bindings: Add native automatic re-export feature

This implements a native version of automatic re-export, similar to the
macro from Reexport.jl. Doing this natively in the binding system has
two key advantages:

1. It properly participates in binding resolution and world ages. If a
   new binding is Revise'd into a re-exported module, this will now
   propagate.

2. The re-exported bindings are allocated lazily, improving performance.

An accessor for this functionality is provided as
`@Base.Experimental.reexport`. However, the only supported argument
to this macro is single `using` statement (unlike the Reexport.jl
version, which has more syntax). It is my expectation that Reexport.jl
will be updated to make use of the underlying funtionality here
directly, the base version of the macro is mostly for testing.

There are a few design warts here - in particular, we inherit
the module name exporting behavior
(JuliaLang/Reexport.jl#39).

However, I think that would be best addressed by not exporting
the module name itself from the modules but rather introducing
the module name as an additional binding on `using` statements.

However, this would be potentially breaking (even if the effect
is unlikely to be seen in practice), so I'm not proposing it here.
The Reexport.jl version of the macro can do whatever it needs to,
including creating an explicit non-exported binding to suppress
the automatic re-export for this if desired.

Partially written by Claude Code.

Author: Keno

base/experimental.jl

@@ -690,4 +690,55 @@ function wait_with_timeout(c::GenericCondition; first::Bool=false, timeout::Real
     end
 end
 
+"""
+    Base.Experimental.@reexport using Module
+
+Automatically re-export all exported names from a module when using it.
+
+# Examples
+
+```jldoctest
+julia> module A
+           export foo
+           foo() = "foo from A"
+       end
+A
+
+julia> module B
+           using Base.Experimental: @reexport
+           @reexport using ..A
+           # Now B exports foo, even though it's defined in A
+       end
+B
+
+julia> using .B
+
+julia> foo()
+"foo from A"
+```
+
+!!! warning
+    This interface is experimental and subject to change or removal without notice.
+"""
+macro reexport(ex)
+    if !Meta.isexpr(ex, :using) || isempty(ex.args)
+        error("@reexport must be used with a `using` statement, e.g., `@reexport using MyModule`")
+    end
+
+    # Check for `using Foo: x, y` syntax (not supported)
+    if any(arg -> Meta.isexpr(arg, :(:)), ex.args)
+        error("@reexport does not support `using Module: names` syntax")
+    end
+
+    # Generate _eval_using calls for each module in the using statement
+    calls = Expr(:block)
+    for mod_path in ex.args
+        push!(calls.args, :($(Core._eval_using)($(__module__), $(QuoteNode(mod_path)), $(Base.JL_MODULE_USING_REEXPORT))))
+    end
+    push!(calls.args, Expr(:latestworld))
+    push!(calls.args, :nothing)
+
+    return esc(calls)
+end
+
 end # module

base/module.jl

@@ -133,9 +133,9 @@ using A.B            => _module_using(Main, Expr(:., :A, :B))
 
 See also [`_using`](@ref Core._using).
 """
-function _eval_using(to::Module, path::Expr)
+function _eval_using(to::Module, path::Expr, flags::UInt8=UInt8(0))
     from = eval_import_path_all(to, path, "using")
-    Core._using(to, from)
+    Core._using(to, from, flags)
     is_package = length(path.args) == 1 && path.args[1] !== :.
     if to == Main && is_package
         Core._import(to, from, nameof(from))

base/runtime_internals.jl

@@ -250,12 +250,15 @@ const PARTITION_KIND_BACKDATED_CONST    = 0xb
 const PARTITION_FLAG_EXPORTED     = 0x10
 const PARTITION_FLAG_DEPRECATED   = 0x20
 const PARTITION_FLAG_DEPWARN      = 0x40
+const PARTITION_FLAG_IMPLICITLY_EXPORTED = 0x80
 
 const PARTITION_MASK_KIND         = 0x0f
 const PARTITION_MASK_FLAG         = 0xf0
 
 const BINDING_FLAG_ANY_IMPLICIT_EDGES = 0x8
 
+const JL_MODULE_USING_REEXPORT = 0x1
+
 is_defined_const_binding(kind::UInt8) = (kind == PARTITION_KIND_CONST || kind == PARTITION_KIND_CONST_IMPORT || kind == PARTITION_KIND_IMPLICIT_CONST || kind == PARTITION_KIND_BACKDATED_CONST)
 is_some_const_binding(kind::UInt8) = (is_defined_const_binding(kind) || kind == PARTITION_KIND_UNDEF_CONST)
 is_some_imported(kind::UInt8) = (kind == PARTITION_KIND_IMPLICIT_GLOBAL || kind == PARTITION_KIND_IMPLICIT_CONST || kind == PARTITION_KIND_EXPLICIT || kind == PARTITION_KIND_IMPORTED)

base/show.jl

@@ -3294,6 +3294,10 @@ function print_partition(io::IO, partition::Core.BindingPartition)
         if (partition.kind & PARTITION_FLAG_EXPORTED) != 0
             print(io, "exported")
         end
+        if (partition.kind & PARTITION_FLAG_IMPLICITLY_EXPORTED) != 0
+            first ? (first = false) : print(io, ",")
+            print(io, "re-exported")
+        end
         if (partition.kind & PARTITION_FLAG_DEPRECATED) != 0
             first ? (first = false) : print(io, ",")
             print(io, "deprecated")

src/builtins.c

@@ -1579,10 +1579,15 @@ JL_CALLABLE(jl_f__import)
 // _using(to::Module, from::Module)
 JL_CALLABLE(jl_f__using)
 {
-    JL_NARGS(_using, 2, 2);
+    JL_NARGS(_using, 2, 3);
     JL_TYPECHK(_using, module, args[0]);
     JL_TYPECHK(_using, module, args[1]);
-    jl_module_using((jl_module_t *)args[0], (jl_module_t *)args[1]);
+    size_t flags = 0;
+    if (nargs == 3) {
+        JL_TYPECHK(_using, uint8, args[2]);
+        flags = jl_unbox_uint8(args[2]);
+    }
+    jl_module_using((jl_module_t *)args[0], (jl_module_t *)args[1], flags);
     return jl_nothing;
 }
 

src/gc-stock.c

@@ -2168,9 +2168,9 @@ STATIC_INLINE void gc_mark_module_binding(jl_ptls_t ptls, jl_module_t *mb_parent
         jl_value_t *obj_parent = (jl_value_t *)mb_parent;
         struct _jl_module_using *objary_begin = (struct _jl_module_using *)mb_parent->usings.items;
         struct _jl_module_using *objary_end = objary_begin + nusings;
-        static_assert(sizeof(struct _jl_module_using) == 3*sizeof(void *), "Mismatch in _jl_module_using size");
+        static_assert(sizeof(struct _jl_module_using) == 4*sizeof(void *), "Mismatch in _jl_module_using size");
         static_assert(offsetof(struct _jl_module_using, mod) == 0, "Expected `mod` at the beginning of _jl_module_using");
-        gc_mark_objarray(ptls, obj_parent, (jl_value_t**)objary_begin, (jl_value_t**)objary_end, 3, nptr);
+        gc_mark_objarray(ptls, obj_parent, (jl_value_t**)objary_begin, (jl_value_t**)objary_end, 4, nptr);
     }
     else {
         gc_mark_push_remset(ptls, (jl_value_t *)mb_parent, nptr);

src/julia.h

@@ -765,6 +765,9 @@ static const uint8_t PARTITION_FLAG_DEPRECATED     = 0x20;
 // implies _DEPRECATED. However, the reverse is not true. Such bindings are usually used for functions,
 // where calling the function itself will provide a (better) deprecation warning/error.
 static const uint8_t PARTITION_FLAG_DEPWARN        = 0x40;
+// _IMPLICITLY_EXPORTED: This binding partition is implicitly exported via @reexport. Unlike _EXPORTED,
+// this flag is set during implicit resolution and can be removed if the resolution changes.
+static const uint8_t PARTITION_FLAG_IMPLICITLY_EXPORTED = 0x80;
 
 #if defined(_COMPILER_MICROSOFT_)
 #define JL_ALIGNED_ATTR(alignment) \
@@ -853,6 +856,8 @@ typedef struct _jl_module_t {
     int8_t max_methods;
     // If cleared no binding partition in this module has PARTITION_FLAG_EXPORTED and min_world > jl_require_world.
     _Atomic(int8_t) export_set_changed_since_require_world;
+    // Set if this module has any reexport usings (used to bypass fast-path in implicit resolution)
+    _Atomic(int8_t) has_reexports;
     jl_mutex_t lock;
     intptr_t hash;
 } jl_module_t;
@@ -861,8 +866,12 @@ struct _jl_module_using {
     jl_module_t *mod;
     size_t min_world;
     size_t max_world;
+    size_t flags;
 };
 
+// Flags for _jl_module_using.flags
+static const uint8_t JL_MODULE_USING_REEXPORT = 0x1;
+
 struct _jl_globalref_t {
     JL_DATA_TYPE
     jl_module_t *mod;
@@ -2038,7 +2047,7 @@ JL_DLLEXPORT jl_binding_partition_t *jl_declare_constant_val(jl_binding_t *b JL_
 JL_DLLEXPORT jl_binding_partition_t *jl_declare_constant_val2(jl_binding_t *b JL_ROOTING_ARGUMENT, jl_module_t *mod, jl_sym_t *var, jl_value_t *val JL_ROOTED_ARGUMENT JL_MAYBE_UNROOTED, enum jl_partition_kind);
 JL_DLLEXPORT void jl_module_import(jl_task_t *ct, jl_module_t *to, jl_module_t *from, jl_sym_t *asname, jl_sym_t *s, int explici);
 JL_DLLEXPORT void jl_import_module(jl_task_t *ct, jl_module_t *m, jl_module_t *import, jl_sym_t *asname);
-JL_DLLEXPORT void jl_module_using(jl_module_t *to, jl_module_t *from);
+JL_DLLEXPORT void jl_module_using(jl_module_t *to, jl_module_t *from, size_t flags);
 int jl_module_public_(jl_module_t *from, jl_sym_t *s, int exported, size_t new_world);
 JL_DLLEXPORT int jl_is_imported(jl_module_t *m, jl_sym_t *s);
 JL_DLLEXPORT int jl_module_exports_p(jl_module_t *m, jl_sym_t *var);

src/julia_internal.h

@@ -913,19 +913,19 @@ typedef struct _modstack_t {
 // The analyzer doesn't like looking through the arraylist, so just model the
 // access for it using this function
 STATIC_INLINE struct _jl_module_using *module_usings_getidx(jl_module_t *m JL_PROPAGATES_ROOT, size_t i) JL_NOTSAFEPOINT {
-    return (struct _jl_module_using *)&(m->usings.items[3*i]);
+    return (struct _jl_module_using *)&(m->usings.items[4*i]);
 }
 STATIC_INLINE jl_module_t *module_usings_getmod(jl_module_t *m JL_PROPAGATES_ROOT, size_t i) JL_NOTSAFEPOINT {
     return module_usings_getidx(m, i)->mod;
 }
 #endif
 
 STATIC_INLINE size_t module_usings_length(jl_module_t *m) JL_NOTSAFEPOINT {
-    return m->usings.len/3;
+    return m->usings.len/4;
 }
 
 STATIC_INLINE size_t module_usings_max(jl_module_t *m) JL_NOTSAFEPOINT {
-    return m->usings.max/3;
+    return m->usings.max/4;
 }
 
 JL_DLLEXPORT jl_sym_t *jl_module_name(jl_module_t *m) JL_NOTSAFEPOINT;
@@ -1026,6 +1026,10 @@ STATIC_INLINE int jl_bkind_is_real_constant(enum jl_partition_kind kind) JL_NOTS
     return kind == PARTITION_KIND_IMPLICIT_CONST || kind == PARTITION_KIND_CONST || kind == PARTITION_KIND_CONST_IMPORT;
 }
 
+STATIC_INLINE int jl_bpart_is_exported(uint8_t flags) JL_NOTSAFEPOINT {
+    return flags & (PARTITION_FLAG_EXPORTED | PARTITION_FLAG_IMPLICITLY_EXPORTED);
+}
+
 JL_DLLEXPORT jl_binding_partition_t *jl_get_binding_partition(jl_binding_t *b JL_PROPAGATES_ROOT, size_t world) JL_GLOBALLY_ROOTED;
 JL_DLLEXPORT jl_binding_partition_t *jl_get_binding_partition_with_hint(jl_binding_t *b JL_PROPAGATES_ROOT, jl_binding_partition_t *previous_part, size_t world) JL_GLOBALLY_ROOTED;
 JL_DLLEXPORT jl_binding_partition_t *jl_get_binding_partition_all(jl_binding_t *b JL_PROPAGATES_ROOT, size_t min_world, size_t max_world) JL_GLOBALLY_ROOTED;