gccrs: Default Rust frontend to stdin when no input file is provided

This fixes an Internal Compiler Error (ICE) where the `crab1` frontend
would segmentation fault if run without any input source files.

gcc/rust/ChangeLog:

	* rust-lang.cc (grs_langhook_post_options): Set pfilename
	to "-" if no input file is provided and num_in_fnames is 0.
	* rust-session-manager.cc (Session::handle_input_files):
	Handle num_files == 0 case explicitly by setting filename to "-".

Signed-off-by: shreyas-omkar <shreyashegdeplus06@gmail.com>

Author: shreyas-omkar

gcc/rust/rust-lang.cc

@@ -286,6 +286,10 @@ grs_langhook_post_options (const char **pfilename ATTRIBUTE_UNUSED)
 {
   // can be used to override other options if required
 
+  // check for input file
+  if (!*pfilename && num_in_fnames == 0)
+    *pfilename = "-";
+
   // satisfies an assert in init_excess_precision in toplev.cc
   if (flag_excess_precision /*_cmdline*/ == EXCESS_PRECISION_DEFAULT)
     flag_excess_precision /*_cmdline*/ = EXCESS_PRECISION_STANDARD;

gcc/rust/rust-session-manager.cc

@@ -443,6 +443,12 @@ Session::handle_excluded_node (std::string arg)
 void
 Session::handle_input_files (int num_files, const char **files)
 {
+  static const char *stdin_file[] = {"-"};
+  if (num_files == 0)
+    {
+      files = stdin_file;
+      num_files = 1;
+    }
   if (num_files != 1)
     rust_fatal_error (UNDEF_LOCATION,
 		      "only one file may be specified on the command line");