[gcc r16-6363] gccrs: respect cfg_attr expansion

[Arthur Cohen]

https://gcc.gnu.org/g:a088603cbbaa2a8ffad2551ca9490045eb415e25

commit r16-6363-ga088603cbbaa2a8ffad2551ca9490045eb415e25
Author: lenny.chiadmi-delage <lenny.chiadmi-delage@epita.fr>
Date:   Fri Dec 19 12:34:45 2025 +0000

    gccrs: respect cfg_attr expansion
    
    Fix cfg_attr expansion and feature gate attribute handling
    
    Fixes Rust-GCC#4245
    
    gcc/rust/ChangeLog:
    
            * checks/errors/feature/rust-feature-gate.cc (FeatureGate::visit): Added
            handling for META_ITEM type attributes to properly process feature gates.
            * expand/rust-cfg-strip.cc (expand_cfg_attrs): Fixed a bug where
            newly inserted cfg_attr attributes wheren't being reprocessed,
            and cleaned up the loop increment logic.
    
    Signed-off-by: lenny.chiadmi-delage <lenny.chiadmi-delage@epita.fr>

Diff:
---
 gcc/rust/checks/errors/feature/rust-feature-gate.cc | 16 ++++++++++++++++
 gcc/rust/expand/rust-cfg-strip.cc                   | 10 ++++++----
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/gcc/rust/checks/errors/feature/rust-feature-gate.cc b/gcc/rust/checks/errors/feature/rust-feature-gate.cc
index 9df822b7e1c7..e5170b77808b 100644
--- a/gcc/rust/checks/errors/feature/rust-feature-gate.cc
+++ b/gcc/rust/checks/errors/feature/rust-feature-gate.cc
@@ -69,6 +69,22 @@ FeatureGate::visit (AST::Crate &crate)
 		    valid_lib_features.emplace (name_str, item->get_locus ());
 		}
 	    }
+	  else if (type == AST::AttrInput::AttrInputType::META_ITEM)
+	    {
+	      const auto &meta_item
+		= static_cast<const AST::AttrInputMetaItemContainer &> (
+		  attr_input);
+	      for (const auto &item : meta_item.get_items ())
+		{
+		  const auto &name_str = item->as_string ();
+
+		  // TODO: detect duplicates
+		  if (auto tname = Feature::as_name (name_str))
+		    valid_lang_features.insert (tname.value ());
+		  else
+		    valid_lib_features.emplace (name_str, item->get_locus ());
+		}
+	    }
 	}
     }
 
diff --git a/gcc/rust/expand/rust-cfg-strip.cc b/gcc/rust/expand/rust-cfg-strip.cc
index 3c5e74e7aae6..e8c6c3789421 100644
--- a/gcc/rust/expand/rust-cfg-strip.cc
+++ b/gcc/rust/expand/rust-cfg-strip.cc
@@ -124,6 +124,12 @@ expand_cfg_attrs (AST::AttrVec &attrs)
 	      attrs.insert (attrs.begin () + i,
 			    std::make_move_iterator (new_attrs.begin ()),
 			    std::make_move_iterator (new_attrs.end ()));
+
+	      /* Decrement i so that the for loop's i++ will bring us back to
+	       * position i, allowing us to reprocess the newly inserted
+	       * attribute (in case it's also a cfg_attr that needs expansion)
+	       */
+	      i--;
 	    }
 
 	  /* do something - if feature (first token in tree) is in fact enabled,
@@ -133,10 +139,6 @@ expand_cfg_attrs (AST::AttrVec &attrs)
 	   * recursive, so check for expanded attributes being recursive and
 	   * possibly recursively call the expand_attrs? */
 	}
-      else
-	{
-	  i++;
-	}
     }
   attrs.shrink_to_fit ();
 }