mruby-compiler: add guard clauses to pattern matching

Add support for if/unless guards in case/in pattern matching:

  case value
  in x if x > 0 then :positive
  in x unless x == 0 then :non_zero
  end

Uses modifier_if/modifier_unless tokens since guards appear after
an expression. Disable peephole optimization for pattern variable
binding to prevent gen_move() from being optimized away when failed
guard jumps target the binding instruction.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-compiler/core/codegen.c

@@ -4271,6 +4271,8 @@ codegen_case_match(codegen_scope *s, node *varnode, int val)
   while (current_in) {
     struct mrb_ast_in_node *in_n = in_node(current_in->car);
     node *pattern = in_n->pattern;
+    node *guard = in_n->guard;
+    mrb_bool guard_is_unless = in_n->guard_is_unless;
     node *body = in_n->body;
 
     uint32_t fail_pos = JMPLINK_START;
@@ -4280,6 +4282,21 @@ codegen_case_match(codegen_scope *s, node *varnode, int val)
       codegen_pattern(s, pattern, head, &fail_pos);
     }
 
+    /* Generate guard clause if present */
+    if (guard) {
+      codegen(s, guard, VAL);
+      pop();  /* pop before jump - cursp() now points to guard result */
+      if (guard_is_unless) {
+        /* unless guard: fail if guard is true */
+        tmp = genjmp2(s, OP_JMPIF, cursp(), fail_pos, 0);
+      }
+      else {
+        /* if guard: fail if guard is false */
+        tmp = genjmp2(s, OP_JMPNOT, cursp(), fail_pos, 0);
+      }
+      fail_pos = tmp;
+    }
+
     /* Generate in-clause body */
     codegen(s, body, val);
     if (val) pop();
@@ -4348,7 +4365,7 @@ codegen_pattern(codegen_scope *s, node *pattern, int target, uint32_t *fail_pos)
         /* Bind the matched value to the variable */
         int idx = lv_idx(s, pat_var->name);
         if (idx > 0) {
-          gen_move(s, idx, target, 0);
+          gen_move(s, idx, target, 1);  /* nopeep=1 to prevent optimization */
         }
       }
       /* Variable pattern always matches (wildcard if name is 0) */

mrbgems/mruby-compiler/core/parse.y

@@ -3327,6 +3327,24 @@ primary         : literal
                       node *in_clause = new_in(p, $5, NULL, $7, FALSE);
                       $$ = new_case_match(p, $2, cons(in_clause, $8));
                     }
+                | keyword_case expr_value opt_terms
+                  keyword_in p_expr modifier_if expr_value then
+                  compstmt
+                  in_clauses
+                  keyword_end
+                    {
+                      node *in_clause = new_in(p, $5, $7, $9, FALSE);
+                      $$ = new_case_match(p, $2, cons(in_clause, $10));
+                    }
+                | keyword_case expr_value opt_terms
+                  keyword_in p_expr modifier_unless expr_value then
+                  compstmt
+                  in_clauses
+                  keyword_end
+                    {
+                      node *in_clause = new_in(p, $5, $7, $9, TRUE);
+                      $$ = new_case_match(p, $2, cons(in_clause, $10));
+                    }
                 | keyword_for for_var keyword_in
                   {COND_PUSH(1);}
                   expr_value do
@@ -3808,6 +3826,16 @@ in_clauses      : opt_else
                       node *in_clause = new_in(p, $2, NULL, $4, FALSE);
                       $$ = cons(in_clause, $5);
                     }
+                | keyword_in p_expr modifier_if expr_value then compstmt in_clauses
+                    {
+                      node *in_clause = new_in(p, $2, $4, $6, FALSE);
+                      $$ = cons(in_clause, $7);
+                    }
+                | keyword_in p_expr modifier_unless expr_value then compstmt in_clauses
+                    {
+                      node *in_clause = new_in(p, $2, $4, $6, TRUE);
+                      $$ = cons(in_clause, $7);
+                    }
                 ;
 
 /* Pattern expressions for case/in */