gccrs: Track fn_once output lang item properly

In order to setup the Output assoicated type we can rely on using generic
argument bindings. So for example when we have the FnOnce trait:

  #[lang = "fn_once"]
  pub trait FnOnce<Args> {
      #[lang = "fn_once_output"]
      type Output;

      extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
  }

Thn we might have a function such as:

  pub fn map<R, F: FnOnce(T) -> R>(self, f: F) -> Option<R> { ... }

This trait bound predicate of FnOnce(T) -> R we setup generics for the
bound as:

  FnOnce<(T), Output=R>

This means we can reuse our generic arguments handling to get this support.

Fixes #2105

gcc/rust/ChangeLog:

	* typecheck/rust-tyty-bounds.cc (TypeCheckBase::get_predicate_from_bound): track output
	* util/rust-hir-map.cc (Mappings::lookup_trait_item_lang_item): new helper
	* util/rust-hir-map.h: add prototype for helper

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2105.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

4 files changed, 57 insertions, 14 deletions

diff --git a/gcc/rust/typecheck/rust-tyty-bounds.cc b/gcc/rust/typecheck/rust-tyty-bounds.cc
index 12e6659..ec64aa7 100644
--- a/gcc/rust/typecheck/rust-tyty-bounds.cc
+++ b/gcc/rust/typecheck/rust-tyty-bounds.cc
@@ -227,22 +227,26 @@ TypeCheckBase::get_predicate_from_bound (HIR::TypePath &type_path,
 	std::vector<std::unique_ptr<HIR::Type>> inputs;
 	inputs.push_back (std::unique_ptr<HIR::Type> (implicit_tuple));
 
+	// resolve the fn_once_output type which assumes there must be an output
+	// set
+	rust_assert (fn.has_return_type ());
+	TypeCheckType::Resolve (fn.get_return_type ().get ());
+
+	HIR::TraitItem *trait_item = mappings->lookup_trait_item_lang_item (
+	  Analysis::RustLangItem::ItemType::FN_ONCE_OUTPUT);
+
+	std::vector<HIR::GenericArgsBinding> bindings;
+	Location output_locus = fn.get_return_type ()->get_locus ();
+	HIR::GenericArgsBinding binding (Identifier (
+					   trait_item->trait_identifier ()),
+					 fn.get_return_type ()->clone_type (),
+					 output_locus);
+	bindings.push_back (std::move (binding));
+
 	args = HIR::GenericArgs ({} /* lifetimes */,
 				 std::move (inputs) /* type_args*/,
-				 {} /* binding_args*/, {} /* const_args */,
-				 final_seg->get_locus ());
-
-	// resolve the fn_once_output type
-	TyTy::BaseType *fn_once_output_ty
-	  = fn.has_return_type ()
-	      ? TypeCheckType::Resolve (fn.get_return_type ().get ())
-	      : TyTy::TupleType::get_unit_type (
-		final_seg->get_mappings ().get_hirid ());
-	context->insert_implicit_type (final_seg->get_mappings ().get_hirid (),
-				       fn_once_output_ty);
-
-	// setup the associated type.. ??
-	// fn_once_output_ty->debug ();
+				 std::move (bindings) /* binding_args*/,
+				 {} /* const_args */, final_seg->get_locus ());
       }
       break;
 
diff --git a/gcc/rust/util/rust-hir-map.cc b/gcc/rust/util/rust-hir-map.cc
index 87a3a73..3326e7e 100644
--- a/gcc/rust/util/rust-hir-map.cc
+++ b/gcc/rust/util/rust-hir-map.cc
@@ -1137,5 +1137,18 @@ Mappings::lookup_builtin_marker ()
   return builtinMarker;
 }
 
+HIR::TraitItem *
+Mappings::lookup_trait_item_lang_item (Analysis::RustLangItem::ItemType item)
+{
+  DefId trait_item_id = UNKNOWN_DEFID;
+  bool trait_item_lang_item_defined = lookup_lang_item (item, &trait_item_id);
+
+  // FIXME
+  // make this an error? what does rustc do when a lang item is not defined?
+  rust_assert (trait_item_lang_item_defined);
+
+  return lookup_trait_item_defid (trait_item_id);
+}
+
 } // namespace Analysis
 } // namespace Rust
diff --git a/gcc/rust/util/rust-hir-map.h b/gcc/rust/util/rust-hir-map.h
index 6e6a1c8..964b626 100644
--- a/gcc/rust/util/rust-hir-map.h
+++ b/gcc/rust/util/rust-hir-map.h
@@ -321,6 +321,9 @@ public:
 
   HIR::ImplBlock *lookup_builtin_marker ();
 
+  HIR::TraitItem *
+  lookup_trait_item_lang_item (Analysis::RustLangItem::ItemType item);
+
 private:
   Mappings ();
 
diff --git a/gcc/testsuite/rust/compile/issue-2105.rs b/gcc/testsuite/rust/compile/issue-2105.rs
new file mode 100644
index 0000000..7d1c9a1
--- /dev/null
+++ b/gcc/testsuite/rust/compile/issue-2105.rs
@@ -0,0 +1,23 @@
+pub enum Option<T> {
+    Some(T),
+    None,
+}
+
+pub use Option::{None, Some};
+
+#[lang = "fn_once"]
+pub trait FnOnce<Args> {
+    #[lang = "fn_once_output"]
+    type Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
+}
+
+impl<T> Option<T> {
+    pub fn map<R, F: FnOnce(T) -> R>(self, f: F) -> Option<R> {
+        match self {
+            Some(value) => Some(f(value)),
+            None => None,
+        }
+    }
+}