document -Zmiri-start-fn; make its logic more clear

README.md

@@ -76,6 +76,9 @@ MIRI_SYSROOT=~/.xargo/HOST cargo run --bin miri tests/run-pass-fullmir/vecs.rs
 Notice that you will have to re-run the last step of the preparations above when
 your toolchain changes (e.g., when you update the nightly).
 
+You can also set `-Zmiri-start-fn` to make miri start evaluation with the
+`start_fn` lang item, instead of starting at the `main` function.
+
 ## Contributing and getting help
 
 Check out the issues on this GitHub repository for some ideas. There's lots that

src/bin/miri.rs

@@ -138,13 +138,8 @@ fn visit_impl_item(&mut self, _impl_item: &'hir hir::ImplItem) {}
         );
     } else if let Some((entry_node_id, _, _)) = *state.session.entry_fn.borrow() {
         let entry_def_id = tcx.hir.local_def_id(entry_node_id);
-        let start_wrapper = tcx.lang_items().start_fn().and_then(|start_fn| {
-            if use_start_fn {
-                Some(start_fn)
-            } else {
-                None
-            }
-        });
+        // Use start_fn lang item if it is available and we have -Zmiri-start-fn set
+        let start_wrapper = if use_start_fn { tcx.lang_items().start_fn() } else { None };
         miri::eval_main(tcx, entry_def_id, start_wrapper);
 
         state.session.abort_if_errors();