gccrs: Add error diag for self params on plain functions

self params dont have a type unless used within impl blocks. Rustc as far as I
can tell in this senario generics a synthetic param of type Self in this senario
so that it keeps consistent error diagnostic for number of parameters but the
logic for what the parameter typpe should be seems unclear.

Fixes #3592

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-item.cc (ASTLoweringItem::visit): add error diagnostic

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3592.rs: New test.

Signed-off-by: Philip Herron <[email protected]>

Author: philberty

gcc/rust/hir/rust-ast-lower-item.cc

@@ -418,10 +418,39 @@ ASTLoweringItem::visit (AST::Function &function)
   std::vector<HIR::FunctionParam> function_params;
   function_params.reserve (function.get_function_params ().size ());
 
+  auto crate_num = mappings.get_current_crate ();
   for (auto &p : function.get_function_params ())
     {
-      if (p->is_variadic () || p->is_self ())
+      if (p->is_variadic ())
 	continue;
+      if (p->is_self ())
+	{
+	  rich_location r (line_table, p->get_locus ());
+	  r.add_range (function.get_locus ());
+	  rust_error_at (
+	    r, "%<self%> parameter is only allowed in associated functions");
+
+	  // rustc creates a synthetic regular fn-param here pointing to a
+	  // generic Self as far as i can see but that seems over the top for
+	  // now.
+	  //
+	  // see this example (invalid code):
+	  //
+	  // pub trait X {
+	  //   fn x() {
+	  //     fn f(&mut self) {}
+	  //     f();
+	  //   }
+	  // }
+	  //
+	  // without a synthetic param we wont get the number of args error as
+	  // well but i think this is fine for now.
+	  //
+	  // problem is what we make the param type to become...
+
+	  continue;
+	}
+
       auto param = static_cast<AST::FunctionParam &> (*p);
 
       auto translated_pattern = std::unique_ptr<HIR::Pattern> (
@@ -445,7 +474,6 @@ ASTLoweringItem::visit (AST::Function &function)
       ASTLoweringBlock::translate (*function.get_definition ().value (),
 				   &terminated));
 
-  auto crate_num = mappings.get_current_crate ();
   Analysis::NodeMapping mapping (crate_num, function.get_node_id (),
 				 mappings.get_next_hir_id (crate_num),
 				 mappings.get_next_localdef_id (crate_num));

gcc/testsuite/rust/compile/issue-3592.rs

@@ -0,0 +1,7 @@
+pub trait X {
+    fn x() {
+        fn f(&mut self) {}
+        // { dg-error ".self. parameter is only allowed in associated functions" "" { target *-*-* } .-1 }
+        f();
+    }
+}