fix #40258: nested string interpolation (#40261)

* fix #40258: nested string interpolation

fixes #40258

* fix interpolation with varargs

* add additional test case

* add additional test cases

Co-authored-by: Nathan Daly <NHDaly@gmail.com>

* keep previous behavior, don't try to be too smart

Co-authored-by: Nathan Daly <NHDaly@gmail.com>
Co-authored-by: Jeff Bezanson <jeff.bezanson@gmail.com>

src/julia-syntax.scm

@@ -2384,11 +2384,12 @@
    'string
    (lambda (e)
      (expand-forms
-      `(call (top string) ,@(map (lambda (s)
-                                   (if (and (pair? s) (eq? (car s) 'string))
-                                       (cadr s)
-                                       s))
-                                 (cdr e)))))
+       `(call (top string)
+              ,@(map (lambda (s)
+                       (if (and (length= s 2) (eq? (car s) 'string) (string? (cadr s)))
+                           (cadr s)
+                           s))
+                     (cdr e)))))
 
    '|::|
    (lambda (e)

test/syntax.jl

@@ -2763,3 +2763,11 @@ macro m_begin_hygiene(a)
 end
 
 @test @m_begin_hygiene([1, 2, 3]) == 1
+
+# issue 40258
+@test "a $("b $("c")")" == "a b c"
+
+@test "$(([[:a, :b], [:c, :d]]...)...)" == "abcd"
+
+@test eval(Expr(:string, "a", Expr(:string, "b", "c"))) == "abc"
+@test eval(Expr(:string, "a", Expr(:string, "b", Expr(:string, "c")))) == "abc"