3 files changed, 12 insertions, 2 deletions
diff --git a/compiler/rustc_monomorphize/messages.ftl b/compiler/rustc_monomorphize/messages.ftl
index e27875853df..94b553a07a7 100644
--- a/compiler/rustc_monomorphize/messages.ftl
+++ b/compiler/rustc_monomorphize/messages.ftl
@@ -20,6 +20,9 @@ monomorphize_recursion_limit =
     reached the recursion limit while instantiating `{$shrunk}`
     .note = `{$def_path_str}` defined here
 
+monomorphize_start_not_found = using `fn main` requires the standard library
+    .help = use `#![no_main]` to bypass the Rust generated entrypoint and declare a platform specific entrypoint yourself, usually with `#[no_mangle]`
+
 monomorphize_symbol_already_defined = symbol `{$symbol}` is already defined
 
 monomorphize_type_length_limit = reached the type-length limit while instantiating `{$shrunk}`
diff --git a/compiler/rustc_monomorphize/src/collector.rs b/compiler/rustc_monomorphize/src/collector.rs
index 44beafa0873..92f001cc321 100644
--- a/compiler/rustc_monomorphize/src/collector.rs
+++ b/compiler/rustc_monomorphize/src/collector.rs
@@ -194,7 +194,7 @@ use rustc_target::abi::Size;
 use std::path::PathBuf;
 
 use crate::errors::{
-    EncounteredErrorWhileInstantiating, LargeAssignmentsLint, NoOptimizedMir, RecursionLimit,
+    self, EncounteredErrorWhileInstantiating, LargeAssignmentsLint, NoOptimizedMir, RecursionLimit,
     TypeLengthLimit,
 };
 
@@ -1272,7 +1272,9 @@ impl<'v> RootCollector<'_, 'v> {
             return;
         };
 
-        let start_def_id = self.tcx.require_lang_item(LangItem::Start, None);
+        let Some(start_def_id) = self.tcx.lang_items().start_fn() else {
+            self.tcx.dcx().emit_fatal(errors::StartNotFound);
+        };
         let main_ret_ty = self.tcx.fn_sig(main_def_id).no_bound_vars().unwrap().output();
 
         // Given that `main()` has no arguments,
diff --git a/compiler/rustc_monomorphize/src/errors.rs b/compiler/rustc_monomorphize/src/errors.rs
index 2ca14673a58..bd89874b5cc 100644
--- a/compiler/rustc_monomorphize/src/errors.rs
+++ b/compiler/rustc_monomorphize/src/errors.rs
@@ -95,6 +95,11 @@ pub struct EncounteredErrorWhileInstantiating {
 }
 
 #[derive(Diagnostic)]
+#[diag(monomorphize_start_not_found)]
+#[help]
+pub struct StartNotFound;
+
+#[derive(Diagnostic)]
 #[diag(monomorphize_unknown_cgu_collection_mode)]
 pub struct UnknownCguCollectionMode<'a> {
     pub mode: &'a str,