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

2 files changed, 16 insertions, 4 deletions

diff --git a/compiler/rustc_middle/src/mir/mono.rs b/compiler/rustc_middle/src/mir/mono.rs
index 2d7ddd105bd..105736b9e24 100644
--- a/compiler/rustc_middle/src/mir/mono.rs
+++ b/compiler/rustc_middle/src/mir/mono.rs
@@ -143,10 +143,8 @@ impl<'tcx> MonoItem<'tcx> {
         };
 
         // Similarly, the executable entrypoint must be instantiated exactly once.
-        if let Some((entry_def_id, _)) = tcx.entry_fn(()) {
-            if instance.def_id() == entry_def_id {
-                return InstantiationMode::GloballyShared { may_conflict: false };
-            }
+        if tcx.is_entrypoint(instance.def_id()) {
+            return InstantiationMode::GloballyShared { may_conflict: false };
         }
 
         // If the function is #[naked] or contains any other attribute that requires exactly-once
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 9d9cc3403dc..66d1335e763 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -3419,6 +3419,20 @@ impl<'tcx> TyCtxt<'tcx> {
     pub fn do_not_recommend_impl(self, def_id: DefId) -> bool {
         self.get_diagnostic_attr(def_id, sym::do_not_recommend).is_some()
     }
+
+    /// Whether this def is one of the special bin crate entrypoint functions that must have a
+    /// monomorphization and also not be internalized in the bin crate.
+    pub fn is_entrypoint(self, def_id: DefId) -> bool {
+        if self.is_lang_item(def_id, LangItem::Start) {
+            return true;
+        }
+        if let Some((entry_def_id, _)) = self.entry_fn(())
+            && entry_def_id == def_id
+        {
+            return true;
+        }
+        false
+    }
 }
 
 /// Parameter attributes that can only be determined by examining the body of a function instead