weak lang items are not allowed to be #[track_caller]

Author: RalfJung

compiler/rustc_passes/messages.ftl

@@ -394,9 +394,18 @@ passes_invalid_macro_export_arguments = `{$name}` isn't a valid `#[macro_export]
 
 passes_invalid_macro_export_arguments_too_many_items = `#[macro_export]` can only take 1 or 0 arguments
 
+passes_lang_item_fn = {$name ->
+    [panic_impl] `#[panic_handler]`
+    *[other] `{$name}` language item
+} function
+
 passes_lang_item_fn_with_target_feature =
-    `{$name}` language item function is not allowed to have `#[target_feature]`
-    .label = `{$name}` language item function is not allowed to have `#[target_feature]`
+    {passes_lang_item_fn} is not allowed to have `#[target_feature]`
+    .label = {passes_lang_item_fn} is not allowed to have `#[target_feature]`
+
+passes_lang_item_fn_with_track_caller =
+    {passes_lang_item_fn} is not allowed to have `#[track_caller]`
+    .label = {passes_lang_item_fn} is not allowed to have `#[target_feature]`
 
 passes_lang_item_on_incorrect_target =
     `{$name}` language item must be applied to a {$expected_target}

compiler/rustc_passes/src/check_attr.rs

@@ -11,9 +11,9 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::StashKey;
 use rustc_errors::{Applicability, DiagCtxt, IntoDiagArg, MultiSpan};
 use rustc_feature::{AttributeDuplicates, AttributeType, BuiltinAttribute, BUILTIN_ATTRIBUTE_MAP};
-use rustc_hir as hir;
 use rustc_hir::def_id::LocalModDefId;
 use rustc_hir::intravisit::{self, Visitor};
+use rustc_hir::{self as hir};
 use rustc_hir::{
     self, FnSig, ForeignItem, HirId, Item, ItemKind, TraitItem, CRATE_HIR_ID, CRATE_OWNER_ID,
 };
@@ -519,7 +519,26 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 self.dcx().emit_err(errors::NakedTrackedCaller { attr_span });
                 false
             }
-            Target::Fn | Target::Method(..) | Target::ForeignFn | Target::Closure => true,
+            Target::Fn => {
+                // `#[track_caller]` is not valid on weak lang items because they are called via
+                // `extern` declarations and `#[track_caller]` would alter their ABI.
+                if let Some((lang_item, _)) = hir::lang_items::extract(attrs)
+                    && let Some(item) = hir::LangItem::from_name(lang_item)
+                    && item.is_weak()
+                {
+                    let sig = self.tcx.hir_node(hir_id).fn_sig().unwrap();
+
+                    self.dcx().emit_err(errors::LangItemWithTrackCaller {
+                        attr_span,
+                        name: lang_item,
+                        sig_span: sig.span,
+                    });
+                    false
+                } else {
+                    true
+                }
+            }
+            Target::Method(..) | Target::ForeignFn | Target::Closure => true,
             // FIXME(#80564): We permit struct fields, match arms and macro defs to have an
             // `#[track_caller]` attribute with just a lint, because we previously
             // erroneously allowed it and some crates used it accidentally, to be compatible

compiler/rustc_passes/src/errors.rs

@@ -819,6 +819,16 @@ pub struct MissingLangItem {
     pub name: Symbol,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_lang_item_fn_with_track_caller)]
+pub struct LangItemWithTrackCaller {
+    #[primary_span]
+    pub attr_span: Span,
+    pub name: Symbol,
+    #[label]
+    pub sig_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_lang_item_fn_with_target_feature)]
 pub struct LangItemWithTargetFeature {

tests/ui/panic-handler/panic-handler-with-target-feature.rs

@@ -9,7 +9,7 @@ use core::panic::PanicInfo;
 
 #[panic_handler]
 #[target_feature(enable = "avx2")]
-//~^ ERROR `panic_impl` language item function is not allowed to have `#[target_feature]`
+//~^ ERROR `#[panic_handler]` function is not allowed to have `#[target_feature]`
 fn panic(info: &PanicInfo) -> ! {
     unimplemented!();
 }

tests/ui/panic-handler/panic-handler-with-target-feature.stderr

@@ -1,11 +1,11 @@
-error: `panic_impl` language item function is not allowed to have `#[target_feature]`
+error: `#[panic_handler]` function is not allowed to have `#[target_feature]`
   --> $DIR/panic-handler-with-target-feature.rs:11:1
    |
 LL | #[target_feature(enable = "avx2")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 LL |
 LL | fn panic(info: &PanicInfo) -> ! {
-   | ------------------------------- `panic_impl` language item function is not allowed to have `#[target_feature]`
+   | ------------------------------- `#[panic_handler]` function is not allowed to have `#[target_feature]`
 
 error: aborting due to 1 previous error
 

tests/ui/panic-handler/panic-handler-with-track-caller.rs

@@ -0,0 +1,14 @@
+//@ compile-flags:-C panic=abort
+//@ only-x86_64
+
+#![no_std]
+#![no_main]
+
+use core::panic::PanicInfo;
+
+#[panic_handler]
+#[track_caller]
+//~^ ERROR `#[panic_handler]` function is not allowed to have `#[track_caller]`
+fn panic(info: &PanicInfo) -> ! {
+    unimplemented!();
+}

tests/ui/panic-handler/panic-handler-with-track-caller.stderr

@@ -0,0 +1,11 @@
+error: `#[panic_handler]` function is not allowed to have `#[track_caller]`
+  --> $DIR/panic-handler-with-track-caller.rs:10:1
+   |
+LL | #[track_caller]
+   | ^^^^^^^^^^^^^^^
+LL |
+LL | fn panic(info: &PanicInfo) -> ! {
+   | ------------------------------- `#[panic_handler]` function is not allowed to have `#[target_feature]`
+
+error: aborting due to 1 previous error
+