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
 ----------------

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