const lowering: respect scope, prohibit non-const const assignment

Modifies the handling of the 3-argument "lowered const" so `const x = y`
participates in scope resolution again (#56613); we treat (const x y) as an
assignment form, producing the "unsupported `const` declaration on local
variable" error if the usual scope rules would not make x global.

At the top level, these should succeed:

    const x = 1
    begin
        const x = 1
    end
    let
        const global x = 1
    end

Each of the following should fail:

    # ERROR: syntax: unsupported `const` declaration on local variable around REPL[1]:2
    let
        const x = 1
    end

    # ERROR: syntax: unsupported `const` declaration on local variable around REPL[1]:2
    function f()
        const x = 1
    end

This also amkes the use of `const global` inside a function body produce the
original, specific error message rather than a generic "World age increment not
at top level" error (#57334):

    # ERROR: syntax: `global const` declaration not allowed inside function around REPL[1]:2
    function f()
        global const x = 1
    end

Issue #57334 revealed we silently accept `const` expressions writing to
variables.  These now fail with a new error message:

    a = [1, 2]
    # ERROR: syntax: cannot declare "a[1]" `const` around REPL[1]:2
    const a[1] = 3

The bulk of these changes track const-ness through the functions called by
expand-assignment, since each of the three types of LHS is affect differently:
- ordinary variables become `const`, producing error if not global
- `X{T} = ...` forces X to be `const`, rather than the defaulting to being
  implicitly const only if in global scope.
- `f() = ...` ignores `const`.

Author: xal-0

src/julia-syntax.scm

@@ -1423,7 +1423,7 @@
           (else
            (error "invalid \"try\" form")))))
 
-(define (expand-unionall-def name type-ex (allow-local #t))
+(define (expand-unionall-def name type-ex (const? #t))
   (if (and (pair? name)
            (eq? (car name) 'curly))
       (let ((name   (cadr name))
@@ -1434,7 +1434,7 @@
           (expand-forms
             `(block
               (= ,rr (where ,type-ex ,@params))
-              (,(if allow-local 'assign-const-if-global 'const) ,name ,rr)
+              (,(if const? 'const 'assign-const-if-global) ,name ,rr)
               (latestworld-if-toplevel)
               ,rr)))
       (expand-forms
@@ -1444,38 +1444,24 @@
   (filter (lambda (x) (not (underscore-symbol? x))) syms))
 
 ;; Expand `[global] const a::T = val`
-(define (expand-const-decl e (mustassgn #f))
-  (if (length= e 3) e
-    (let ((arg (cadr e)))
-      (if (atom? arg)
-          (if mustassgn
-            (error "expected assignment after \"const\"")
-            e)
-          (case (car arg)
-            ((global)
-              (expand-const-decl `(const ,(cadr arg)) #t))
-            ((=)
-              (cond
-                ;; `const f() = ...` - The `const` here is inoperative, but the syntax happened to work in earlier versions, so simply strip `const`.
-                ;; TODO: Consider whether to keep this in 2.0.
-                ((eventually-call? (cadr arg))
-                 (expand-forms arg))
-                ((and (pair? (cadr arg)) (eq? (caadr arg) 'curly))
-                 (expand-unionall-def (cadr arg) (caddr arg)))
-                ((and (pair? (cadr arg)) (eq? (caadr arg) 'tuple) (not (has-parameters? (cdr (cadr arg)))))
-                  ;; We need this case because `(f(), g()) = (1, 2)` goes through here, which cannot go via the `local` lowering below,
-                  ;; because the symbols come out wrong. Sigh... So much effort for such a syntax corner case.
-                  (expand-tuple-destruct (cdr (cadr arg)) (caddr arg) (lambda (assgn) `(,(car e) ,assgn))))
-                (else
-                 (let ((rr (make-ssavalue)))
-                   (expand-forms `(block
-                           (= ,rr ,(caddr arg))
-                           (scope-block (block (hardscope)
-                            (local (= ,(cadr arg) ,rr))
-                            ,.(map (lambda (v) `(,(car e) (globalref (thismodule) ,v) ,v)) (filter-not-underscore (lhs-vars (cadr arg))))
-                            (latestworld)
-                            ,rr))))))))
-            (else (error "expected assignment after \"const\"")))))))
+(define (expand-const-decl e)
+  (define (check-assignment asgn)
+    (unless (and (pair? asgn) (eq? (car asgn) '=))
+      ;; (const (global x)) is possible due to a parser quirk
+      (error "expected assignment after \"const\"")))
+  (if (length= e 3)
+      `(const ,(cadr e) ,(expand-forms (caddr e)))
+      (let ((arg (cadr e)))
+        (case (car arg)
+          ((global) (let ((asgn (cadr arg)))
+                      (check-assignment asgn)
+                      `(block
+                        ,.(map (lambda (v) `(global ,v))
+                               (filter-not-underscore (lhs-vars (cadr asgn))))
+                        ,(expand-assignment asgn #t))))
+          ((=)      (check-assignment arg)
+                    (expand-assignment arg #t))
+          (else     (error "expected assignment after \"const\""))))))
 
 (define (expand-atomic-decl e)
   (error "unimplemented or unsupported atomic declaration"))
@@ -1532,35 +1518,54 @@
                   (eq? (car (cadr lhs)) 'call)))))
   (define (assignment-to-function lhs e)  ;; convert '= expr to 'function expr
     (cons 'function (cdr e)))
+  (define (maybe-wrap-const x)
+    (if const? `(const ,x) x))
   (cond
    ((function-lhs? lhs)
+    ;; `const f() = ...` - The `const` here is inoperative, but the syntax
+    ;; happened to work in earlier versions, so simply strip `const`.
     (expand-forms (assignment-to-function lhs e)))
    ((and (pair? lhs)
          (eq? (car lhs) 'curly))
-    (expand-unionall-def (cadr e) (caddr e)))
+    (expand-unionall-def (cadr e) (caddr e) const?))
    ((assignment? (caddr e))
     ;; chain of assignments - convert a=b=c to `b=c; a=c`
     (let loop ((lhss (list lhs))
                (rhs  (caddr e)))
       (if (and (assignment? rhs) (not (function-lhs? (cadr rhs))))
           (loop (cons (cadr rhs) lhss) (caddr rhs))
-          (let ((rr (if (symbol-like? rhs) rhs (make-ssavalue))))
+          (let ((rr (if (symbol-like? rhs) rhs (make-ssavalue)))
+                (lhss (reverse lhss)))
             (expand-forms
              `(block ,.(if (eq? rr rhs) '() `((= ,rr ,(if (assignment? rhs)
                                                           (assignment-to-function (cadr rhs) rhs)
                                                           rhs))))
+                     ;; In const x = y = z, only x becomes const
+                     ,(maybe-wrap-const `(= ,(car lhss) ,rr))
                      ,@(map (lambda (l) `(= ,l ,rr))
-                            lhss)
+                            (cdr lhss))
                      (unnecessary ,rr)))))))
    ((or (and (symbol-like? lhs) (valid-name? lhs))
         (globalref? lhs))
-    (sink-assignment lhs (expand-forms (caddr e))))
+    ;; TODO: We currently call (latestworld) after every (const _ _), but this
+    ;; may need to be moved elsewhere if we want to avoid making one const
+    ;; visible before side effects have been performed (#57484)
+    (if const?
+        (let ((rr (make-ssavalue)))
+          `(block
+            ,(sink-assignment rr (expand-forms (caddr e)))
+            (const ,lhs ,rr)
+            (latestworld)
+            (unnecessary ,rr)))
+        (sink-assignment lhs (expand-forms (caddr e)))))
    ((atom? lhs)
     (error (string "invalid assignment location \"" (deparse lhs) "\"")))
    (else
     (case (car lhs)
       ((|.|)
        ;; a.b =
+       (when const?
+         (error (string "cannot declare \"" (deparse lhs) "\" `const`")))
        (let* ((a   (cadr lhs))
               (b  (caddr lhs))
               (rhs (caddr e)))
@@ -1582,15 +1587,17 @@
              (x    (caddr e)))
          (if (has-parameters? lhss)
              ;; property destructuring
-             (expand-property-destruct lhss x)
+             (expand-property-destruct lhss x maybe-wrap-const)
              ;; multiple assignment
-             (expand-tuple-destruct lhss x))))
+             (expand-tuple-destruct lhss x maybe-wrap-const))))
       ((typed_hcat)
        (error "invalid spacing in left side of indexed assignment"))
       ((typed_vcat typed_ncat)
        (error "unexpected \";\" in left side of indexed assignment"))
       ((ref)
        ;; (= (ref a . idxs) rhs)
+       (when const?
+         (error (string "cannot declare \"" (deparse lhs) "\" `const`")))
        (let ((a    (cadr lhs))
              (idxs (cddr lhs))
              (rhs  (caddr e)))
@@ -1620,10 +1627,24 @@
              (T (caddr lhs))
              (rhs (caddr e)))
          (let ((e (remove-argument-side-effects x)))
-           (expand-forms
-            `(block ,@(cdr e)
-                    (decl ,(car e) ,T)
-                    (= ,(car e) ,rhs))))))
+           (if const?
+               ;; This could go through convert-assignment in the closure
+               ;; conversion pass, but since constants don't have declared types
+               ;; the way other variables do, we insert convert() here.
+               (expand-forms
+                ;; TODO: This behaviour (`const _:T = ...` does not call convert,
+                ;; but still evaluates RHS) should be documented.
+                `(const ,(car e) ,(if (underscore-symbol? (car e))
+                                rhs
+                                (convert-for-type-decl rhs T #t #f))))
+               (expand-forms
+                `(block ,@(cdr e)
+                        ;; TODO: When x is a complex expression, this acts as a
+                        ;; typeassert rather than a declaration.
+                        ,.(if (underscore-symbol? (car e))
+                              '() ; Assignment to _ will ultimately be discarded---don't declare anything
+                              `((decl ,(car e) ,T)))
+                        ,(maybe-wrap-const `(= ,(car e) ,rhs))))))))
       ((vcat ncat)
        ;; (= (vcat . args) rhs)
        (error "use \"(a, b) = ...\" to assign multiple values"))
@@ -2365,7 +2386,7 @@
          (gensy))
         (else (make-ssavalue))))
 
-(define (expand-property-destruct lhs x)
+(define (expand-property-destruct lhs x (wrap identity))
   (if (not (length= lhs 1))
       (error (string "invalid assignment location \"" (deparse `(tuple ,lhs)) "\"")))
   (let* ((lhss (cdar lhs))
@@ -2380,7 +2401,7 @@
                                 (cadr field))
                                (else
                                 (error (string "invalid assignment location \"" (deparse `(tuple ,lhs)) "\""))))))
-               (expand-forms `(= ,field (call (top getproperty) ,xx (quote ,prop))))))
+               (expand-forms (wrap `(= ,field (call (top getproperty) ,xx (quote ,prop)))))))
            lhss)
        (unnecessary ,xx))))
 
@@ -2401,7 +2422,6 @@
   (if (null? lhss)
       '()
       (let* ((lhs  (car lhss))
-             (wrapfirst (lambda (x i) (if (= i 1) (wrap x) x)))
              (lhs- (cond ((or (symbol? lhs) (ssavalue? lhs))
                           lhs)
                          ((vararg? lhs)
@@ -2413,7 +2433,10 @@
                                              (make-ssavalue))))))
                          ;; can't use ssavalues if it's a function definition
                          ((eventually-call? lhs) (gensy))
-                         (else (make-ssavalue)))))
+                         (else (make-ssavalue))))
+             ;; If we use an intermediary lhs, don't wrap `const`.
+             (wrap-subassign (if (eq? lhs lhs-) wrap identity))
+             (wrapfirst (lambda (x i) (if (= i 1) (wrap-subassign x) x))))
         (if (and (vararg? lhs) (any vararg? (cdr lhss)))
             (error "multiple \"...\" on lhs of assignment"))
         (if (not (eq? lhs lhs-))
@@ -2425,7 +2448,7 @@
                 (if (underscore-symbol? (cadr lhs-))
                     '()
                     (list (expand-forms
-                            (wrap `(= ,(cadr lhs-) (call (top rest) ,xx ,@(if (eq? i 1) '() `(,st))))))))
+                            (wrap-subassign `(= ,(cadr lhs-) (call (top rest) ,xx ,@(if (eq? i 1) '() `(,st))))))))
                 (let ((tail (if (eventually-call? lhs) (gensy) (make-ssavalue))))
                   (cons (expand-forms
                           (lower-tuple-assignment
@@ -2998,7 +3021,7 @@
             ;; like v = val, except that if `v` turns out global(either
             ;; implicitly or by explicit `global`), it gains an implicit `const`
             (set! vars (cons (cadr e) vars)))
-          ((=)
+          ((= const)
            (let ((v (decl-var (cadr e))))
              (find-assigned-vars- (caddr e))
              (if (or (ssavalue? v) (globalref? v) (underscore-symbol? v))
@@ -3127,7 +3150,7 @@
         ((eq? (car e) 'assign-const-if-global)
            (if (eq? (var-kind (cadr e) scope) 'local)
                (if (length= e 2) (null) `(= ,@(cdr e)))
-               `(const ,@(cdr e))))
+               (resolve-scopes- `(const ,@(cdr e)) scope sp loc)))
         ((memq (car e) '(local local-def))
          (check-valid-name (cadr e))
          ;; remove local decls
@@ -3280,7 +3303,7 @@
            ,(resolve-scopes- (caddr  e) scope)
            ,(resolve-scopes- (cadddr e) scope (method-expr-static-parameters e))))
         (else
-         (if (and (eq? (car e) '=) (symbol? (cadr e))
+         (if (and (memq (car e) '(= const)) (symbol? (cadr e))
                   scope (null? (lam:args (scope:lam scope)))
                   (warn-var?! (cadr e) scope)
                   (= *scopewarn-opt* 1))
@@ -3400,7 +3423,7 @@
         ((local-def) ;; a local that we know has an assignment that dominates all usages
          (let ((vi (get tab (cadr e) #f)))
               (vinfo:set-never-undef! vi #t)))
-        ((=)
+        ((= const)
          (let ((vi (and (symbol? (cadr e)) (get tab (cadr e) #f))))
            (if vi ; if local or captured
                (begin (if (vinfo:asgn vi)
@@ -4017,7 +4040,10 @@ f(x) = yt(x)
                      '(null)
                      `(newvar ,(cadr e))))))
           ((const)
-           (put! globals (binding-to-globalref (cadr e)) #f)
+           ;; Check we've expanded surface `const` (1 argument form)
+           (assert (and (length= e 3)))
+           (when (globalref? (cadr e))
+             (put! globals (cadr e) #f))
            e)
           ((atomic) e)
           ((isdefined) ;; convert isdefined expr to function for closure converted variables
@@ -4369,7 +4395,6 @@ f(x) = yt(x)
         (first-line #t)
         (current-loc #f)
         (rett #f)
-        (global-const-error #f)
         (vinfo-table (vinfo-to-table (car (lam:vinfo lam))))
         (arg-map #f)          ;; map arguments to new names if they are assigned
         (label-counter 0)     ;; counter for generating label addresses
@@ -4582,18 +4607,19 @@ f(x) = yt(x)
                                  (cdr cnd)
                                  (list cnd))))))
           tests))
-    (define (emit-assignment-or-setglobal lhs rhs)
-      (if (globalref? lhs)
+    (define (emit-assignment-or-setglobal lhs rhs (op '=))
+      ;; (const (globalref _ _) _) does not use setglobal!
+      (if (and (globalref? lhs) (eq? op '=))
         (emit `(call (top setglobal!) ,(cadr lhs) (inert ,(caddr lhs)) ,rhs))
-        (emit `(= ,lhs ,rhs))))
-    (define (emit-assignment lhs rhs)
+        (emit `(,op ,lhs ,rhs))))
+    (define (emit-assignment lhs rhs (op '=))
       (if rhs
           (if (valid-ir-rvalue? lhs rhs)
-              (emit-assignment-or-setglobal lhs rhs)
+              (emit-assignment-or-setglobal lhs rhs op)
               (let ((rr (make-ssavalue)))
                 (emit `(= ,rr ,rhs))
-                (emit-assignment-or-setglobal lhs rr)))
-          (emit-assignment-or-setglobal lhs `(null))) ; in unreachable code (such as after return), still emit the assignment so that the structure of those uses is preserved
+                (emit-assignment-or-setglobal lhs rr op)))
+          (emit-assignment-or-setglobal lhs `(null) op)) ; in unreachable code (such as after return), still emit the assignment so that the structure of those uses is preserved
       #f)
     ;; the interpreter loop. `break-labels` keeps track of the labels to jump to
     ;; for all currently closing break-blocks.
@@ -4659,7 +4685,12 @@ f(x) = yt(x)
                (cond (tail (emit-return tail callex))
                      (value callex)
                      (else (emit callex)))))
-            ((=)
+            ((= const)
+             (when (eq? (car e) 'const)
+               (when (local-in? (cadr e) lam)
+                 (error (string "unsupported `const` declaration on local variable" (format-loc current-loc))))
+               (when (pair? (cadr lam))
+                 (error (string "`global const` declaration not allowed inside function" (format-loc current-loc)))))
              (let ((lhs (cadr e)))
                (if (and (symbol? lhs) (underscore-symbol? lhs))
                    (compile (caddr e) break-labels value tail)
@@ -4672,10 +4703,10 @@ f(x) = yt(x)
                                        rhs (make-ssavalue))))
                            (if (not (eq? rr rhs))
                                (emit `(= ,rr ,rhs)))
-                           (emit-assignment-or-setglobal lhs rr)
+                           (emit-assignment-or-setglobal lhs rr (car e))
                            (if tail (emit-return tail rr))
                            rr)
-                         (emit-assignment lhs rhs))))))
+                         (emit-assignment lhs rhs (car e)))))))
             ((block)
              (let* ((last-fname filename)
                     (fnm        (first-non-meta e))
@@ -4918,14 +4949,6 @@ f(x) = yt(x)
             ((moved-local)
              (set-car! (lam:vinfo lam) (append (car (lam:vinfo lam)) `((,(cadr e) Any 2))))
              #f)
-            ((const)
-             (if (local-in? (cadr e) lam)
-                 (error (string "unsupported `const` declaration on local variable" (format-loc current-loc)))
-                 (if (pair? (cadr lam))
-                     ;; delay this error to allow "misplaced struct" errors to happen first
-                     (if (not global-const-error)
-                         (set! global-const-error current-loc))
-                     (emit e))))
             ((atomic) (error "misplaced atomic declaration"))
             ((isdefined throw_undef_if_not) (if tail (emit-return tail e) e))
             ((boundscheck) (if tail (emit-return tail e) e))
@@ -5056,8 +5079,6 @@ f(x) = yt(x)
                       (let ((pexc (pop-exc-expr src-catch-tokens target-catch-tokens)))
                         (if pexc (set-cdr! point (cons pexc (cdr point)))))))))
               handler-goto-fixups)
-    (if global-const-error
-        (error (string "`global const` declaration not allowed inside function" (format-loc global-const-error))))
     (let* ((stmts (reverse! code))
            (di    (definitely-initialized-vars stmts vi))
            (body  (cons 'block (filter (lambda (e)