fix slurping into function definition (#40738)

Discovered while working on #40737. Currently, this throws an
unintuitive error:
```julia
julia> a, f()... = 1, 2, 3
ERROR: syntax: ssavalue with no def
Stacktrace:
 [1] top-level scope
   @ REPL[1]:1
```

Might as well fix this properly - Why not allow function definitions to
slurp a little from time to time? ;)

(cherry picked from commit 5c49a0d)

src/julia-syntax.scm

@@ -1456,7 +1456,7 @@
                        (cons R elts)))
                 ((vararg? L)
                  (if (null? (cdr lhss))
-                     (let ((temp (make-ssavalue)))
+                     (let ((temp (if (eventually-call? (cadr L)) (gensy) (make-ssavalue))))
                        `(block ,@(reverse stmts)
                                (= ,temp (tuple ,@rhss))
                                ,@(reverse after)
@@ -2155,9 +2155,13 @@
                       ((eq? l x) #t)
                       (else (in-lhs? x (cdr lhss)))))))
         ;; in-lhs? also checks for invalid syntax, so always call it first
-        (let* ((xx  (if (or (and (not (in-lhs? x lhss)) (symbol? x))
-                            (ssavalue? x))
-                        x (make-ssavalue)))
+        (let* ((xx  (cond ((or (and (not (in-lhs? x lhss)) (symbol? x))
+                               (ssavalue? x))
+                            x)
+                          ((and (pair? lhss) (vararg? (last lhss))
+                                (eventually-call? (cadr (last lhss))))
+                           (gensy))
+                          (else (make-ssavalue))))
                (ini (if (eq? x xx) '() (list (sink-assignment xx (expand-forms x)))))
                (n   (length lhss))
                ;; skip last assignment if it is an all-underscore vararg

test/syntax.jl

@@ -2937,3 +2937,15 @@ end
     @test eval(Meta.parse("`a\\\r\nb`")) == `ab`
     @test eval(Meta.parse("`a\\\rb`")) == `ab`
 end
+
+@testset "slurping into function def" begin
+    x, f()... = [1, 2, 3]
+    @test x == 1
+    @test f() == [2, 3]
+    # test that call to `Base.rest` is outside the definition of `f`
+    @test f() === f()
+
+    x, f()... = 1, 2, 3
+    @test x == 1
+    @test f() == (2, 3)
+end