gccrs: Fix ICE cloning trait functions without return types

Fixes #3972.

Trait functions without an explicit return type can have a null
`return_type` in `TraitFunctionDecl`. When such declarations are copied,
the copy constructor and assignment operator currently try to clone the
return type unconditionally, and this can lead to an ICE.

Handle this case by keeping `nullptr` when there is no return type to
clone. Also add a regression test for the example from #3972.

gcc/rust/ChangeLog:

	* hir/tree/rust-hir-item.cc (TraitFunctionDecl::TraitFunctionDecl):
	Handle null return types in copy constructor.
	(TraitFunctionDecl::operator=): Likewise.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3972.rs: New test.

Signed-off-by: lishin <lishin1008@gmail.com>

Author: Lishin1215

gcc/rust/hir/tree/rust-hir-item.cc

@@ -630,7 +630,8 @@ TraitFunctionDecl::TraitFunctionDecl (
 TraitFunctionDecl::TraitFunctionDecl (TraitFunctionDecl const &other)
   : qualifiers (other.qualifiers), function_name (other.function_name),
     function_params (other.function_params),
-    return_type (other.return_type->clone_type ()),
+    return_type (other.return_type != nullptr ? other.return_type->clone_type ()
+					      : nullptr),
     where_clause (other.where_clause), self (other.self)
 {
   generic_params.reserve (other.generic_params.size ());
@@ -644,7 +645,9 @@ TraitFunctionDecl::operator= (TraitFunctionDecl const &other)
   function_name = other.function_name;
   qualifiers = other.qualifiers;
   function_params = other.function_params;
-  return_type = other.return_type->clone_type ();
+  return_type
+    = other.return_type != nullptr ? other.return_type->clone_type () : nullptr;
+
   where_clause = other.where_clause;
   self = other.self;
 

gcc/testsuite/rust/compile/issue-3972.rs

@@ -0,0 +1,19 @@
+// { dg-options "-frust-compile-until=lowering" }
+#![feature(no_core)]
+#![no_core]
+
+struct Expr<const N: u32>;
+
+trait Trait0 {
+    fn required(
+        _: Expr<
+            {
+                trait Trait0 {
+                    fn required();
+                }
+
+                0
+            },
+        >,
+    );
+}