gccrs: Fix ICE when compiling path which resolves to trait constant

Fixes #3552

gcc/rust/ChangeLog:

	* backend/rust-compile-resolve-path.cc (HIRCompileBase::query_compile): check for Expr trait
	* hir/rust-hir-dump.cc (Dump::visit): expr is optional

gcc/testsuite/ChangeLog:

	* rust/compile/issue-3552.rs: New test.

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

Author: philberty

gcc/rust/backend/rust-compile-resolve-path.cc

@@ -301,6 +301,27 @@ HIRCompileBase::query_compile (HirId ref, TyTy::BaseType *lookup,
 	    trait->get_mappings ().get_defid (), &trait_ref);
 	  rust_assert (ok);
 
+	  if (trait_item.value ()->get_item_kind ()
+	      == HIR::TraitItem::TraitItemKind::CONST)
+	    {
+	      auto &c
+		= *static_cast<HIR::TraitItemConst *> (trait_item.value ());
+	      if (!c.has_expr ())
+		{
+		  rich_location r (line_table, expr_locus);
+		  r.add_range (trait->get_locus ());
+		  r.add_range (c.get_locus ());
+		  rust_error_at (r, "no default expression on trait constant");
+		  return error_mark_node;
+		}
+
+	      return CompileExpr::Compile (c.get_expr (), ctx);
+	    }
+
+	  if (trait_item.value ()->get_item_kind ()
+	      != HIR::TraitItem::TraitItemKind::FUNC)
+	    return error_mark_node;
+
 	  // the type resolver can only resolve type bounds to their trait
 	  // item so its up to us to figure out if this path should resolve
 	  // to an trait-impl-block-item or if it can be defaulted to the

gcc/rust/hir/rust-hir-dump.cc

@@ -1932,7 +1932,9 @@ Dump::visit (TraitItemConst &e)
 
   put_field ("name", e.get_name ().as_string ());
   visit_field ("type", e.get_type ());
-  visit_field ("expr", e.get_expr ());
+  if (e.has_expr ())
+    visit_field ("expr", e.get_expr ());
+
   end ("TraitItemConst");
 }
 

gcc/testsuite/rust/compile/issue-3552.rs

@@ -0,0 +1,14 @@
+trait Foo {
+    const BAR: u32;
+}
+
+const TRAIT_REF_BAR: u32 = <Foo>::BAR;
+// { dg-error "no default expression on trait constant" "" { target *-*-* } .-1 }
+
+struct GlobalTraitRef;
+
+impl Foo for GlobalTraitRef {
+    const BAR: u32 = TRAIT_REF_BAR;
+}
+
+fn main() {}