Implement pre-defining structs/unions

spec/lib_body_transformer_spec.cr

@@ -261,4 +261,18 @@ describe LibBodyTransformer do
       end
       fun some_struct_with_other_struct_pointer(handle : StructBar*)
     )
+
+    # Verify that an additional struct does not get generated
+    assert_transform "simple",
+    %(
+      struct SomeStruct1
+        x : LibC::Int
+      end
+      fun just_some_struct_1
+    ), %(
+      struct SomeStruct1
+        x : LibC::Int
+      end
+      fun just_some_struct_1 : SomeStruct1
+    )
 end

src/crystal_lib/lib_body_transformer.cr

@@ -81,6 +81,15 @@ class CrystalLib::LibBodyTransformer < Crystal::Transformer
     check_pending_definitions(node)
   end
 
+  def transform(node : Crystal::CStructOrUnionDef)
+    @mapper.predefined_structs_or_unions << node
+
+    # There will be no pending definitions, since the node will
+    # be skipped and replaced with the current one when going through
+    # its usage in the header
+    node
+  end
+
   def map_type(type)
     @mapper.map(type)
   end

src/crystal_lib/type_mapper.cr

@@ -5,13 +5,15 @@ class CrystalLib::TypeMapper
     original_name : String
 
   getter pending_definitions
+  getter predefined_structs_or_unions
 
   @typedef_name : String?
 
   def initialize(@prefix_matcher : PrefixMatcher? = nil)
     @pending_definitions = [] of Crystal::ASTNode
     @pending_structs = [] of PendingStruct
     @generated = {} of UInt64 => Crystal::ASTNode
+    @predefined_structs_or_unions = [] of Crystal::CStructOrUnionDef
 
     # When completing a struct's fields we keep that struct and the field name in
     # case we find a nested struct, such as in:#
@@ -163,18 +165,25 @@ class CrystalLib::TypeMapper
     untouched_struct_name = check_anonymous_name(type.unscoped_name)
     struct_name = crystal_type_name(untouched_struct_name)
 
-    if type.fields.empty?
-      # For an empty struct we just return an alias to Void
-      struct_def = Crystal::Alias.new(path(struct_name), path(["Void"])).tap(&.doc = type.doc)
+    # Try finding a struct or union that has already been defined. If such
+    # a struct/union exist, use their definition instead and don't insert anything
+    found_predefined = @predefined_structs_or_unions.find { |predefined| predefined.name == struct_name }
+    if found_predefined
+      struct_def = found_predefined
     else
-      struct_def = Crystal::CStructOrUnionDef.new(struct_name, union: type.kind == :union).tap(&.doc = type.doc)
+      if type.fields.empty?
+        # For an empty struct we just return an alias to Void
+        struct_def = Crystal::Alias.new(path(struct_name), path(["Void"])).tap(&.doc = type.doc)
+      else
+        struct_def = Crystal::CStructOrUnionDef.new(struct_name, union: type.kind == :union).tap(&.doc = type.doc)
+
+        # Leave struct body for later, because of possible recursiveness
+        @pending_structs << PendingStruct.new(struct_def, type, untouched_struct_name)
+      end
 
-      # Leave struct body for later, because of possible recursiveness
-      @pending_structs << PendingStruct.new(struct_def, type, untouched_struct_name)
+      @pending_definitions << struct_def unless @generated.has_key?(type.object_id)
     end
 
-    @pending_definitions << struct_def unless @generated.has_key?(type.object_id)
-
     path(struct_name)
   end