Replace R-assignment by single-line pattern matching.

Since `R-assignment` in CRuby is abandoned.  Single-line pattern matching
in `mruby` only matches single local variable at the moment. Currently
it works as a right assignment to a local variable. It will be enhanced
in the future.

doc/mruby3.md

@@ -32,7 +32,8 @@ pull-request.
 
 We have ported some new syntax from CRuby.
 
-* R-assignment (`12 => x`)
+* Single line pattern matching (`12 => x`);
+  mruby matches only with local variables at the moment
 * Numbered block parameter (`x.map{_1 * 2}`)
 * End-less `def` (`def double(x) = x*2`)
 

mrbgems/mruby-compiler/core/parse.y

@@ -1442,7 +1442,7 @@ heredoc_end(parser_state *p)
 
 %type <nd> singleton string string_fragment string_rep string_interp xstring regexp
 %type <nd> literal numeric cpath symbol defn_head defs_head
-%type <nd> top_compstmt top_stmts top_stmt rassign
+%type <nd> top_compstmt top_stmts top_stmt
 %type <nd> bodystmt compstmt stmts stmt expr arg primary command command_call method_call
 %type <nd> expr_value arg_rhs primary_value
 %type <nd> if_tail opt_else case_body cases opt_rescue exc_list exc_var opt_ensure
@@ -1685,30 +1685,16 @@ stmt            : keyword_alias fsym {p->lstate = EXPR_FNAME;} fsym
                     {
                       $$ = new_masgn(p, $1, new_array(p, $3));
                     }
-                | rassign
+                | arg tASSOC tIDENTIFIER
+                    {
+                      node *lhs = new_lvar(p, $3);
+                      void_expr_error(p, $1);
+                      assignable(p, lhs);
+                      $$ = new_asgn(p, lhs, $1);
+                    }
                 | expr
                 ;
 
-rassign         : arg tASSOC lhs
-                    {
-                      void_expr_error(p, $1);
-                      $$ = new_asgn(p, $3, $1);
-                    }
-                | arg tASSOC mlhs
-                    {
-                      void_expr_error(p, $1);
-                      $$ = new_masgn(p, $3, $1);
-                    }
-                | rassign tASSOC lhs
-                    {
-                      $$ = new_asgn(p, $3, $1);
-                    }
-                | rassign tASSOC mlhs
-                    {
-                      $$ = new_masgn(p, $3, $1);
-                    }
-                ;
-
 command_asgn    : lhs '=' command_rhs
                     {
                       $$ = new_asgn(p, $1, $3);

test/t/syntax.rb

@@ -345,13 +345,9 @@ assert('splat object in assignment') do
   assert_equal [2], (a = *o)
 end
 
-assert('right-ward assignment') do
+assert('one-line pattern match') do
   1 => a
   assert_equal(1, a)
-  13.divmod(5) => a,b
-  assert_equal([2,3], [a, b])
-  13.divmod(5) => a,b => c, d
-  assert_equal([2,3,2,3], [a, b, c, d])
 end
 
 assert('splat object in case statement') do