Re: [COMMITTED 41/77] gccrs: add static mut refs lint

Arthur Cohen <[email protected]> Fri, 7 Aug 2026 14:43:46 +0200
Newsgroups	gmane.comp.gcc.rust,gmane.comp.gcc.patches
Message-ID	<[email protected]>
Note that upstream the commit changelog is correct and the mention of 
`rust-lang.cc` has been removed.

Thanks,

Arthur


On 8/7/26 2:39 PM, [email protected] wrote:
> From: Lucas Ly Ba <[email protected]>
> 
> Warn on taking a reference to a mutable static, which is discouraged as
> it can easily lead to undefined behaviour.
> 
> gcc/rust/ChangeLog:
> 
> 	* checks/lints/unused/rust-unused-checker.cc (UnusedChecker::visit):
> 	New.
> 	* checks/lints/unused/rust-unused-checker.h (UnusedChecker::visit):
> 	New.
> 	* rust-lang.cc (grs_langhook_init_options_struct): Enable warn_unused.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* rust/compile/static-mut-refs_0.rs: New test.
> 
> Signed-off-by: Lucas Ly Ba <[email protected]>
> ---
>   .../lints/unused/rust-unused-checker.cc       | 20 +++++++++++++++++++
>   .../checks/lints/unused/rust-unused-checker.h |  1 +
>   .../rust/compile/static-mut-refs_0.rs         | 13 ++++++++++++
>   3 files changed, 34 insertions(+)
>   create mode 100644 gcc/testsuite/rust/compile/static-mut-refs_0.rs
> 
> diff --git a/gcc/rust/checks/lints/unused/rust-unused-checker.cc b/gcc/rust/checks/lints/unused/rust-unused-checker.cc
> index 0d091e6473a..fffe61dd129 100644
> --- a/gcc/rust/checks/lints/unused/rust-unused-checker.cc
> +++ b/gcc/rust/checks/lints/unused/rust-unused-checker.cc
> @@ -347,5 +347,25 @@ UnusedChecker::visit (HIR::LetStmt &stmt)
>     walk (stmt);
>   }
>   
> +void
> +UnusedChecker::visit (HIR::BorrowExpr &expr)
> +{
> +  // The static_mut_refs lint: taking a reference to a mutable static is
> +  // discouraged as it can easily lead to undefined behaviour.
> +  NodeId ast_node_id = expr.get_expr ().get_mappings ().get_nodeid ();
> +  if (auto def
> +      = nr_context.lookup (ast_node_id, Resolver2_0::Namespace::Values))
> +    if (auto id = mappings.lookup_node_to_hir (*def))
> +      if (auto item = mappings.lookup_hir_item (*id))
> +	if (item.value ()->get_item_kind () == HIR::Item::ItemKind::Static)
> +	  {
> +	    auto &static_item = static_cast<HIR::StaticItem &> (*item.value ());
> +	    if (static_item.is_mut ())
> +	      rust_warning_at (expr.get_locus (), OPT_Wunused,
> +			       "creating a reference to a mutable static");
> +	  }
> +  walk (expr);
> +}
> +
>   } // namespace Analysis
>   } // namespace Rust
> diff --git a/gcc/rust/checks/lints/unused/rust-unused-checker.h b/gcc/rust/checks/lints/unused/rust-unused-checker.h
> index 46e012e7907..161f59b8a66 100644
> --- a/gcc/rust/checks/lints/unused/rust-unused-checker.h
> +++ b/gcc/rust/checks/lints/unused/rust-unused-checker.h
> @@ -52,6 +52,7 @@ private:
>     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::BorrowExpr &expr) override;
>     virtual void visit_loop_label (HIR::LoopLabel &label) override;
>   };
>   } // namespace Analysis
> diff --git a/gcc/testsuite/rust/compile/static-mut-refs_0.rs b/gcc/testsuite/rust/compile/static-mut-refs_0.rs
> new file mode 100644
> index 00000000000..5dcc5978a74
> --- /dev/null
> +++ b/gcc/testsuite/rust/compile/static-mut-refs_0.rs
> @@ -0,0 +1,13 @@
> +// { dg-additional-options "-frust-unused-check-2.0" }
> +#![feature(no_core, lang_items)]
> +#![no_core]
> +
> +#[lang = "sized"]
> +pub trait Sized {}
> +
> +static mut S: i32 = 0;
> +
> +pub unsafe fn f() {
> +    let _y = &S;
> +// { dg-warning "reference to a mutable static" "" { target *-*-* } .-1 }
> +}
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