Give undefined constants their own binding kind (#56968)

As noted in
#56649 (comment),
there needs to be a separate kind for undefined constants, so we know
whether or not it is safe to call `partition_restriction`.

base/runtime_internals.jl

@@ -228,10 +228,12 @@ const BINDING_KIND_IMPORTED     = 0x5
 const BINDING_KIND_FAILED       = 0x6
 const BINDING_KIND_DECLARED     = 0x7
 const BINDING_KIND_GUARD        = 0x8
+const BINDING_KIND_UNDEF_CONST  = 0x9
 
-is_some_const_binding(kind::UInt8) = (kind == BINDING_KIND_CONST || kind == BINDING_KIND_CONST_IMPORT)
+is_defined_const_binding(kind::UInt8) = (kind == BINDING_KIND_CONST || kind == BINDING_KIND_CONST_IMPORT)
+is_some_const_binding(kind::UInt8) = (is_defined_const_binding(kind) || kind == BINDING_KIND_UNDEF_CONST)
 is_some_imported(kind::UInt8) = (kind == BINDING_KIND_IMPLICIT || kind == BINDING_KIND_EXPLICIT || kind == BINDING_KIND_IMPORTED)
-is_some_guard(kind::UInt8) = (kind == BINDING_KIND_GUARD || kind == BINDING_KIND_DECLARED || kind == BINDING_KIND_FAILED)
+is_some_guard(kind::UInt8) = (kind == BINDING_KIND_GUARD || kind == BINDING_KIND_DECLARED || kind == BINDING_KIND_FAILED || kind == BINDING_KIND_UNDEF_CONST)
 
 function lookup_binding_partition(world::UInt, b::Core.Binding)
     ccall(:jl_get_binding_partition, Ref{Core.BindingPartition}, (Any, UInt), b, world)

base/show.jl

@@ -1045,7 +1045,7 @@ function check_world_bounded(tn::Core.TypeName)
     isdefined(bnd, :partitions) || return nothing
     partition = @atomic bnd.partitions
     while true
-        if is_some_const_binding(binding_kind(partition)) && partition_restriction(partition) <: tn.wrapper
+        if is_defined_const_binding(binding_kind(partition)) && partition_restriction(partition) <: tn.wrapper
             max_world = @atomic partition.max_world
             max_world == typemax(UInt) && return nothing
             return Int(partition.min_world):Int(max_world)
@@ -3364,9 +3364,11 @@ function print_partition(io::IO, partition::Core.BindingPartition)
     end
     print(io, " - ")
     kind = binding_kind(partition)
-    if is_some_const_binding(kind)
+    if is_defined_const_binding(kind)
         print(io, "constant binding to ")
         print(io, partition_restriction(partition))
+    elseif kind == BINDING_KIND_UNDEF_CONST
+        print(io, "undefined const binding")
     elseif kind == BINDING_KIND_GUARD
         print(io, "undefined binding - guard entry")
     elseif kind == BINDING_KIND_FAILED

src/julia.h

@@ -623,7 +623,7 @@ typedef struct _jl_weakref_t {
 
 // N.B: Needs to be synced with runtime_internals.jl
 enum jl_partition_kind {
-    // Constant: This binding partition is a constant declared using `const`
+    // Constant: This binding partition is a constant declared using `const _ = ...`
     //  ->restriction holds the constant value
     BINDING_KIND_CONST        = 0x0,
     // Import Constant: This binding partition is a constant declared using `import A`
@@ -649,7 +649,11 @@ enum jl_partition_kind {
     BINDING_KIND_DECLARED     = 0x7,
     // Guard: The binding was looked at, but no global or import was resolved at the time
     //  ->restriction is NULL.
-    BINDING_KIND_GUARD        = 0x8
+    BINDING_KIND_GUARD        = 0x8,
+    // Undef Constant: This binding partition is a constant declared using `const`, but
+    // without a value.
+    //  ->restriction is NULL
+    BINDING_KIND_UNDEF_CONST  = 0x9
 };
 
 #ifdef _P64

src/julia_internal.h

@@ -924,8 +924,13 @@ EXTERN_INLINE_DECLARE enum jl_partition_kind decode_restriction_kind(jl_ptr_kind
     uint8_t bits = (pku & 0x7);
     jl_value_t *val = (jl_value_t*)(pku & ~0x7);
 
-    if (val == NULL && bits == BINDING_KIND_IMPLICIT) {
-        return BINDING_KIND_GUARD;
+    if (val == NULL) {
+        if (bits == BINDING_KIND_IMPLICIT) {
+            return BINDING_KIND_GUARD;
+        }
+        if (bits == BINDING_KIND_CONST) {
+            return BINDING_KIND_UNDEF_CONST;
+        }
     }
 
     return (enum jl_partition_kind)bits;
@@ -947,10 +952,14 @@ STATIC_INLINE jl_value_t *decode_restriction_value(jl_ptr_kind_union_t JL_PROPAG
 STATIC_INLINE jl_ptr_kind_union_t encode_restriction(jl_value_t *val, enum jl_partition_kind kind) JL_NOTSAFEPOINT
 {
 #ifdef _P64
-    if (kind == BINDING_KIND_GUARD || kind == BINDING_KIND_DECLARED || kind == BINDING_KIND_FAILED)
+    if (kind == BINDING_KIND_GUARD || kind == BINDING_KIND_DECLARED || kind == BINDING_KIND_FAILED || kind == BINDING_KIND_UNDEF_CONST)
         assert(val == NULL);
+    else if (kind == BINDING_KIND_IMPLICIT || kind == BINDING_KIND_CONST)
+        assert(val != NULL);
     if (kind == BINDING_KIND_GUARD)
         kind = BINDING_KIND_IMPLICIT;
+    else if (kind == BINDING_KIND_UNDEF_CONST)
+        kind = BINDING_KIND_CONST;
     assert((((uintptr_t)val) & 0x7) == 0);
     return ((jl_ptr_kind_union_t)val) | kind;
 #else
@@ -964,6 +973,10 @@ STATIC_INLINE int jl_bkind_is_some_import(enum jl_partition_kind kind) JL_NOTSAF
 }
 
 STATIC_INLINE int jl_bkind_is_some_constant(enum jl_partition_kind kind) JL_NOTSAFEPOINT {
+    return kind == BINDING_KIND_CONST || kind == BINDING_KIND_CONST_IMPORT || kind == BINDING_KIND_UNDEF_CONST;
+}
+
+STATIC_INLINE int jl_bkind_is_defined_constant(enum jl_partition_kind kind) JL_NOTSAFEPOINT {
     return kind == BINDING_KIND_CONST || kind == BINDING_KIND_CONST_IMPORT;
 }
 

src/module.c

@@ -400,7 +400,10 @@ JL_DLLEXPORT jl_value_t *jl_get_binding_value_if_resolved(jl_binding_t *b)
 JL_DLLEXPORT jl_value_t *jl_bpart_get_restriction_value(jl_binding_partition_t *bpart)
 {
     jl_ptr_kind_union_t pku = jl_atomic_load_relaxed(&bpart->restriction);
-    return decode_restriction_value(pku);
+    jl_value_t *v = decode_restriction_value(pku);
+    if (!v)
+        jl_throw(jl_undefref_exception);
+    return v;
 }
 
 typedef struct _modstack_t {

src/toplevel.c

@@ -157,7 +157,7 @@ static jl_value_t *jl_eval_module_expr(jl_module_t *parent_module, jl_expr_t *ex
     else {
         jl_binding_t *b = jl_get_module_binding(parent_module, name, 1);
         jl_binding_partition_t *bpart = jl_get_binding_partition(b, ct->world_age);
-        jl_ptr_kind_union_t pku = encode_restriction(NULL, BINDING_KIND_CONST);
+        jl_ptr_kind_union_t pku = encode_restriction(NULL, BINDING_KIND_UNDEF_CONST);
         jl_ptr_kind_union_t new_pku = encode_restriction((jl_value_t*)newm, BINDING_KIND_CONST);
         if (!jl_atomic_cmpswap(&bpart->restriction, &pku, new_pku)) {
             if (decode_restriction_kind(pku) != BINDING_KIND_CONST) {
@@ -749,8 +749,7 @@ JL_DLLEXPORT jl_binding_partition_t *jl_declare_constant_val2(jl_binding_t *b, j
     while (1) {
         if (jl_bkind_is_some_constant(decode_restriction_kind(pku))) {
             if (!val) {
-                JL_GC_POP();
-                return bpart;
+                break;
             }
             jl_value_t *old = decode_restriction_value(pku);
             JL_GC_PROMISE_ROOTED(old);
@@ -787,7 +786,7 @@ JL_DLLEXPORT jl_binding_partition_t *jl_declare_constant_val2(jl_binding_t *b, j
 
 JL_DLLEXPORT jl_binding_partition_t *jl_declare_constant_val(jl_binding_t *b, jl_module_t *mod, jl_sym_t *var, jl_value_t *val)
 {
-    return jl_declare_constant_val2(b, mod, var, val, BINDING_KIND_CONST);
+    return jl_declare_constant_val2(b, mod, var, val, val ? BINDING_KIND_CONST : BINDING_KIND_UNDEF_CONST);
 }
 
 JL_DLLEXPORT void jl_eval_const_decl(jl_module_t *m, jl_value_t *arg, jl_value_t *val)