gccrs: Emit error when borrowing immutable variable as mutable

Fixes #4289

Rust rules strictly forbid creating a mutable reference ('&mut T') to an
immutable binding. Previously, the compiler failed to validate the
mutability of the source variable when using a 'ref mut' pattern.
This patch adds verification logic to TypeCheckStmt to check the
mutability status of the variable definition.

gcc/rust/ChangeLog:

	* typecheck/rust-hir-type-check-stmt.cc (TypeCheckStmt::visit):
	Add check to ensure 'ref mut' patterns bind to mutable variables.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4289.rs: New test.

Signed-off-by: Jayant Chauhan <[email protected]>

Author: Pasta-coder

gcc/rust/expand/rust-derive-eq.cc

@@ -207,9 +207,15 @@ DeriveEq::visit_enum (Enum &item)
 void
 DeriveEq::visit_union (Union &item)
 {
-  rust_error_at (item.get_locus (), "derive(Eq) cannot be used on unions");
+  auto types = std::vector<std::unique_ptr<Type>> ();
+
+  for (auto &field : item.get_variants ())
+    types.emplace_back (field.get_field_type ().reconstruct ());
+
+  expanded = eq_impls (assert_receiver_is_total_eq_fn (std::move (types)),
+		       item.get_identifier ().as_string (),
+		       item.get_generic_params ());
 }
 
 } // namespace AST
 } // namespace Rust
-g