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.

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}`

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,

compiler/rustc_monomorphize/src/errors.rs

@@ -94,6 +94,11 @@ pub struct EncounteredErrorWhileInstantiating {
     pub formatted_item: String,
 }
 
+#[derive(Diagnostic)]
+#[diag(monomorphize_start_not_found)]
+#[help]
+pub struct StartNotFound;
+
 #[derive(Diagnostic)]
 #[diag(monomorphize_unknown_cgu_collection_mode)]
 pub struct UnknownCguCollectionMode<'a> {

compiler/rustc_passes/messages.ftl

@@ -575,6 +575,11 @@ passes_outside_loop =
 
 passes_outside_loop_suggestion = consider labeling this block to be able to break within it
 
+passes_panic_unwind_without_std =
+    unwinding panics are not supported without std
+    .note = since the core library is usually precompiled with panic="unwind", rebuilding your crate with panic="abort" may not be enough to fix the problem
+    .help = using nightly cargo, use -Zbuild-std with panic="abort" to avoid unwinding
+
 passes_params_not_allowed =
     referencing function parameters is not allowed in naked functions
     .help = follow the calling convention in asm block to use parameters

compiler/rustc_passes/src/errors.rs

@@ -812,6 +812,12 @@ pub struct UnknownExternLangItem {
 #[diag(passes_missing_panic_handler)]
 pub struct MissingPanicHandler;
 
+#[derive(Diagnostic)]
+#[diag(passes_panic_unwind_without_std)]
+#[help]
+#[note]
+pub struct PanicUnwindWithoutStd;
+
 #[derive(Diagnostic)]
 #[diag(passes_missing_lang_item)]
 #[note]

compiler/rustc_passes/src/weak_lang_items.rs

@@ -9,7 +9,9 @@ use rustc_middle::middle::lang_items::required;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::config::CrateType;
 
-use crate::errors::{MissingLangItem, MissingPanicHandler, UnknownExternLangItem};
+use crate::errors::{
+    MissingLangItem, MissingPanicHandler, PanicUnwindWithoutStd, UnknownExternLangItem,
+};
 
 /// Checks the crate for usage of weak lang items, returning a vector of all the
 /// language items required by this crate, but not defined yet.
@@ -76,6 +78,8 @@ fn verify(tcx: TyCtxt<'_>, items: &lang_items::LanguageItems) {
         if missing.contains(&item) && required(tcx, item) && items.get(item).is_none() {
             if item == LangItem::PanicImpl {
                 tcx.dcx().emit_err(MissingPanicHandler);
+            } else if item == LangItem::EhPersonality {
+                tcx.dcx().emit_err(PanicUnwindWithoutStd);
             } else {
                 tcx.dcx().emit_err(MissingLangItem { name: item.name() });
             }