[gcc r17-2235] gccrs: add unused visibilities lint

https://gcc.gnu.org/g:1295b399622995da35979c9afde76e1237ce90c7

commit r17-2235-g1295b399622995da35979c9afde76e1237ce90c7
Author: Lucas Ly Ba <[email protected]>
Date:   Sat Jun 27 23:45:56 2026 +0200

    gccrs: add unused visibilities lint
    
    Warn on a visibility qualifier applied to a `const _` item, as it has no
    effect.
    
    gcc/rust/ChangeLog:
    
            * checks/lints/unused/rust-unused-checker.cc (UnusedChecker::visit):
            Warn on a visibility qualifier on a `const _` item.
    
    gcc/testsuite/ChangeLog:
    
            * rust/compile/unused-visibilities_0.rs: New test.
    
    Signed-off-by: Lucas Ly Ba <[email protected]>

Diff:
---
 gcc/rust/checks/lints/unused/rust-unused-checker.cc | 6 ++++++
 gcc/testsuite/rust/compile/unused-visibilities_0.rs | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/gcc/rust/checks/lints/unused/rust-unused-checker.cc b/gcc/rust/checks/lints/unused/rust-unused-checker.cc
index 571e31e430ae..6fd6c6efe1ca 100644
--- a/gcc/rust/checks/lints/unused/rust-unused-checker.cc
+++ b/gcc/rust/checks/lints/unused/rust-unused-checker.cc
@@ -58,6 +58,12 @@ UnusedChecker::visit (HIR::ConstantItem &item)
     rust_warning_at (item.get_locus (), OPT_Wunused_variable,
 		     "unused variable %qs",
 		     item.get_identifier ().as_string ().c_str ());
+
+  // The unused_visibilities lint: a visibility qualifier on a `const _` item
+  // has no effect.
+  if (var_name == "_" && item.get_visibility ().is_public ())
+    rust_warning_at (item.get_locus (), OPT_Wunused_variable,
+		     "visibility qualifier on a %<const _%> item is unused");
 }
 
 void
diff --git a/gcc/testsuite/rust/compile/unused-visibilities_0.rs b/gcc/testsuite/rust/compile/unused-visibilities_0.rs
new file mode 100644
index 000000000000..d158ab657b23
--- /dev/null
+++ b/gcc/testsuite/rust/compile/unused-visibilities_0.rs
@@ -0,0 +1,6 @@
+// { dg-additional-options "-frust-unused-check-2.0" }
+#![feature(no_core)]
+#![no_core]
+
+pub const _: i32 = 0;
+// { dg-warning "visibility qualifier on a .const _. item is unused" "" { target *-*-* } .-1 }