Evaluate the enum's discriminant in a const context

gcc/rust/ChangeLog:

	* backend/rust-compile-resolve-path.cc: Evaluate the enum's discriminant in a const context

gcc/testsuite/ChangeLog:

	* rust/compile/enum_discriminant1.rs: New test.

Signed-off-by: Ryutaro Okada <[email protected]>

Author: sakupan102

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

@@ -105,7 +105,9 @@ ResolvePathRef::attempt_constructor_expression_lookup (
 
   // make the ctor for the union
   HIR::Expr &discrim_expr = variant->get_discriminant ();
+  ctx->push_const_context ();
   tree discrim_expr_node = CompileExpr::Compile (discrim_expr, ctx);
+  ctx->pop_const_context ();
   tree folded_discrim_expr = fold_expr (discrim_expr_node);
   tree qualifier = folded_discrim_expr;
 

gcc/testsuite/rust/compile/enum_discriminant1.rs

@@ -0,0 +1,7 @@
+enum Foo {
+    Bar = 3 + 12,
+}
+
+fn test() -> Foo { // { dg-warning "function is never used" }
+    return Foo::Bar;
+}