deprecate rvalues consisting only of underscores

NEWS.md

@@ -118,6 +118,9 @@ Language changes
   * `reduce(+, [...])` and `reduce(*, [...])` no longer widen the iterated over arguments to
     system word size. `sum` and `prod` still preserve this behavior. ([#22825])
 
+  * Like `_`, variable names consisting only of underscores can be assigned,
+    but accessing their values is deprecated ([#24221]).
+
 Breaking changes
 ----------------
 

base/client.jl

@@ -208,8 +208,8 @@ function parse_input_line(s::String; filename::String="none")
     # expr
     ex = ccall(:jl_parse_input_line, Any, (Ptr{UInt8}, Csize_t, Ptr{UInt8}, Csize_t),
                s, sizeof(s), filename, sizeof(filename))
-    if ex === :_
-        # remove with 0.6 deprecation
+    if ex isa Symbol && all(equalto('_'), string(ex))
+        # remove with 0.7 deprecation
         expand(Main, ex)  # to get possible warning about using _ as an rvalue
     end
     return ex

src/flisp/julia_extensions.c

@@ -210,6 +210,16 @@ value_t fl_julia_strip_op_suffix(fl_context_t *fl_ctx, value_t *args, uint32_t n
     return opnew_symbol;
 }
 
+/* check whether arg is a symbol that consists solely of underscores. */
+value_t fl_julia_underscore_symbolp(fl_context_t *fl_ctx, value_t *args, uint32_t nargs)
+{
+    argcount(fl_ctx, "underscore-symbol?", nargs, 1);
+    if (!issymbol(args[0])) return fl_ctx->F;
+    char *op = symbol_name(fl_ctx, args[0]);
+    while (*op == '_') ++op;
+    return *op ? fl_ctx->F : fl_ctx->T;
+}
+
 #include "julia_charmap.h"
 
 utf8proc_int32_t jl_charmap_map(utf8proc_int32_t c, void *ctx)
@@ -298,6 +308,7 @@ static const builtinspec_t julia_flisp_func_info[] = {
     { "identifier-start-char?", fl_julia_identifier_start_char },
     { "op-suffix-char?", fl_julia_op_suffix_char },
     { "strip-op-suffix", fl_julia_strip_op_suffix },
+    { "underscore-symbol?", fl_julia_underscore_symbolp },
     { NULL, NULL }
 };
 

src/jlfrontend.scm

@@ -100,8 +100,8 @@
                   (or (memq (car e) '(toplevel line module import importall using export
                                                error incomplete))
                       (and (eq? (car e) 'global) (every symbol? (cdr e))))))
-         (if (eq? e '_)
-             (syntax-deprecation #f "_ as an rvalue" ""))
+         (if (underscore-symbol? e)
+             (syntax-deprecation #f "underscores as an rvalue" ""))
          e)
         (else
          (let ((last *in-expand*))

src/julia-syntax.scm

@@ -3515,11 +3515,11 @@ f(x) = yt(x)
           (let ((e1 (if (and arg-map (symbol? e))
                         (get arg-map e e)
                         e)))
-            (if (and value (or (eq? e '_)
+            (if (and value (or (underscore-symbol? e)
                                (and (pair? e) (or (eq? (car e) 'outerref)
                                                   (eq? (car e) 'globalref))
-                                    (eq? (cadr e) '_))))
-                (syntax-deprecation #f (string "_ as an rvalue" (linenode-string current-loc))
+                                    (underscore-symbol? (cadr e)))))
+                (syntax-deprecation #f (string "underscores as an rvalue" (linenode-string current-loc))
                                     ""))
             (if (and (not *warn-all-loop-vars*) (has? deprecated-loop-vars e))
                 (begin (deprecation-message