expander.py: trace line numbers back to the original source file

Using the "#line" directives, it's possible to allow compilers to
report line numbers from the original source file. At least GCC
and Clang support this. It's useful for tracing the compilation
error locations back to the original file. Example:

  #include <iostream>
  #include <atcoder/lazysegtree>
  int main() {
    lazy_segtree<S, op, e, F, mapping, composition, id> seg(arr);
  }

== The current default behaviour: ==

$ ./expander.py example.cpp
$ g++ combined.cpp
combined.cpp: In function ‘int main()’:
combined.cpp:263:3: error: ‘lazy_segtree’ was not declared in this
                    scope; did you mean ‘atcoder::lazy_segtree’?

== With the new '--origname' option: ==

$ ./expander.py --origname=example.cpp example.cpp
$ g++ combined.cpp
example.cpp: In function ‘int main()’:
example.cpp:4:3: error: ‘lazy_segtree’ was not declared in this
                 scope; did you mean ‘atcoder::lazy_segtree’?

Author: ssvb

expander.py

@@ -63,14 +63,18 @@ def expand_acl(self, acl_file_path: Path) -> List[str]:
             result.append(line)
         return result
 
-    def expand(self, source: str) -> str:
+    def expand(self, source: str, origname) -> str:
         self.included = set()
         result = []  # type: List[str]
+        linenum = 0
         for line in source.splitlines():
+            linenum += 1
             m = self.atcoder_include.match(line)
             if m:
                 acl_path = self.find_acl(m.group(1))
                 result.extend(self.expand_acl(acl_path))
+                if origname:
+                    result.append('#line ' + str(linenum + 1) + ' "' + origname + '"')
                 continue
 
             result.append(line)
@@ -88,6 +92,8 @@ def expand(self, source: str) -> str:
     parser.add_argument('-c', '--console',
                         action='store_true', help='Print to Console')
     parser.add_argument('--lib', help='Path to Atcoder Library')
+    parser.add_argument('--origname', help='report line numbers from the original ' +
+                                           'source file in GCC/Clang error messages')
     opts = parser.parse_args()
 
     lib_paths = []
@@ -99,7 +105,7 @@ def expand(self, source: str) -> str:
     lib_paths.append(Path.cwd())
     expander = Expander(lib_paths)
     source = open(opts.source).read()
-    output = expander.expand(source)
+    output = expander.expand(source, opts.origname)
 
     if opts.console:
         print(output)