ast: Implement reconstruct() logic for Path hierarchy

This patch implements the `reconstruct` pattern for the `Path` hierarchy and its
constituents (segments, generic args, qualified paths).

Previously, derive expansion macros (like `derive(PartialEq)`) had to manually
rebuild paths field-by-field to ensure fresh NodeIDs. This change moves that
logic into the AST classes themselves, allowing paths to be deeply reconstructed
uniformly. This ensures that expanded code has unique NodeIDs, which is critical
for correct name resolution.

gcc/rust/ChangeLog:

	* ast/rust-path.h (GenericArgsBinding::reconstruct): New method.
	(GenericArg::reconstruct): New method.
	(GenericArgs::reconstruct): New method.
	(PathExprSegment::reconstruct): New method.
	(PathInExpression::reconstruct): New method.
	(TypePathSegmentGeneric::reconstruct_impl): New method.
	(TypePathFunction::reconstruct): New method.
	(TypePathSegmentFunction::reconstruct_impl): New method.
	(QualifiedPathType::reconstruct): New method.
	(QualifiedPathInExpression::reconstruct): New method.
	(QualifiedPathInType::reconstruct_impl): New method.
	* expand/rust-derive-cmp-common.cc (EnumMatchBuilder::tuple): Use path.reconstruct().
	(EnumMatchBuilder::strukt): Use path.reconstruct().

Signed-off-by: Jayant Chauhan <0001jayant@gmail.com>

Author: Pasta-coder

gcc/rust/ast/rust-path.h

@@ -84,6 +84,15 @@ struct GenericArgsBinding
     // and also identifier is empty, but cheaper computation
   }
 
+  GenericArgsBinding reconstruct () const
+  {
+    std::unique_ptr<Type> new_type = nullptr;
+    if (type)
+      new_type = type->reconstruct ();
+
+    return GenericArgsBinding (identifier, std::move (new_type), locus);
+  }
+
   // Creates an error state generic args binding.
   static GenericArgsBinding create_error ()
   {
@@ -190,6 +199,22 @@ class GenericArg
     return GenericArg (nullptr, nullptr, std::move (path), Kind::Either, locus);
   }
 
+  GenericArg reconstruct () const
+  {
+    switch (kind)
+      {
+      case Kind::Type:
+	return create_type (type->reconstruct ());
+      case Kind::Const:
+	// FIXME: Use reconstruct_expr when available
+	return create_const (expression->clone_expr ());
+      case Kind::Either:
+      default:
+	// For ambiguous or error states, copy constructs are sufficient
+	return GenericArg (*this);
+      }
+  }
+
   GenericArg (const GenericArg &other)
     : path (other.path), kind (other.kind), locus (other.locus)
   {
@@ -460,6 +485,23 @@ struct GenericArgs
 
   ~GenericArgs () = default;
 
+  GenericArgs reconstruct () const
+  {
+    std::vector<GenericArg> new_args;
+    new_args.reserve (generic_args.size ());
+    for (const auto &arg : generic_args)
+      new_args.push_back (arg.reconstruct ());
+
+    std::vector<GenericArgsBinding> new_bindings;
+    new_bindings.reserve (binding_args.size ());
+    for (const auto &binding : binding_args)
+      new_bindings.push_back (binding.reconstruct ());
+
+    // Lifetimes are values, so they can be copied directly
+    return GenericArgs (lifetime_args, std::move (new_args),
+			std::move (new_bindings), locus);
+  }
+
   // overloaded assignment operator to vector clone
   GenericArgs &operator= (GenericArgs const &other)
   {
@@ -563,6 +605,11 @@ class PathExprSegment
 
   NodeId get_node_id () const { return node_id; }
 
+  PathExprSegment reconstruct () const
+  {
+    return PathExprSegment (segment_name, locus, generic_args.reconstruct ());
+  }
+
   bool is_super_path_seg () const
   {
     return !has_generic_args () && get_ident_segment ().is_super_path_seg ();
@@ -705,6 +752,20 @@ class PathInExpression : public Path, public ExprWithoutBlock
     return convert_to_simple_path (has_opening_scope_resolution);
   }
 
+  std::unique_ptr<PathInExpression> reconstruct () const
+  {
+    std::vector<PathExprSegment> new_segments;
+    new_segments.reserve (segments.size ());
+    for (const auto &seg : segments)
+      new_segments.push_back (seg.reconstruct ());
+
+    auto *new_path
+      = new PathInExpression (std::move (new_segments), outer_attrs, locus,
+			      has_opening_scope_resolution);
+
+    return std::unique_ptr<PathInExpression> (new_path);
+  }
+
   location_t get_locus () const override final { return locus; }
 
   void accept_vis (ASTVisitor &vis) override;
@@ -1004,6 +1065,14 @@ class TypePathSegmentGeneric : public TypePathSegment
   {
     return new TypePathSegmentGeneric (*this);
   }
+
+  TypePathSegmentGeneric *reconstruct_impl () const override
+  {
+    return new TypePathSegmentGeneric (get_ident_segment (),
+				       has_separating_scope_resolution,
+				       generic_args.reconstruct (),
+				       get_locus ());
+  }
 };
 
 // A function as represented in a type path
@@ -1112,6 +1181,21 @@ struct TypePathFunction
     return return_type;
   }
 
+  TypePathFunction reconstruct () const
+  {
+    std::vector<std::unique_ptr<Type>> new_inputs;
+    new_inputs.reserve (inputs.size ());
+    for (const auto &e : inputs)
+      new_inputs.push_back (e->reconstruct ());
+
+    std::unique_ptr<Type> new_ret = nullptr;
+    if (return_type)
+      new_ret = return_type->reconstruct ();
+
+    return TypePathFunction (std::move (new_inputs), locus,
+			     std::move (new_ret));
+  }
+
   location_t get_locus () const { return locus; }
 };
 
@@ -1159,6 +1243,14 @@ class TypePathSegmentFunction : public TypePathSegment
   {
     return new TypePathSegmentFunction (*this);
   }
+
+  TypePathSegmentFunction *reconstruct_impl () const override
+  {
+    return new TypePathSegmentFunction (get_ident_segment (),
+					has_separating_scope_resolution,
+					function_path.reconstruct (),
+					get_locus ());
+  }
 };
 
 class TypePath : public TypeNoBounds
@@ -1327,6 +1419,20 @@ struct QualifiedPathType
   QualifiedPathType (QualifiedPathType &&other) = default;
   QualifiedPathType &operator= (QualifiedPathType &&other) = default;
 
+  QualifiedPathType reconstruct () const
+  {
+    auto new_type = type_to_invoke_on->reconstruct ();
+
+    // trait_path is stored by value, but reconstruct returns a unique_ptr.
+    // We must dereference it to pass to the constructor.
+    // This is safe because the constructor makes its own copy/move.
+    auto new_trait_path_ptr = trait_path.reconstruct ();
+    TypePath *concrete_ptr
+      = static_cast<TypePath *> (new_trait_path_ptr.get ());
+
+    return QualifiedPathType (std::move (new_type), locus, *concrete_ptr);
+  }
+
   // Returns whether the qualified path type has a rebind as clause.
   bool has_as_clause () const { return !trait_path.is_error (); }
 
@@ -1433,6 +1539,20 @@ class QualifiedPathInExpression : public Path, public ExprWithoutBlock
     return Expr::Kind::QualifiedPathInExpression;
   }
 
+  std::unique_ptr<QualifiedPathInExpression> reconstruct () const
+  {
+    std::vector<PathExprSegment> new_segments;
+    new_segments.reserve (segments.size ());
+    for (const auto &seg : segments)
+      new_segments.push_back (seg.reconstruct ());
+
+    auto *new_path = new QualifiedPathInExpression (path_type.reconstruct (),
+						    std::move (new_segments),
+						    outer_attrs, locus);
+
+    return std::unique_ptr<QualifiedPathInExpression> (new_path);
+  }
+
 protected:
   /* Use covariance to implement clone function as returning this object
    * rather than base */
@@ -1474,7 +1594,7 @@ class QualifiedPathInType : public TypeNoBounds
   }
   QualifiedPathInType *reconstruct_impl () const override
   {
-    return new QualifiedPathInType (path_type,
+    return new QualifiedPathInType (path_type.reconstruct (),
 				    associated_segment->reconstruct (),
 				    reconstruct_vec (segments), locus);
   }

gcc/rust/expand/rust-derive-cmp-common.cc

@@ -96,9 +96,8 @@ EnumMatchBuilder::tuple (EnumItem &variant_raw)
       });
     }
 
-  // TODO: Replace with `reconstruct()` instead of building these twice
   auto self_variant_path = builder.variant_path (enum_path, variant_path);
-  auto other_variant_path = builder.variant_path (enum_path, variant_path);
+  auto other_variant_path = self_variant_path.reconstruct ();
 
   auto self_pattern_items = std::unique_ptr<TupleStructItems> (
     new TupleStructItemsNoRest (std::move (self_patterns)));
@@ -111,7 +110,7 @@ EnumMatchBuilder::tuple (EnumItem &variant_raw)
 			  false, false, builder.loc));
   auto other_pattern = std::unique_ptr<Pattern> (new ReferencePattern (
     std::unique_ptr<Pattern> (new TupleStructPattern (
-      other_variant_path, std::move (other_pattern_items))),
+      *other_variant_path, std::move (other_pattern_items))),
     false, false, builder.loc));
 
   auto tuple_items = std::make_unique<TuplePatternItemsNoRest> (
@@ -159,9 +158,8 @@ EnumMatchBuilder::strukt (EnumItem &variant_raw)
       });
     }
 
-  // TODO: Replace with `reconstruct()` instead of building these twice
   auto self_variant_path = builder.variant_path (enum_path, variant_path);
-  auto other_variant_path = builder.variant_path (enum_path, variant_path);
+  auto other_variant_path = self_variant_path.reconstruct ();
 
   auto self_elts = StructPatternElements (std::move (self_fields));
   auto other_elts = StructPatternElements (std::move (other_fields));
@@ -172,7 +170,7 @@ EnumMatchBuilder::strukt (EnumItem &variant_raw)
     false, false, builder.loc));
   auto other_pattern = std::unique_ptr<Pattern> (
     new ReferencePattern (std::unique_ptr<Pattern> (
-			    new StructPattern (other_variant_path, builder.loc,
+			    new StructPattern (*other_variant_path, builder.loc,
 					       std::move (other_elts))),
 			  false, false, builder.loc));