parser: show where another declaration is located (fixes #26483)

Author: medvednikov

vlib/v/parser/struct.v

@@ -4,6 +4,7 @@
 module parser
 
 import v.ast
+import v.errors
 import v.token
 import v.util
 
@@ -479,8 +480,35 @@ fn (mut p Parser) struct_decl(is_anon bool) ast.StructDecl {
 	}
 	// allow duplicate c struct declarations
 	if ret == -1 && language != .c && !p.pref.is_fmt {
-		p.error_with_pos('cannot register struct `${name}`, another type with this name exists',
-			name_pos)
+		msg := 'cannot register struct `${name}`, another type with this name exists'
+		mut existing_sym, mut existing_idx := p.table.find_sym_and_type_idx(name)
+		if existing_idx <= 0 && name.starts_with('main.') {
+			existing_sym, existing_idx = p.table.find_sym_and_type_idx(name.trim_string_left('main.'))
+		}
+		if existing_idx > 0 {
+			if existing_name_pos := existing_sym.info.get_name_pos() {
+				existing_file_path := if existing_name_pos.file_idx < 0 {
+					p.file_path
+				} else {
+					p.table.filelist[existing_name_pos.file_idx]
+				}
+				error_file_path := if name_pos.file_idx < 0 {
+					p.file_path
+				} else {
+					p.table.filelist[name_pos.file_idx]
+				}
+				p.error_with_error(errors.Error{
+					file_path: error_file_path
+					pos:       name_pos
+					reporter:  .parser
+					message:   msg
+					details:   util.formatted_error('details:', 'another declaration was found here',
+						existing_file_path, existing_name_pos)
+				})
+				return ast.StructDecl{}
+			}
+		}
+		p.error_with_pos(msg, name_pos)
 		return ast.StructDecl{}
 	}
 	p.expr_mod = ''

vlib/v/parser/tests/duplicate_type_b.out

@@ -3,3 +3,8 @@ vlib/v/parser/tests/duplicate_type_b.vv:3:8: error: cannot register struct `Foo`
     2 |
     3 | struct Foo {}
       |        ~~~
+Details: vlib/v/parser/tests/duplicate_type_b.vv:1:1: details: another declaration was found here
+    1 | interface Foo {}
+      | ^
+    2 |
+    3 | struct Foo {}

vlib/v/parser/tests/prohibit_redeclaration_of_builtin_types.out

@@ -1,3 +1,10 @@
 vlib/v/parser/tests/prohibit_redeclaration_of_builtin_types.vv:1:8: error: cannot register struct `Option`, another type with this name exists
     1 | struct Option {}
       |        ~~~~~~
+Details: vlib/builtin/chan_option_result.v:142:8: details: another declaration was found here
+  140 |
+  141 | // Option is the base of V's internal option return system.
+  142 | struct Option {
+      |        ~~~~~~
+  143 |     state u8 // 0 - ok; 2 - none; 1 - ?
+  144 |     err   IError = none__