Stop mentioning internal lang items in no_std binary errors

When writing a no_std binary, you'll be greeted with nonsensical errors
mentioning lang items like eh_personality and start. That's pretty bad
because it makes you think that you need to define them somewhere! But
oh no, now you're getting the `internal_features` lint telling you that
you shouldn't use them! But you need a no_std binary! What now?

No problem! Writing a no_std binary is super easy. Just use panic=abort
and supply your own platform specific entrypoint symbol (like `main`)
and you're good to go. Would be nice if the compiler told you that,
right?

This makes it so that it does do that.

1 files changed, 4 insertions, 2 deletions

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,