#16071: restrict typed variable declaration to local x::T and global x::T…

[jamesonquinn]

This is my first Julia pull request, so tell me if I'm doing it wrong. This is a fix to #16071. My deprecation message is:

          (syntax-deprecation s
            (string "Type statement on left hand side of assignment.")
            (string "Use with `local` or `global` first.")))

Also, this doesn't catch the case where the LHS is in parentheses or otherwise complicated (such as tuple assignment where only the first tuple item is typed.)

[fcard]

When deprecating something, you should look for uses of it on the codebase and fix them. Also, tests!

[vtjnash]

It's easier and more accurate to do this during lowering (julia-syntax.scm) rather than parsing. That also allows macros to continue using the syntax, and keeps the parser simpler (context-unaware), but changes the interpretation of that syntax later. For example, here I think we would just need to catch the branch where this construct is handled during lowering:

diff --git a/src/julia-syntax.scm b/src/julia-syntax.scm
index 770aaae..c52b34e 100644
--- a/src/julia-syntax.scm
+++ b/src/julia-syntax.scm
@@ -1828,7 +1828,13 @@
           (let ((x (cadr lhs))
                 (T (caddr lhs))
                 (rhs (caddr e)))
-            (let ((e (remove-argument-side-effects x)))
+            (let* ((e (remove-argument-side-effects x))
+                   (str-x (deparse x))
+                   (str-T (deparse T))
+                   (str-lhs (string str-x "::" str-T " = ...")))
+              (if (symbol? x)
+                  (syntax-deprecation #f str-lhs (string "local " str-lhs " = ..."))
+                  (syntax-deprecation #f str-lhs (string "typeassert(" str-x ", " str-T "); " str-x " = ...")))
               (expand-forms
                `(block ,@(cdr e)
                        (decl ,(car e) ,T)

(fwiw, your PR looked fine otherwise, just starting off in the wrong direction on where to make the change)