gccrs: add path statement lint

gcc/rust/checks/lints/unused/rust-unused-checker.cc

@@ -347,5 +347,14 @@ UnusedChecker::visit (HIR::LetStmt &stmt)
   walk (stmt);
 }
 
+void
+UnusedChecker::visit (HIR::ExprStmt &stmt)
+{
+  if (stmt.get_expr ().get_expression_type () == HIR::Expr::ExprType::Path)
+    rust_warning_at (stmt.get_locus (), OPT_Wunused_value,
+		     "path statement with no effect");
+  walk (stmt);
+}
+
 } // namespace Analysis
 } // namespace Rust

gcc/rust/checks/lints/unused/rust-unused-checker.h

@@ -52,6 +52,7 @@ class UnusedChecker : public HIR::DefaultHIRVisitor
   virtual void visit (HIR::MatchExpr &expr) override;
   virtual void visit (HIR::ExternBlock &block) override;
   virtual void visit (HIR::LetStmt &stmt) override;
+  virtual void visit (HIR::ExprStmt &stmt) override;
   virtual void visit_loop_label (HIR::LoopLabel &label) override;
 };
 } // namespace Analysis

gcc/rust/rust-lang.cc

@@ -136,6 +136,8 @@ grs_langhook_init_options_struct (struct gcc_options *opts)
 
   /* We need to warn on unused variables by default */
   opts->x_warn_unused_variable = 1;
+  /* Experimental lints under -frust-unused-check-2.0 warn by default */
+  opts->x_warn_unused_value = 1;
   /* For const variables too */
   opts->x_warn_unused_const_variable = 1;
   /* And finally unused result for #[must_use] */

gcc/testsuite/rust/compile/path-statements_0.rs

@@ -0,0 +1,13 @@
+// { dg-additional-options "-frust-unused-check-2.0" }
+#![feature(no_core)]
+#![no_core]
+
+pub fn foo() {
+    let x = 5;
+    x;
+// { dg-warning "path statement with no effect" "" { target *-*-* } .-1 }
+}
+
+pub fn bar(y: i32) -> i32 {
+    y
+}