scanner: disallow a shebang line, that is not at the top of a file (#…

…21029)

vlib/v/scanner/scanner.v

@@ -959,6 +959,16 @@ pub fn (mut s Scanner) text_scan() token.Token {
 				if nextc == `!` {
 					// treat shebang line (#!) as a comment
 					comment := s.text[start - 1..s.pos].trim_space()
+					if s.line_nr != 1 {
+						comment_pos := token.Pos{
+							line_nr: s.line_nr - 1
+							len: comment.len
+							pos: start
+							col: s.current_column() - comment.len
+						}
+						s.error_with_pos('a shebang is only valid at the top of the file',
+							comment_pos)
+					}
 					// s.fgenln('// shebang line "$s.line_comment"')
 					return s.new_token(.comment, comment, comment.len + 2)
 				}

vlib/v/scanner/tests/shebang_outside_vsh_err.out

@@ -0,0 +1,7 @@
+vlib/v/scanner/tests/shebang_outside_vsh_err.vv:3:1: error: a shebang is only valid at the top of the file
+    1 | module foo
+    2 | 
+    3 | #!/usr/bin/env -S v
+      | ^
+    4 | 
+    5 | import os

vlib/v/scanner/tests/shebang_outside_vsh_err.vv

@@ -0,0 +1,5 @@
+module foo
+
+#!/usr/bin/env -S v
+
+import os