[gcc r17-2259] gccrs: add missing_abi lint

https://gcc.gnu.org/g:847d8287d372b0f7b3e46272a051adab1bf85f55

commit r17-2259-g847d8287d372b0f7b3e46272a051adab1bf85f55
Author: Lucas Ly Ba <[email protected]>
Date:   Sun Jun 28 18:04:29 2026 +0200

    gccrs: add missing_abi lint
    
    Warn on an `extern` block written without an explicit ABI string, such as
    a bare `extern { ... }`, which silently defaults to the C ABI. Rust
    deprecates this in favour of spelling the ABI out, e.g. `extern "C"`.
    
    The HIR `ExternBlock` only stored the resolved ABI enum, which always
    collapses a missing ABI to the C default, so it could not tell a bare
    `extern` from `extern "C"`. Carry the AST's `has_abi` flag through
    lowering so the lint can distinguish the two.
    
    gcc/rust/ChangeLog:
    
            * hir/tree/rust-hir-item.h (ExternBlock): Track whether the ABI
            was explicit and expose it through has_abi.
            * hir/tree/rust-hir-item.cc (ExternBlock::ExternBlock): Likewise.
            (ExternBlock::operator=): Likewise.
            * hir/rust-ast-lower-base.cc (ASTLoweringBase::lower_extern_block):
            Forward the AST has_abi flag to the HIR node.
            * checks/lints/unused/rust-unused-checker.cc (UnusedChecker::visit):
            New.
            * checks/lints/unused/rust-unused-checker.h (UnusedChecker::visit):
            New.
    
    gcc/testsuite/ChangeLog:
    
            * rust/compile/missing-abi_0.rs: New test.
    
    Signed-off-by: Lucas Ly Ba <[email protected]>

Diff:
---
 gcc/rust/checks/lints/unused/rust-unused-checker.cc | 10 ++++++++++
 gcc/rust/checks/lints/unused/rust-unused-checker.h  |  1 +
 gcc/rust/hir/rust-ast-lower-base.cc                 |  5 +++--
 gcc/rust/hir/tree/rust-hir-item.cc                  |  8 +++++---
 gcc/rust/hir/tree/rust-hir-item.h                   |  6 +++++-
 gcc/testsuite/rust/compile/missing-abi_0.rs         | 12 ++++++++++++
 6 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/gcc/rust/checks/lints/unused/rust-unused-checker.cc b/gcc/rust/checks/lints/unused/rust-unused-checker.cc
index 30fb37cfb99b..643d262d2d5f 100644
--- a/gcc/rust/checks/lints/unused/rust-unused-checker.cc
+++ b/gcc/rust/checks/lints/unused/rust-unused-checker.cc
@@ -188,6 +188,16 @@ UnusedChecker::visit (HIR::LifetimeParam &lft)
   walk (lft);
 }
 
+void
+UnusedChecker::visit (HIR::ExternBlock &block)
+{
+  if (!block.has_abi ())
+    rust_warning_at (block.get_locus (), OPT_Wunused_variable,
+		     "extern declarations without an explicit ABI are "
+		     "deprecated");
+  walk (block);
+}
+
 void
 UnusedChecker::visit_loop_label (HIR::LoopLabel &label)
 {
diff --git a/gcc/rust/checks/lints/unused/rust-unused-checker.h b/gcc/rust/checks/lints/unused/rust-unused-checker.h
index c8a272041a9d..2f2557ce3d89 100644
--- a/gcc/rust/checks/lints/unused/rust-unused-checker.h
+++ b/gcc/rust/checks/lints/unused/rust-unused-checker.h
@@ -50,6 +50,7 @@ private:
   virtual void visit (HIR::LifetimeParam &lft) override;
   virtual void visit (HIR::StructPatternFieldIdentPat &field) override;
   virtual void visit (HIR::MatchExpr &expr) override;
+  virtual void visit (HIR::ExternBlock &block) override;
   virtual void visit_loop_label (HIR::LoopLabel &label) override;
 };
 } // namespace Analysis
diff --git a/gcc/rust/hir/rust-ast-lower-base.cc b/gcc/rust/hir/rust-ast-lower-base.cc
index a1f84c9f0779..ffa7a0f39db5 100644
--- a/gcc/rust/hir/rust-ast-lower-base.cc
+++ b/gcc/rust/hir/rust-ast-lower-base.cc
@@ -1071,8 +1071,9 @@ ASTLoweringBase::lower_extern_block (AST::ExternBlock &extern_block)
     }
 
   HIR::ExternBlock *hir_extern_block
-    = new HIR::ExternBlock (mapping, abi, std::move (extern_items),
-			    std::move (vis), extern_block.get_inner_attrs (),
+    = new HIR::ExternBlock (mapping, abi, extern_block.has_abi (),
+			    std::move (extern_items), std::move (vis),
+			    extern_block.get_inner_attrs (),
 			    extern_block.get_outer_attrs (),
 			    extern_block.get_locus ());
 
diff --git a/gcc/rust/hir/tree/rust-hir-item.cc b/gcc/rust/hir/tree/rust-hir-item.cc
index 309118cff434..d5c9ad154c8d 100644
--- a/gcc/rust/hir/tree/rust-hir-item.cc
+++ b/gcc/rust/hir/tree/rust-hir-item.cc
@@ -1001,17 +1001,18 @@ ExternalTypeItem::ExternalTypeItem (ExternalTypeItem const &other)
 {}
 
 ExternBlock::ExternBlock (
-  Analysis::NodeMapping mappings, ABI abi,
+  Analysis::NodeMapping mappings, ABI abi, bool explicit_abi,
   std::vector<std::unique_ptr<ExternalItem>> extern_items, Visibility vis,
   AST::AttrVec inner_attrs, AST::AttrVec outer_attrs, location_t locus)
   : VisItem (std::move (mappings), std::move (vis), std::move (outer_attrs)),
     WithInnerAttrs (std::move (inner_attrs)), abi (abi),
-    extern_items (std::move (extern_items)), locus (locus)
+    explicit_abi (explicit_abi), extern_items (std::move (extern_items)),
+    locus (locus)
 {}
 
 ExternBlock::ExternBlock (ExternBlock const &other)
   : VisItem (other), WithInnerAttrs (other.inner_attrs), abi (other.abi),
-    locus (other.locus)
+    explicit_abi (other.explicit_abi), locus (other.locus)
 {
   extern_items.reserve (other.extern_items.size ());
   for (const auto &e : other.extern_items)
@@ -1023,6 +1024,7 @@ ExternBlock::operator= (ExternBlock const &other)
 {
   VisItem::operator= (other);
   abi = other.abi;
+  explicit_abi = other.explicit_abi;
   inner_attrs = other.inner_attrs;
   locus = other.locus;
 
diff --git a/gcc/rust/hir/tree/rust-hir-item.h b/gcc/rust/hir/tree/rust-hir-item.h
index 588cf5f90096..a956e0b530f4 100644
--- a/gcc/rust/hir/tree/rust-hir-item.h
+++ b/gcc/rust/hir/tree/rust-hir-item.h
@@ -2638,6 +2638,7 @@ protected:
 class ExternBlock : public VisItem, public WithInnerAttrs
 {
   ABI abi;
+  bool explicit_abi;
   std::vector<std::unique_ptr<ExternalItem>> extern_items;
   location_t locus;
 
@@ -2647,9 +2648,12 @@ public:
   // Returns whether extern block has extern items.
   bool has_extern_items () const { return !extern_items.empty (); }
 
+  // Returns whether the extern block was given an explicit ABI string.
+  bool has_abi () const { return explicit_abi; }
+
   ABI get_abi () const { return abi; }
 
-  ExternBlock (Analysis::NodeMapping mappings, ABI abi,
+  ExternBlock (Analysis::NodeMapping mappings, ABI abi, bool explicit_abi,
 	       std::vector<std::unique_ptr<ExternalItem>> extern_items,
 	       Visibility vis, AST::AttrVec inner_attrs,
 	       AST::AttrVec outer_attrs, location_t locus);
diff --git a/gcc/testsuite/rust/compile/missing-abi_0.rs b/gcc/testsuite/rust/compile/missing-abi_0.rs
new file mode 100644
index 000000000000..e8c130bd94f1
--- /dev/null
+++ b/gcc/testsuite/rust/compile/missing-abi_0.rs
@@ -0,0 +1,12 @@
+// { dg-additional-options "-frust-unused-check-2.0" }
+#![feature(no_core)]
+#![no_core]
+
+extern {
+// { dg-warning "extern declarations without an explicit ABI are deprecated" "" { target *-*-* } .-1 }
+    fn foo();
+}
+
+extern "C" {
+    fn bar();
+}