rust: Fix ICE with infer type used in struct attribute

When visiting a struct declaration, add check for when a struct's
attribute is declared as an infer type, emitting an error if true.

	Fixes #3583

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-item.cc (TypeCheckItem::visit):
	Add check for infer type on struct's attribute.

gcc/testsuite/ChangeLog:

	* rust/compile/infer-type-issue-3583.rs: New test.

Signed-off-by: João Novo <joao.c.novo@tecnico.ulisboa.pt>

Author: joao-novo

gcc/rust/typecheck/rust-hir-type-check-item.cc

@@ -266,6 +266,13 @@ TypeCheckItem::visit (HIR::StructStruct &struct_decl)
     {
       TyTy::BaseType *field_type
 	= TypeCheckType::Resolve (field.get_field_type ());
+      if (field_type->contains_infer ())
+	{
+	  rust_error_at (field_type->get_locus (),
+			 "the placeholder %<_%> is not allowed within types on "
+			 "item signatures for structs");
+	  return;
+	}
       auto *ty_field
 	= new TyTy::StructFieldType (field.get_mappings ().get_hirid (),
 				     field.get_field_name ().as_string (),

gcc/testsuite/rust/compile/infer-type-issue-3583.rs

@@ -0,0 +1,19 @@
+#![feature(no_core)]
+#![no_core]
+
+struct Test10 {
+    a: _, // { dg-error "the placeholder ... is not allowed within types on item" }
+}
+
+struct Test11 {
+    a: _, // { dg-error "the placeholder ... is not allowed within types on item" }
+    b: i32,
+}
+
+struct Test12 {
+    a: (_, _), // { dg-error "the placeholder ... is not allowed within types on item" }
+}
+
+struct Test13 {
+    a: [_; _], // { dg-error "the placeholder ... is not allowed within types on item" }
+}