,6 +462,7 @@ void jl_init_common_symbols(void)
     jl_acquire_release_sym = jl_symbol("acquire_release");
     jl_sequentially_consistent_sym = jl_symbol("sequentially_consistent");
     jl_uninferred_sym = jl_symbol("uninferred");
+    jl_latestworld_sym = jl_symbol("latestworld");
 }
 
 JL_DLLEXPORT void jl_lisp_prompt(void)

src/interpreter.c

@@ -643,6 +643,9 @@ static jl_value_t *eval_body(jl_array_t *stmts, interpreter_state *s, size_t ip,
                     jl_eval_const_decl(s->module, jl_exprarg(stmt, 0), val);
                     s->locals[jl_source_nslots(s->src) + s->ip] = jl_nothing;
                 }
+                else if (head == jl_latestworld_sym) {
+                    ct->world_age = jl_atomic_load_acquire(&jl_world_counter);
+                }
                 else if (jl_is_toplevel_only_expr(stmt)) {
                     jl_toplevel_eval(s->module, stmt);
                 }

src/julia-syntax.scm

@@ -4512,6 +4512,7 @@ f(x) = yt(x)
           ((struct_type)       "\"struct\" expression")
           ((method)            "method definition")
           ((set_binding_type!) (string "type declaration for global \"" (deparse (cadr e)) "\""))
+          ((latestworld)          "World age increment")
           (else                (string "\"" h "\" expression"))))
       (if (not (null? (cadr lam)))
           (error (string (head-to-text (car e)) " not at top level"))))
@@ -4979,8 +4980,13 @@ f(x) = yt(x)
                (if tail (emit-return tail val))
                val))
 
+            ((latestworld-if-toplevel)
+             (if (null? (cadr lam))
+               (emit `(latestworld)))
+             '(null))
+
             ;; other top level expressions
-            ((import using export public)
+            ((import using export public latestworld)
              (check-top-level e)
              (emit e)
              (let ((have-ret? (and (pair? code) (pair? (car code)) (eq? (caar code) 'return))))

src/julia_internal.h

@@ -1856,6 +1856,7 @@ extern JL_DLLEXPORT jl_sym_t *jl_release_sym;
 extern JL_DLLEXPORT jl_sym_t *jl_acquire_release_sym;
 extern JL_DLLEXPORT jl_sym_t *jl_sequentially_consistent_sym;
 extern JL_DLLEXPORT jl_sym_t *jl_uninferred_sym;
+extern JL_DLLEXPORT jl_sym_t *jl_latestworld_sym;
 
 JL_DLLEXPORT enum jl_memory_order jl_get_atomic_order(jl_sym_t *order, char loading, char storing);
 JL_DLLEXPORT enum jl_memory_order jl_get_atomic_order_checked(jl_sym_t *order, char loading, char storing);

src/toplevel.c

@@ -607,7 +607,8 @@ int jl_is_toplevel_only_expr(jl_value_t *e) JL_NOTSAFEPOINT
          ((jl_expr_t*)e)->head == jl_const_sym ||
          ((jl_expr_t*)e)->head == jl_toplevel_sym ||
          ((jl_expr_t*)e)->head == jl_error_sym ||
-         ((jl_expr_t*)e)->head == jl_incomplete_sym);
+         ((jl_expr_t*)e)->head == jl_incomplete_sym ||
+         ((jl_expr_t*)e)->head == jl_latestworld_sym);
 }
 
 int jl_needs_lowering(jl_value_t *e) JL_NOTSAFEPOINT