mruby-compiler: preserve line number across multi-file boundary

When mrbc compiles multiple input files (e.g. `mrbc -g -o out.mrb
a.rb b.rb`), the bison parser's one-token lookahead can buffer the
final token of one file before partial_hook switches to the next.
By the time bison reduces that token into an AST node,
`mrb_parser_set_filename` has already reset `p->lineno` to 0, so
init_var_header recorded lineno=0 for the previous file's last
statement and codegen propagated the previous instruction's line.

Save the lineno into `prev_file_lineno` immediately before the
reset so init_var_header can restore the correct value when it
detects the lookahead edge case (lineno==0 && filename_index>0).

close #1316

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

include/mruby/compile.h

@@ -134,6 +134,7 @@ struct mrb_parser_state {
   mrb_sym* filename_table;
   uint16_t filename_table_length;
   uint16_t current_filename_index;
+  uint16_t prev_file_lineno;    /* saved lineno before partial_hook file switch */
 
   /* Variable-sized node management */
   mrb_ast_node *nvars;

mrbgems/mruby-bin-mrbc/bintest/mrbc.rb

@@ -28,3 +28,20 @@ assert('embedded document with invalid terminator') do
   assert_equal "#{a.path}:3:0: embedded document meets end of file", result.chomp
   assert_equal 1, $?.exitstatus
 end
+
+assert('debug info preserves line/filename across multiple inputs. #1316') do
+  a = Tempfile.new(['a', '.rb'])
+  b = Tempfile.new(['b', '.rb'])
+  out = Tempfile.new(['out', '.mrb'])
+  a.write("# line 1\n# line 2\nputs \"from a\"\n# line 4\nundefined_in_a\n")
+  a.flush
+  b.write("# b line 1\nputs \"from b\"\n")
+  b.flush
+  `#{cmd('mrbc')} -g -o #{out.path} #{a.path} #{b.path}`
+  assert_equal 0, $?.exitstatus
+  result = `#{cmd('mruby')} -b #{out.path} 2>&1`
+  # Error should point at a.rb line 5 (the `undefined_in_a` line),
+  # not b.rb or a different line within a.rb.
+  assert_include result, "#{a.path}:5:"
+  assert_not_include result, b.path
+end

mrbgems/mruby-compiler/core/parse.y

@@ -151,9 +151,13 @@ init_var_header(struct mrb_ast_var_header *header, parser_state *p, enum node_ty
   header->filename_index = p->current_filename_index;
   header->node_type = (uint8_t)type;
 
-  /* Handle file boundary edge case */
+  /* Handle file boundary edge case: this node is reduced from a token that
+     was buffered by bison lookahead before partial_hook switched the file,
+     so attribute it to the previous file at its last known lineno rather
+     than the new file at lineno=0. */
   if (p->lineno == 0 && p->current_filename_index > 0) {
     header->filename_index--;
+    header->lineno = p->prev_file_lineno;
   }
 }
 
@@ -7658,6 +7662,10 @@ mrb_parser_set_filename(struct mrb_parser_state *p, const char *f)
 
   sym = mrb_intern_cstr(p->mrb, f);
   p->filename_sym = sym;
+  /* Save current lineno so that AST nodes produced from a bison lookahead
+     across the file boundary (in partial_hook) can recover the correct
+     line in init_var_header instead of recording lineno=0. */
+  p->prev_file_lineno = p->lineno;
   p->lineno = (p->filename_table_length > 0)? 0 : 1;
 
   for (i = 0; i < p->filename_table_length; i++) {