[gcc r17-3093] gccrs: add dyn drop lint

https://gcc.gnu.org/g:f9eb87556d10353a8ca678c4c570772a6f2df043

commit r17-3093-gf9eb87556d10353a8ca678c4c570772a6f2df043
Author: Lucas Ly Ba <[email protected]>
Date:   Sat Jun 27 23:12:31 2026 +0200

    gccrs: add dyn drop lint
    
    Warn on a trait object that carries a `Drop` bound, as values are
    dropped automatically and the bound has no effect.
    
    gcc/rust/ChangeLog:
    
            * typecheck/rust-hir-type-check-type.cc (TypeCheckType::visit): Warn on
            a trait object with a `Drop` bound.
            * rust-lang.cc (grs_langhook_init_options_struct): Enable warn_unused.
    
    gcc/testsuite/ChangeLog:
    
            * rust/compile/dyn-drop_0.rs: New test.
    
    Signed-off-by: Lucas Ly Ba <[email protected]>

Diff:
---
 gcc/rust/rust-lang.cc                          |  2 ++
 gcc/rust/typecheck/rust-hir-type-check-type.cc | 10 ++++++++++
 gcc/testsuite/rust/compile/dyn-drop_0.rs       | 14 ++++++++++++++
 3 files changed, 26 insertions(+)

diff --git a/gcc/rust/rust-lang.cc b/gcc/rust/rust-lang.cc
index dde3ef204ce0..dc39bc053d0e 100644
--- a/gcc/rust/rust-lang.cc
+++ b/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 = 1;
   /* For const variables too */
   opts->x_warn_unused_const_variable = 1;
   /* And finally unused result for #[must_use] */
diff --git a/gcc/rust/typecheck/rust-hir-type-check-type.cc b/gcc/rust/typecheck/rust-hir-type-check-type.cc
index 634d04988616..c6fbb2ae1bd7 100644
--- a/gcc/rust/typecheck/rust-hir-type-check-type.cc
+++ b/gcc/rust/typecheck/rust-hir-type-check-type.cc
@@ -655,6 +655,16 @@ TypeCheckType::visit (HIR::TraitObjectType &type)
 	specified_bounds.push_back (std::move (predicate));
     }
 
+  // The dyn_drop lint: a trait object with a `Drop` bound is pointless, as
+  // values are dropped automatically regardless of the bound.
+  if (flag_unused_check_2_0)
+    if (auto drop = mappings.lookup_lang_item (LangItem::Kind::DROP))
+      for (auto &bound : specified_bounds)
+	if (bound.get_id () == drop.value ())
+	  rust_warning_at (type.get_locus (), OPT_Wunused,
+			   "this trait object has a %<Drop%> bound, which has "
+			   "no effect");
+
   RustIdent ident{CanonicalPath::create_empty (), type.get_locus ()};
   translated
     = new TyTy::DynamicObjectType (type.get_mappings ().get_hirid (), ident,
diff --git a/gcc/testsuite/rust/compile/dyn-drop_0.rs b/gcc/testsuite/rust/compile/dyn-drop_0.rs
new file mode 100644
index 000000000000..07bc30d7b2a2
--- /dev/null
+++ b/gcc/testsuite/rust/compile/dyn-drop_0.rs
@@ -0,0 +1,14 @@
+// { dg-additional-options "-frust-unused-check-2.0" }
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+pub trait Sized {}
+
+#[lang = "drop"]
+pub trait Drop {
+    fn drop(&mut self);
+}
+
+pub fn f(_x: &dyn Drop) {}
+// { dg-warning "trait object has a .Drop. bound" "" { target *-*-* } .-1 }