Don't skip argument parsing when running rustc with no arguments

Setting up the argument parser to parse no arguments is a tiny bit of wasted
work, but avoids an otherwise-unnecessary special case.

In particular, this lets us avoid having to deal with multiple different APIs
to determine whether the compiler is nightly or not.

Author: Zalathar

compiler/rustc_driver_impl/src/lib.rs

@@ -1191,15 +1191,6 @@ fn print_flag_list<T>(cmdline_opt: &str, flag_list: &[OptionDesc<T>]) {
 /// be public when using rustc as a library, see
 /// <https://github.com/rust-lang/rust/commit/2b4c33817a5aaecabf4c6598d41e190080ec119e>
 pub fn handle_options(early_dcx: &EarlyDiagCtxt, args: &[String]) -> Option<getopts::Matches> {
-    if args.is_empty() {
-        // user did not write `-v` nor `-Z unstable-options`, so do not
-        // include that extra information.
-        let nightly_build =
-            rustc_feature::UnstableFeatures::from_environment(None).is_nightly_build();
-        usage(false, false, nightly_build);
-        return None;
-    }
-
     // Parse with *all* options defined in the compiler, we don't worry about
     // option stability here we just want to parse as much as possible.
     let mut options = getopts::Options::new();
@@ -1245,7 +1236,7 @@ pub fn handle_options(early_dcx: &EarlyDiagCtxt, args: &[String]) -> Option<geto
     //   (unstable option being used on stable)
     nightly_options::check_nightly_options(early_dcx, &matches, &config::rustc_optgroups());
 
-    if matches.opt_present("h") || matches.opt_present("help") {
+    if args.is_empty() || matches.opt_present("h") || matches.opt_present("help") {
         // Only show unstable options in --help if we accept unstable options.
         let unstable_enabled = nightly_options::is_unstable_enabled(&matches);
         let nightly_build = nightly_options::match_is_nightly_build(&matches);