delegation: Do not crash on qpaths without a trait

compiler/rustc_expand/messages.ftl

@@ -61,6 +61,9 @@ expand_feature_removed =
 expand_glob_delegation_outside_impls =
     glob delegation is only supported in impls
 
+expand_glob_delegation_traitless_qpath =
+    qualified path without a trait in glob delegation
+
 expand_helper_attribute_name_invalid =
     `{$name}` cannot be a name of derive helper attribute
 

compiler/rustc_expand/src/errors.rs

@@ -449,6 +449,13 @@ pub(crate) struct GlobDelegationOutsideImpls {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(expand_glob_delegation_traitless_qpath)]
+pub(crate) struct GlobDelegationTraitlessQpath {
+    #[primary_span]
+    pub span: Span,
+}
+
 // This used to be the `proc_macro_back_compat` lint (#83125). It was later
 // turned into a hard error.
 #[derive(Diagnostic)]

compiler/rustc_expand/src/expand.rs

@@ -1,8 +1,9 @@
 use crate::base::*;
 use crate::config::StripUnconfigured;
 use crate::errors::{
-    EmptyDelegationMac, GlobDelegationOutsideImpls, IncompleteParse, RecursionLimitReached,
-    RemoveExprNotSupported, RemoveNodeNotSupported, UnsupportedKeyValue, WrongFragmentKind,
+    EmptyDelegationMac, GlobDelegationOutsideImpls, GlobDelegationTraitlessQpath, IncompleteParse,
+    RecursionLimitReached, RemoveExprNotSupported, RemoveNodeNotSupported, UnsupportedKeyValue,
+    WrongFragmentKind,
 };
 use crate::mbe::diagnostics::annotate_err_with_kind;
 use crate::module::{mod_dir_path, parse_external_mod, DirOwnership, ParsedExternalMod};
@@ -1989,6 +1990,8 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
                 }
                 None if let Some((deleg, item)) = node.delegation() => {
                     let Some(suffixes) = &deleg.suffixes else {
+                        let traitless_qself =
+                            matches!(&deleg.qself, Some(qself) if qself.position == 0);
                         let item = match node.to_annotatable() {
                             Annotatable::ImplItem(item) => item,
                             ann @ (Annotatable::Item(_)
@@ -2000,6 +2003,11 @@ impl<'a, 'b> InvocationCollector<'a, 'b> {
                             }
                             _ => unreachable!(),
                         };
+                        if traitless_qself {
+                            let span = item.span;
+                            self.cx.dcx().emit_err(GlobDelegationTraitlessQpath { span });
+                            return Default::default();
+                        }
                         return self.collect_glob_delegation(item, Node::KIND).make_ast::<Node>();
                     };
 

tests/ui/delegation/glob-traitless-qpath.rs

@@ -0,0 +1,11 @@
+#![feature(fn_delegation)]
+#![allow(incomplete_features)]
+
+struct S;
+
+impl S {
+    reuse <u8>::*; //~ ERROR qualified path without a trait in glob delegation
+    reuse <()>::*; //~ ERROR qualified path without a trait in glob delegation
+}
+
+fn main() {}

tests/ui/delegation/glob-traitless-qpath.stderr

@@ -0,0 +1,14 @@
+error: qualified path without a trait in glob delegation
+  --> $DIR/glob-traitless-qpath.rs:7:5
+   |
+LL |     reuse <u8>::*;
+   |     ^^^^^^^^^^^^^^
+
+error: qualified path without a trait in glob delegation
+  --> $DIR/glob-traitless-qpath.rs:8:5
+   |
+LL |     reuse <()>::*;
+   |     ^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+