Emit error on lonely self use declaration

gcc/rust/resolve/rust-early-name-resolver-2.0.cc

@@ -438,7 +438,9 @@ Early::finalize_rebind_import (const Early::ImportPair &mapping)
 	// We don't want to insert `self` with `use module::self`
 	if (path.get_final_segment ().is_lower_self_seg ())
 	  {
-	    rust_assert (segments.size () > 1);
+	    // Erroneous `self` or `{self}` use declaration
+	    if (segments.size () == 1)
+	      break;
 	    declared_name = segments[segments.size () - 2].as_string ();
 	  }
 	else
@@ -459,6 +461,19 @@ Early::finalize_rebind_import (const Early::ImportPair &mapping)
 void
 Early::visit (AST::UseDeclaration &decl)
 {
+  // We do not want to visit the use trees, we're only looking for top level
+  // rebind. eg. `use something;` or `use something::other;`
+  if (decl.get_tree ()->get_kind () == AST::UseTree::Kind::Rebind)
+    {
+      auto &rebind = static_cast<AST::UseTreeRebind &> (*decl.get_tree ());
+      if (rebind.get_path ().get_final_segment ().is_lower_self_seg ())
+	{
+	  collect_error (
+	    Error (decl.get_locus (), ErrorCode::E0429,
+		   "%<self%> imports are only allowed within a { } list"));
+	}
+    }
+
   auto &imports = toplevel.get_imports_to_resolve ();
   auto current_import = imports.find (decl.get_node_id ());
   if (current_import != imports.end ())
@@ -484,5 +499,31 @@ Early::visit (AST::UseDeclaration &decl)
   DefaultResolver::visit (decl);
 }
 
+void
+Early::visit (AST::UseTreeList &use_list)
+{
+  if (!use_list.has_path ())
+    {
+      for (auto &&tree : use_list.get_trees ())
+	{
+	  if (tree->get_kind () == AST::UseTree::Kind::Rebind)
+	    {
+	      auto &rebind = static_cast<AST::UseTreeRebind &> (*tree);
+	      auto path_size = rebind.get_path ().get_segments ().size ();
+	      if (path_size == 1
+		  && rebind.get_path ()
+		       .get_final_segment ()
+		       .is_lower_self_seg ())
+		{
+		  collect_error (Error (rebind.get_locus (), ErrorCode::E0431,
+					"%<self%> import can only appear in an "
+					"import list with a non-empty prefix"));
+		}
+	    }
+	}
+    }
+  DefaultResolver::visit (use_list);
+}
+
 } // namespace Resolver2_0
 } // namespace Rust

gcc/rust/resolve/rust-early-name-resolver-2.0.h

@@ -61,6 +61,7 @@ class Early : public DefaultResolver
   void visit (AST::Function &) override;
   void visit (AST::StructStruct &) override;
   void visit (AST::UseDeclaration &) override;
+  void visit (AST::UseTreeList &) override;
 
   struct ImportData
   {

gcc/testsuite/rust/compile/use_self_alone.rs

@@ -0,0 +1,2 @@
+use self;
+// { dg-error ".self. imports are only allowed within a { } list" "" { target *-*-* } .-1 }

gcc/testsuite/rust/compile/use_self_alone_in_list.rs

@@ -0,0 +1,7 @@
+struct B;
+
+use {B as B2, self};
+// { dg-error ".self. import can only appear in an import list with a non-empty prefix" "" { target *-*-* } .-1 }
+
+use {self};
+// { dg-error ".self. import can only appear in an import list with a non-empty prefix" "" { target *-*-* } .-1 }