Struct desugaring: "Undo decision to publish incomplete types..."

JuliaLang/julia#56497; Add self-referencing struct shim

I have doubts about how long this solution will stay in the base repository, and
     how complete it is (doesn't seem to work with M1.M2.S), but we are testing
     for it here.

Also change the expected value of a test changed in the same PR.

Author: mlechu

src/desugaring.jl

@@ -3825,6 +3825,24 @@ function _constructor_min_initalized(ex::SyntaxTree)
     end
 end
 
+# Let S be a struct we're defining in module M.  Below is a hack to allow its
+# field types to refer to S as M.S.  See #56497.
+function insert_struct_shim(ctx, fieldtypes, name)
+    function replace_type(ex)
+        if kind(ex) == K"." &&
+            numchildren(ex) == 2 &&
+            kind(ex[2]) == K"Symbol" &&
+            ex[2].name_val == name.name_val
+            @ast ctx ex [K"call" "struct_name_shim"::K"core" ex[1] ex[2] ctx.mod::K"Value" name]
+        elseif numchildren(ex) > 0
+            @ast ctx ex [ex.kind map(replace_type, children(ex))...]
+        else
+            ex
+        end
+    end
+    map(replace_type, fieldtypes)
+end
+
 function expand_struct_def(ctx, ex, docs)
     @chk numchildren(ex) == 2
     type_sig = ex[1]
@@ -3952,16 +3970,17 @@ function expand_struct_def(ctx, ex, docs)
                                 ]
                             end
                         ]
-                        # Otherwise do an assignment to trigger an error
-                        [K"const" global_struct_name struct_name]
                     ]
-                    [K"const" global_struct_name struct_name]
                 ]
                 [K"call"(type_body)
                     "_typebody!"::K"core"
                     struct_name
-                    [K"call" "svec"::K"core" field_types...]
+                    [K"call" "svec"::K"core" insert_struct_shim(ctx, field_types, struct_name)...]
                 ]
+                [K"constdecl"
+                    global_struct_name
+                    newtype_var
+                 ]
                 # Default constructors
                 if isempty(inner_defs)
                     default_inner_constructors(ctx, ex, global_struct_name,

test/typedefs.jl

@@ -251,7 +251,7 @@ end
 """)
 @test fieldtypes(test_mod.M36104.T36104) == (Vector{test_mod.M36104.T36104},)
 @test_throws ErrorException("expected") JuliaLowering.include_string(test_mod, """struct X36104; x::error("expected"); end""")
-@test isdefined(test_mod, :X36104)
+@test !isdefined(test_mod, :X36104)
 JuliaLowering.include_string(test_mod, "struct X36104; x::Int; end")
 @test fieldtypes(test_mod.X36104) == (Int,)
 JuliaLowering.include_string(test_mod, "primitive type P36104 8 end")