gccrs: avoid ICE on generic const expressions in path resolution

ResolvePathRef::resolve_with_node_id assumed CONST types were always
concrete values and asserted otherwise. Generic const expressions such
as { N + 1 } are symbolic and cannot be evaluated prior to
monomorphization, leading to an internal compiler error.

Reject non-value const kinds gracefully and emit a diagnostic instead
of aborting.

Fixes #4302

gcc/rust/ChangeLog:

	* backend/rust-compile-resolve-path.cc
	(ResolvePathRef::resolve_with_node_id): Reject non-value
	generic const expressions and emit a diagnostic instead of ICE.

gcc/testsuite/ChangeLog:

	* rust/compile/const-generic-ice-4302.rs: New test.

Signed-off-by: Jayant Chauhan <0001jayant@gmail.com>

Author: Pasta-coder

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

@@ -193,7 +193,14 @@ ResolvePathRef::resolve_with_node_id (
       auto d = lookup->destructure ();
       rust_assert (d->get_kind () == TyTy::TypeKind::CONST);
       auto c = d->as_const_type ();
-      rust_assert (c->const_kind () == TyTy::BaseConstType::ConstKind::Value);
+
+      if (c->const_kind () != TyTy::BaseConstType::ConstKind::Value)
+	{
+	  rust_error_at (
+	    expr_locus,
+	    "cannot evaluate constant expression with generic parameters");
+	  return error_mark_node;
+	}
       auto val = static_cast<TyTy::ConstValueType *> (c);
       return val->get_value ();
     }

gcc/testsuite/rust/compile/const-generic-ice-4302.rs

@@ -0,0 +1,6 @@
+pub struct Foo<const N: usize>;
+
+pub fn foo<const N: usize>() -> Foo<{ N + 1 }> { // { dg-error "cannot evaluate constant expression with generic parameters" }
+    Foo
+}
+// { dg-excess-errors "Noisy error cascade" }