Rollup merge of #144094 - saethlin:codegen-the-main-fn, r=petrochenkov

Ensure we codegen the main fn

This fixes two bugs. The one that was identified in the linked issue is that when we have a `main` function, mono collection didn't consider it as an extra collection root.

The other is that since CGU partitioning doesn't  know about the call edges between the entrypoint functions, naively it can put them in different CGUs and mark them all as internal. Which would result in LLVM just deleting all of them. There was an existing hack to exclude `lang = "start"` from internalization, which I've extended to include `main`.

Fixes https://github.com/rust-lang/rust/issues/144052

1 files changed, 9 insertions, 0 deletions

diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index d435e4e77b9..1bfd83d97ac 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -1575,6 +1575,15 @@ impl<'v> RootCollector<'_, 'v> {
             return;
         };
 
+        let main_instance = Instance::mono(self.tcx, main_def_id);
+        if self.tcx.should_codegen_locally(main_instance) {
+            self.output.push(create_fn_mono_item(
+                self.tcx,
+                main_instance,
+                self.tcx.def_span(main_def_id),
+            ));
+        }
+
         let Some(start_def_id) = self.tcx.lang_items().start_fn() else {
             self.tcx.dcx().emit_fatal(errors::StartNotFound);
         };