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-rw-r--r--compiler/rustc_monomorphize/messages.ftl3
-rw-r--r--compiler/rustc_monomorphize/src/collector.rs6
-rw-r--r--compiler/rustc_monomorphize/src/errors.rs5
-rw-r--r--compiler/rustc_passes/messages.ftl5
-rw-r--r--compiler/rustc_passes/src/errors.rs6
-rw-r--r--compiler/rustc_passes/src/weak_lang_items.rs6
-rw-r--r--src/tools/tidy/src/ui_tests.rs2
-rw-r--r--tests/ui/extern-flag/empty-extern-arg.stderr6
-rw-r--r--tests/ui/lang-items/required-lang-item.rs7
-rw-r--r--tests/ui/lang-items/required-lang-item.stderr6
-rw-r--r--tests/ui/no_std/no-std-no-start-binary.rs13
-rw-r--r--tests/ui/no_std/no-std-no-start-binary.stderr6
-rw-r--r--tests/ui/no_std/no-std-unwind-binary.rs15
-rw-r--r--tests/ui/no_std/no-std-unwind-binary.stderr7
-rw-r--r--tests/ui/panic-handler/weak-lang-item.rs2
-rw-r--r--tests/ui/panic-handler/weak-lang-item.stderr6
16 files changed, 85 insertions, 16 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,
diff --git a/compiler/rustc_passes/messages.ftl b/compiler/rustc_passes/messages.ftl
index be50aad1303..d41cc724408 100644
--- a/compiler/rustc_passes/messages.ftl
+++ b/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
diff --git a/compiler/rustc_passes/src/errors.rs b/compiler/rustc_passes/src/errors.rs
index d934e959a41..cf3c7cc4ace 100644
--- a/compiler/rustc_passes/src/errors.rs
+++ b/compiler/rustc_passes/src/errors.rs
@@ -813,6 +813,12 @@ pub struct UnknownExternLangItem {
 pub struct MissingPanicHandler;
 
 #[derive(Diagnostic)]
+#[diag(passes_panic_unwind_without_std)]
+#[help]
+#[note]
+pub struct PanicUnwindWithoutStd;
+
+#[derive(Diagnostic)]
 #[diag(passes_missing_lang_item)]
 #[note]
 #[help]
diff --git a/compiler/rustc_passes/src/weak_lang_items.rs b/compiler/rustc_passes/src/weak_lang_items.rs
index db3d442676e..4eb0c6c275e 100644
--- a/compiler/rustc_passes/src/weak_lang_items.rs
+++ b/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() });
             }
diff --git a/src/tools/tidy/src/ui_tests.rs b/src/tools/tidy/src/ui_tests.rs
index 32e271d4a56..ba569078f3d 100644
--- a/src/tools/tidy/src/ui_tests.rs
+++ b/src/tools/tidy/src/ui_tests.rs
@@ -11,7 +11,7 @@ use std::path::{Path, PathBuf};
 const ENTRY_LIMIT: usize = 900;
 // FIXME: The following limits should be reduced eventually.
 const ISSUES_ENTRY_LIMIT: usize = 1849;
-const ROOT_ENTRY_LIMIT: usize = 868;
+const ROOT_ENTRY_LIMIT: usize = 869;
 
 const EXPECTED_TEST_FILE_EXTENSIONS: &[&str] = &[
     "rs",     // test source files
diff --git a/tests/ui/extern-flag/empty-extern-arg.stderr b/tests/ui/extern-flag/empty-extern-arg.stderr
index 54b5e66fc21..79efcc5d8b0 100644
--- a/tests/ui/extern-flag/empty-extern-arg.stderr
+++ b/tests/ui/extern-flag/empty-extern-arg.stderr
@@ -2,10 +2,10 @@ error: extern location for std does not exist:
 
 error: `#[panic_handler]` function required, but not found
 
-error: language item required, but not found: `eh_personality`
+error: unwinding panics are not supported without std
    |
-   = note: this can occur when a binary crate with `#![no_std]` is compiled for a target where `eh_personality` is defined in the standard library
-   = help: you may be able to compile for a target that doesn't need `eh_personality`, specify a target with `--target` or in `.cargo/config`
+   = help: using nightly cargo, use -Zbuild-std with panic="abort" to avoid unwinding
+   = 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
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/lang-items/required-lang-item.rs b/tests/ui/lang-items/required-lang-item.rs
index 3b17c5b7255..7f8933ac2ad 100644
--- a/tests/ui/lang-items/required-lang-item.rs
+++ b/tests/ui/lang-items/required-lang-item.rs
@@ -1,11 +1,10 @@
-// build-fail
+// edition: 2018
 
 #![feature(lang_items, no_core)]
 #![no_core]
+#![no_main]
 
 #[lang="copy"] pub trait Copy { }
 #[lang="sized"] pub trait Sized { }
 
-// error-pattern:requires `start` lang_item
-
-fn main() {}
+async fn x() {} //~ ERROR requires `ResumeTy` lang_item
diff --git a/tests/ui/lang-items/required-lang-item.stderr b/tests/ui/lang-items/required-lang-item.stderr
index bb53d336bb2..13c07ee6529 100644
--- a/tests/ui/lang-items/required-lang-item.stderr
+++ b/tests/ui/lang-items/required-lang-item.stderr
@@ -1,4 +1,8 @@
-error: requires `start` lang_item
+error: requires `ResumeTy` lang_item
+  --> $DIR/required-lang-item.rs:10:14
+   |
+LL | async fn x() {}
+   |              ^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/no_std/no-std-no-start-binary.rs b/tests/ui/no_std/no-std-no-start-binary.rs
new file mode 100644
index 00000000000..ce1c871f6a6
--- /dev/null
+++ b/tests/ui/no_std/no-std-no-start-binary.rs
@@ -0,0 +1,13 @@
+// compile-flags: -Cpanic=abort --emit link
+// error-pattern:using `fn main` requires the standard library
+
+// Make sure that we don't emit an error message mentioning internal lang items.
+
+#![no_std]
+
+#[panic_handler]
+fn handler(_info: &core::panic::PanicInfo<'_>) -> ! {
+    loop {}
+}
+
+fn main() {}
diff --git a/tests/ui/no_std/no-std-no-start-binary.stderr b/tests/ui/no_std/no-std-no-start-binary.stderr
new file mode 100644
index 00000000000..dd06c234da2
--- /dev/null
+++ b/tests/ui/no_std/no-std-no-start-binary.stderr
@@ -0,0 +1,6 @@
+error: 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]`
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/no_std/no-std-unwind-binary.rs b/tests/ui/no_std/no-std-unwind-binary.rs
new file mode 100644
index 00000000000..7a9dfd7a48d
--- /dev/null
+++ b/tests/ui/no_std/no-std-unwind-binary.rs
@@ -0,0 +1,15 @@
+// error-pattern:unwinding panics are not supported without std
+// needs-unwind
+// compile-flags: -Cpanic=unwind
+
+// Make sure that we don't emit an error message mentioning internal lang items.
+
+#![no_std]
+#![no_main]
+
+#[panic_handler]
+fn handler(_info: &core::panic::PanicInfo<'_>) -> ! {
+    loop {}
+}
+
+fn main() {}
diff --git a/tests/ui/no_std/no-std-unwind-binary.stderr b/tests/ui/no_std/no-std-unwind-binary.stderr
new file mode 100644
index 00000000000..a3b54fe33ab
--- /dev/null
+++ b/tests/ui/no_std/no-std-unwind-binary.stderr
@@ -0,0 +1,7 @@
+error: unwinding panics are not supported without std
+   |
+   = help: using nightly cargo, use -Zbuild-std with panic="abort" to avoid unwinding
+   = 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
+
+error: aborting due to 1 previous error
+
diff --git a/tests/ui/panic-handler/weak-lang-item.rs b/tests/ui/panic-handler/weak-lang-item.rs
index 14a07a9ef1b..296a2c1514f 100644
--- a/tests/ui/panic-handler/weak-lang-item.rs
+++ b/tests/ui/panic-handler/weak-lang-item.rs
@@ -1,6 +1,6 @@
 // aux-build:weak-lang-items.rs
 // error-pattern: `#[panic_handler]` function required, but not found
-// error-pattern: language item required, but not found: `eh_personality`
+// error-pattern: unwinding panics are not supported without std
 // needs-unwind since it affects the error output
 // ignore-emscripten missing eh_catch_typeinfo lang item
 
diff --git a/tests/ui/panic-handler/weak-lang-item.stderr b/tests/ui/panic-handler/weak-lang-item.stderr
index 202f3309d03..de351d2c3e4 100644
--- a/tests/ui/panic-handler/weak-lang-item.stderr
+++ b/tests/ui/panic-handler/weak-lang-item.stderr
@@ -12,10 +12,10 @@ LL | extern crate core as other_core;
 
 error: `#[panic_handler]` function required, but not found
 
-error: language item required, but not found: `eh_personality`
+error: unwinding panics are not supported without std
    |
-   = note: this can occur when a binary crate with `#![no_std]` is compiled for a target where `eh_personality` is defined in the standard library
-   = help: you may be able to compile for a target that doesn't need `eh_personality`, specify a target with `--target` or in `.cargo/config`
+   = help: using nightly cargo, use -Zbuild-std with panic="abort" to avoid unwinding
+   = 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
 
 error: aborting due to 3 previous errors
generated by cgit-pink 1.4.1-3-g733a5 (git 2.37.1) at 2025-10-26 23:18:19 +0000