3 files changed, 49 insertions, 5 deletions
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index 2d94354e3e1..12118a0268b 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -118,7 +118,7 @@ impl CheckAttrVisitor<'_> {
                 sym::coverage => self.check_coverage(hir_id, attr, span, target),
                 sym::non_exhaustive => self.check_non_exhaustive(hir_id, attr, span, target),
                 sym::marker => self.check_marker(hir_id, attr, span, target),
-                sym::target_feature => self.check_target_feature(hir_id, attr, span, target),
+                sym::target_feature => self.check_target_feature(hir_id, attr, span, target, attrs),
                 sym::thread_local => self.check_thread_local(attr, span, target),
                 sym::track_caller => {
                     self.check_track_caller(hir_id, attr.span, attrs, span, target)
@@ -591,10 +591,36 @@ impl CheckAttrVisitor<'_> {
         attr: &Attribute,
         span: Span,
         target: Target,
+        attrs: &[Attribute],
     ) -> bool {
         match target {
-            Target::Fn
-            | Target::Method(MethodKind::Trait { body: true } | MethodKind::Inherent) => true,
+            Target::Fn => {
+                // `#[target_feature]` is not allowed in language items.
+                if let Some((lang_item, _)) = hir::lang_items::extract(attrs)
+                    // Calling functions with `#[target_feature]` is
+                    // not unsafe on WASM, see #84988
+                    && !self.tcx.sess.target.is_like_wasm
+                    && !self.tcx.sess.opts.actually_rustdoc
+                {
+                    let hir::Node::Item(item) = self.tcx.hir().get(hir_id) else {
+                        unreachable!();
+                    };
+                    let hir::ItemKind::Fn(sig, _, _) = item.kind else {
+                        // target is `Fn`
+                        unreachable!();
+                    };
+
+                    self.tcx.sess.emit_err(errors::LangItemWithTargetFeature {
+                        attr_span: attr.span,
+                        name: lang_item,
+                        sig_span: sig.span,
+                    });
+                    false
+                } else {
+                    true
+                }
+            }
+            Target::Method(MethodKind::Trait { body: true } | MethodKind::Inherent) => true,
             // FIXME: #[target_feature] was previously erroneously allowed on statements and some
             // crates used this, so only emit a warning.
             Target::Statement => {
diff --git a/compiler/rustc_passes/src/errors.rs b/compiler/rustc_passes/src/errors.rs
index 8b65e301b74..2ec6a0b9241 100644
--- a/compiler/rustc_passes/src/errors.rs
+++ b/compiler/rustc_passes/src/errors.rs
@@ -809,6 +809,16 @@ pub struct MissingLangItem {
 }
 
 #[derive(Diagnostic)]
+#[diag(passes_lang_item_fn_with_target_feature)]
+pub struct LangItemWithTargetFeature {
+    #[primary_span]
+    pub attr_span: Span,
+    pub name: Symbol,
+    #[label]
+    pub sig_span: Span,
+}
+
+#[derive(Diagnostic)]
 #[diag(passes_lang_item_on_incorrect_target, code = "E0718")]
 pub struct LangItemOnIncorrectTarget {
     #[primary_span]
diff --git a/compiler/rustc_passes/src/lang_items.rs b/compiler/rustc_passes/src/lang_items.rs
index 476394f30cc..7e837243918 100644
--- a/compiler/rustc_passes/src/lang_items.rs
+++ b/compiler/rustc_passes/src/lang_items.rs
@@ -20,7 +20,8 @@ use rustc_hir::lang_items::{extract, GenericRequirement};
 use rustc_hir::{LangItem, LanguageItems, Target};
 use rustc_middle::ty::TyCtxt;
 use rustc_session::cstore::ExternCrate;
-use rustc_span::{symbol::kw::Empty, Span};
+use rustc_span::symbol::kw::Empty;
+use rustc_span::{sym, Span};
 
 use rustc_middle::query::Providers;
 
@@ -157,7 +158,14 @@ impl<'tcx> LanguageItemCollector<'tcx> {
             self.tcx.hir().get_by_def_id(item_def_id)
         {
             let (actual_num, generics_span) = match kind.generics() {
-                Some(generics) => (generics.params.len(), generics.span),
+                Some(generics) => (
+                    generics
+                        .params
+                        .iter()
+                        .filter(|p| !self.tcx.has_attr(p.def_id, sym::rustc_host))
+                        .count(),
+                    generics.span,
+                ),
                 None => (0, *item_span),
             };