get_attr should check that no duplicates are allowed

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -386,7 +386,8 @@ pub fn from_fn_attrs<'ll, 'tcx>(
     ) {
         let span = cx
             .tcx
-            .get_attr(instance.def_id(), sym::target_feature)
+            .get_attrs(instance.def_id(), sym::target_feature)
+            .next()
             .map_or_else(|| cx.tcx.def_span(instance.def_id()), |a| a.span);
         let msg = format!(
             "the target features {} must all be either enabled or disabled together",

compiler/rustc_feature/src/builtin_attrs.rs

@@ -823,6 +823,14 @@ pub fn is_builtin_only_local(name: Symbol) -> bool {
     BUILTIN_ATTRIBUTE_MAP.get(&name).map_or(false, |attr| attr.only_local)
 }
 
+pub fn is_valid_for_get_attr(name: Symbol) -> bool {
+    BUILTIN_ATTRIBUTE_MAP.get(&name).map_or(false, |attr| match attr.duplicates {
+        WarnFollowing | ErrorFollowing | ErrorPreceding | FutureWarnFollowing
+        | FutureWarnPreceding => true,
+        DuplicatesOk | WarnFollowingWordOnly => false,
+    })
+}
+
 pub static BUILTIN_ATTRIBUTE_MAP: LazyLock<FxHashMap<Symbol, &BuiltinAttribute>> =
     LazyLock::new(|| {
         let mut map = FxHashMap::default();

compiler/rustc_feature/src/lib.rs

@@ -151,7 +151,7 @@ pub use active::{Features, ACTIVE_FEATURES, INCOMPATIBLE_FEATURES};
 pub use builtin_attrs::AttributeDuplicates;
 pub use builtin_attrs::{
     deprecated_attributes, find_gated_cfg, is_builtin_attr_name, is_builtin_only_local,
-    AttributeGate, AttributeTemplate, AttributeType, BuiltinAttribute, GatedCfg,
+    is_valid_for_get_attr, AttributeGate, AttributeTemplate, AttributeType, BuiltinAttribute,
-    BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
+    GatedCfg, BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
 };
 pub use removed::{REMOVED_FEATURES, STABLE_REMOVED_FEATURES};

compiler/rustc_middle/src/ty/mod.rs

@@ -2274,7 +2274,11 @@ impl<'tcx> TyCtxt<'tcx> {
     }
 
     pub fn get_attr(self, did: DefId, attr: Symbol) -> Option<&'tcx ast::Attribute> {
-        self.get_attrs(did, attr).next()
+        if cfg!(debug_assertions) && !rustc_feature::is_valid_for_get_attr(attr) {
+            bug!("get_attr: unexpected called with DefId `{:?}`, attr `{:?}`", did, attr);
+        } else {
+            self.get_attrs(did, attr).next()
+        }
     }
 
     /// Determines whether an item is annotated with an attribute.

compiler/rustc_typeck/src/check/check.rs

@@ -1459,7 +1459,7 @@ fn check_enum<'tcx>(tcx: TyCtxt<'tcx>, vs: &'tcx [hir::Variant<'tcx>], def_id: L
     def.destructor(tcx); // force the destructor to be evaluated
 
     if vs.is_empty() {
-        if let Some(attr) = tcx.get_attr(def_id.to_def_id(), sym::repr) {
+        if let Some(attr) = tcx.get_attrs(def_id.to_def_id(), sym::repr).next() {
             struct_span_err!(
                 tcx.sess,
                 attr.span,

src/test/ui/attributes/issue-100631.rs

@@ -0,0 +1,8 @@
+// issue #100631, make sure `TyCtxt::get_attr` only called by case that compiler
+// can reasonably deal with multiple attributes.
+// `repr` will use `TyCtxt::get_attrs` since it's `DuplicatesOk`.
+#[repr(C)] //~ ERROR: unsupported representation for zero-variant enum [E0084]
+#[repr(C)]
+enum Foo {}
+
+fn main() {}

src/test/ui/attributes/issue-100631.stderr

@@ -0,0 +1,12 @@
+error[E0084]: unsupported representation for zero-variant enum
+  --> $DIR/issue-100631.rs:4:1
+   |
+LL | #[repr(C)]
+   | ^^^^^^^^^^
+LL | #[repr(C)]
+LL | enum Foo {}
+   | -------- zero-variant enum
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0084`.