Always disallow assignments to constants

We have historically allowed the following without warning or error:

```
const x = 1
x = 1
```

As well as
```
const x = 1
x = 2
```
with a UB warning.

In 1.12, we made the second case error, as part of the general binding
partition changes, but did not touch the first case, because it did not
have the warning.

I think this made a reasonable amount of sense, because we essentially
treated constants as special kinds of mutable globals (to which assignment
happened to be UB).

However, in the 1.12+ design, constants and globals are quite sepearate
beasts, and I think it makes sense to extend the error to the egal case
also, even if it is technically more breaking.

In fact, I already thought that's what I did when I implemented the new
effect model for global assignment, causing #57566. I can't think of a
legitimate reason to keep this special case. For those who want to do
binding replacement, the `const` keyword is mandatory, so the assignment
is now literally always a semantic no-op or an error.

Fixes #57566.

Author: Keno

src/module.c

@@ -545,14 +545,19 @@ JL_DLLEXPORT void jl_check_binding_currently_writable(jl_binding_t *b, jl_module
 {
     jl_binding_partition_t *bpart = jl_get_binding_partition(b, jl_current_task->world_age);
     enum jl_partition_kind kind = jl_binding_kind(bpart);
-    if (kind != BINDING_KIND_GLOBAL && kind != BINDING_KIND_DECLARED && !jl_bkind_is_some_constant(kind)) {
+    if (kind != BINDING_KIND_GLOBAL && kind != BINDING_KIND_DECLARED) {
         if (jl_bkind_is_some_guard(kind)) {
             jl_errorf("Global %s.%s does not exist and cannot be assigned.\n"
                         "Note: Julia 1.9 and 1.10 inadvertently omitted this error check (#56933).\n"
                         "Hint: Declare it using `global %s` inside `%s` before attempting assignment.",
                         jl_symbol_name(m->name), jl_symbol_name(s),
                         jl_symbol_name(s), jl_symbol_name(m->name));
-        } else {
+        }
+        else if (jl_bkind_is_some_constant(kind)) {
+            jl_errorf("invalid assignment to constant %s.%s. This redefinition may be permitted using the `const` keyword.",
+                        jl_symbol_name(m->name), jl_symbol_name(s));
+        }
+        else {
             jl_module_t *from = jl_binding_dbgmodule(b, m, s);
             if (from == m)
                 jl_errorf("cannot assign a value to imported variable %s.%s",
@@ -1449,16 +1454,6 @@ jl_value_t *jl_check_binding_assign_value(jl_binding_t *b JL_PROPAGATES_ROOT, jl
     JL_GC_PUSH1(&rhs); // callee-rooted
     jl_binding_partition_t *bpart = jl_get_binding_partition(b, jl_current_task->world_age);
     enum jl_partition_kind kind = jl_binding_kind(bpart);
-    if (jl_bkind_is_some_constant(kind)) {
-        jl_value_t *old = bpart->restriction;
-        JL_GC_PROMISE_ROOTED(old);
-        if (jl_egal(rhs, old)) {
-            JL_GC_POP();
-            return NULL;
-        }
-        jl_errorf("invalid assignment to constant %s.%s. This redefinition may be permitted using the `const` keyword.",
-                    jl_symbol_name(mod->name), jl_symbol_name(var));
-    }
     assert(kind == BINDING_KIND_DECLARED || kind == BINDING_KIND_GLOBAL);
     jl_value_t *old_ty = kind == BINDING_KIND_DECLARED ? (jl_value_t*)jl_any_type : bpart->restriction;
     JL_GC_PROMISE_ROOTED(old_ty);

test/syntax.jl

@@ -3969,8 +3969,13 @@ end
 
 # Test trying to define a constant and then trying to assign to the same value
 module AssignConstValueTest
+    using Test
     const x = 1
-    x = 1
+    @test_throws ErrorException @eval x = 1
+    @test_throws ErrorException @eval begin
+        @Base.Experimental.force_compile
+        global x = 1
+    end
 end
 @test isconst(AssignConstValueTest, :x)