Move describe_lints calls.

Currently we have an inconsistency between the "input" and "no input"
cases:
- no input: `rustc --print=sysroot -Whelp` prints the lint help.
- input:    `rustc --print=sysroot -Whelp a.rs` prints the sysroot.

It makes sense to print the lint help in both cases, because that's what
happens with `--help`/`-Zhelp`/`-Chelp`.

In fact, the `describe_lints` in the "input" case happens amazingly
late, after *parsing*. This is because, with plugins, lints used to be
registered much later, when the global context was created. But rust-lang#117649
moved lint registration much earlier, during session construction.

So this commit moves the `describe_lints` call to a single spot for both
for both the "input" and "no input" cases, as early as possible. This is
still not as early as `--help`/`-Zhelp`/`-Chelp`, because `-Whelp` must
wait until the session is constructed.

Author: nnethercote

compiler/rustc_driver_impl/src/lib.rs

@@ -361,13 +361,18 @@ fn run_compiler(
 
     interface::run_compiler(config, |compiler| {
         let sess = compiler.session();
+
+        // This implements `-Whelp`. It should be handled very early, like
+        // `--help`/`-Zhelp`/`-Chelp`. This is the earliest it can run, because
+        // it must happen after lints are registered, during session creation.
+        if sess.opts.describe_lints {
+            describe_lints(sess);
+            return sess.compile_status();
+        }
+
         let handler = EarlyErrorHandler::new(sess.opts.error_format);
 
         if !has_input {
-            if sess.opts.describe_lints {
-                describe_lints(compiler.session());
-                return sess.compile_status();
-            }
             let should_stop = print_crate_info(
                 &handler,
                 &**compiler.codegen_backend(),
@@ -425,11 +430,6 @@ fn run_compiler(
                 return early_exit();
             }
 
-            if sess.opts.describe_lints {
-                describe_lints(sess);
-                return early_exit();
-            }
-
             // Make sure name resolution and macro expansion is run.
             queries.global_ctxt()?.enter(|tcx| tcx.resolver_for_lowering(()));