deprecate const on local variables, which used to be ignored.

part of #5148

NEWS.md

@@ -63,6 +63,9 @@ Language changes
     (`need_to_handle_undef_sparam = Set{Any}(m.sig for m in Test.detect_unbound_args(Base, recursive=true))`)
     is equal (`==`) to some known set (`expected = Set()`). ([#23117])
 
+  * `const` declarations on local variables were previously ignored. They now give a
+    warning, so that this syntax can be disallowed or given a new meaning in a
+    future version ([#5148]).
 
 Breaking changes
 ----------------

doc/src/manual/variables-and-scoping.md

@@ -475,7 +475,8 @@ their values (or even their types) might change at almost any time. If a global
 not change, adding a `const` declaration solves this performance problem.
 
 Local constants are quite different. The compiler is able to determine automatically when a local
-variable is constant, so local constant declarations are not necessary, and are currently just ignored.
+variable is constant, so local constant declarations are not necessary, and in fact are currently
+not supported.
 
 Special top-level assignments, such as those performed by the `function` and `struct` keywords,
 are constant by default.

src/julia-syntax.scm

@@ -3115,11 +3115,7 @@ f(x) = yt(x)
                  (if (vinfo:never-undef vi)
                      '(null)
                      `(newvar ,(cadr e))))))
-          ((const)
-           (if (or (assq (cadr e) (car  (lam:vinfo lam)))
-                   (assq (cadr e) (cadr (lam:vinfo lam))))
-               '(null)
-               e))
+          ((const) e)
           ((isdefined) ;; convert isdefined expr to function for closure converted variables
            (let* ((sym (cadr e))
                   (vi (and (symbol? sym) (assq sym (car  (lam:vinfo lam)))))
@@ -3661,7 +3657,14 @@ f(x) = yt(x)
             ((local-def) #f)
             ((local) #f)
             ((implicit-global) #f)
-            ((const) (emit e))
+            ((const)
+             (if (or (assq (cadr e) (car  (lam:vinfo lam)))
+                     (assq (cadr e) (cadr (lam:vinfo lam))))
+                 (begin
+                   (syntax-deprecation #f (string "`const` declaration on local variable" (linenode-string current-loc))
+                                       "")
+                   '(null))
+                 (emit e)))
             ((isdefined) (if tail (emit-return e) e))
 
             ;; top level expressions returning values