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 <0001jayant@gmail.com>

Author: Pasta-coder

gcc/rust/typecheck/rust-hir-type-check-stmt.cc

@@ -88,6 +88,39 @@ TypeCheckStmt::visit (HIR::LetStmt &stmt)
     {
       init_expr_locus = stmt.get_init_expr ().get_locus ();
       init_expr_ty = TypeCheckExpr::Resolve (stmt.get_init_expr ());
+
+      if (stmt_pattern.get_pattern_kind ()
+	  == HIR::Pattern::PatternKind::Identifier)
+	{
+	  auto &ident = static_cast<HIR::IdentifierPattern &> (stmt_pattern);
+
+	  if (ident.is_ref () && ident.is_mut ())
+	    {
+	      HIR::Expr &init = stmt.get_init_expr ();
+
+	      if (init.get_expression_kind () == HIR::Expr::ExprKind::Path)
+		{
+		  NodeId ref_id = init.get_mappings ().get_nodeid ();
+		  NodeId def_id = UNKNOWN_NODEID;
+
+		  if (resolver->lookup_resolved_name (ref_id, &def_id))
+		    {
+		      bool is_mutable = false;
+		      Resolver::Rib::Definition def;
+		      if (resolver->get_name_scope ().lookup (def_id, &def))
+			{
+			  is_mutable = def.is_mutable;
+			}
+		      if (!is_mutable)
+			{
+			  rust_error_at (stmt_pattern.get_locus (),
+					 "cannot borrow immutable local "
+					 "variable as mutable");
+			}
+		    }
+		}
+	    }
+	}
       if (init_expr_ty->get_kind () == TyTy::TypeKind::ERROR)
 	return;
 

gcc/testsuite/rust/compile/issue-4289.rs

@@ -0,0 +1,10 @@
+// { dg-options "-fsyntax-only" }
+pub fn a() {
+    let v = 10;
+    let ref mut r = v; // { dg-error "cannot borrow immutable local variable as mutable" }
+}
+
+pub fn b() {
+    let mut v2 = 10;
+    let ref mut r2 = v2; // Should compile fine
+}