godot-ffi: lazy-function-tables broken with experimental-threads

[feefladder]

Not sure if already documented,
I was trying to build a project using gdext_coroutines and then the behaviour was really weird: this bug would only show up on branch="master" and not on rev=<last commit hash>

The compiler complains that the lazily initiated static BINDING is not Sync:

long compiler output

error[E0277]: `RefCell<table_servers_classes::InnerTable>` cannot be shared between threads safely
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/multi_threaded.rs:27:25
    |
27  |         static BINDING: BindingStorage = BindingStorage {
    |                         ^^^^^^^^^^^^^^ `RefCell<table_servers_classes::InnerTable>` cannot be shared between threads safely
    |
    = help: within `table_servers_classes::ClassServersMethodTable`, the trait `Sync` is not implemented for `RefCell<table_servers_classes::InnerTable>`, which is required by `BindingStorage: Sync`
    = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
note: required because it appears within the type `table_servers_classes::ClassServersMethodTable`
   --> /home/user/git/afgrust/rust/target/debug/build/godot-ffi-ef451ebf4bfe53c8/out/table_servers_classes.rs:7:12
    |
7   | pub struct ClassServersMethodTable {
    |            ^^^^^^^^^^^^^^^^^^^^^^^
note: required for `manual_init_cell::ManualInitCell<table_servers_classes::ClassServersMethodTable>` to implement `Sync`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/toolbox.rs:529:33
    |
529 |     unsafe impl<T: Send + Sync> Sync for ManualInitCell<T> {}
    |                           ----  ^^^^     ^^^^^^^^^^^^^^^^^
    |                           |
    |                           unsatisfied trait bound introduced here
note: required because it appears within the type `binding::GodotBinding`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/mod.rs:32:19
    |
32  | pub(crate) struct GodotBinding {
    |                   ^^^^^^^^^^^^
note: required for `manual_init_cell::ManualInitCell<binding::GodotBinding>` to implement `Sync`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/toolbox.rs:529:33
    |
529 |     unsafe impl<T: Send + Sync> Sync for ManualInitCell<T> {}
    |                           ----  ^^^^     ^^^^^^^^^^^^^^^^^
    |                           |
    |                           unsatisfied trait bound introduced here
note: required because it appears within the type `BindingStorage`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/multi_threaded.rs:19:19
    |
19  | pub(super) struct BindingStorage {
    |                   ^^^^^^^^^^^^^^
    = note: shared static variables must have a type that implements `Sync`

error[E0277]: `RefCell<table_scene_classes::InnerTable>` cannot be shared between threads safely
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/multi_threaded.rs:27:25
    |
27  |         static BINDING: BindingStorage = BindingStorage {
    |                         ^^^^^^^^^^^^^^ `RefCell<table_scene_classes::InnerTable>` cannot be shared between threads safely
    |
    = help: within `table_scene_classes::ClassSceneMethodTable`, the trait `Sync` is not implemented for `RefCell<table_scene_classes::InnerTable>`, which is required by `BindingStorage: Sync`
    = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
note: required because it appears within the type `table_scene_classes::ClassSceneMethodTable`
   --> /home/user/git/afgrust/rust/target/debug/build/godot-ffi-ef451ebf4bfe53c8/out/table_scene_classes.rs:7:12
    |
7   | pub struct ClassSceneMethodTable {
    |            ^^^^^^^^^^^^^^^^^^^^^
note: required for `manual_init_cell::ManualInitCell<table_scene_classes::ClassSceneMethodTable>` to implement `Sync`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/toolbox.rs:529:33
    |
529 |     unsafe impl<T: Send + Sync> Sync for ManualInitCell<T> {}
    |                           ----  ^^^^     ^^^^^^^^^^^^^^^^^
    |                           |
    |                           unsatisfied trait bound introduced here
note: required because it appears within the type `binding::GodotBinding`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/mod.rs:32:19
    |
32  | pub(crate) struct GodotBinding {
    |                   ^^^^^^^^^^^^
note: required for `manual_init_cell::ManualInitCell<binding::GodotBinding>` to implement `Sync`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/toolbox.rs:529:33
    |
529 |     unsafe impl<T: Send + Sync> Sync for ManualInitCell<T> {}
    |                           ----  ^^^^     ^^^^^^^^^^^^^^^^^
    |                           |
    |                           unsatisfied trait bound introduced here
note: required because it appears within the type `BindingStorage`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/multi_threaded.rs:19:19
    |
19  | pub(super) struct BindingStorage {
    |                   ^^^^^^^^^^^^^^
    = note: shared static variables must have a type that implements `Sync`

error[E0277]: `RefCell<table_editor_classes::InnerTable>` cannot be shared between threads safely
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/multi_threaded.rs:27:25
    |
27  |         static BINDING: BindingStorage = BindingStorage {
    |                         ^^^^^^^^^^^^^^ `RefCell<table_editor_classes::InnerTable>` cannot be shared between threads safely
    |
    = help: within `table_editor_classes::ClassEditorMethodTable`, the trait `Sync` is not implemented for `RefCell<table_editor_classes::InnerTable>`, which is required by `BindingStorage: Sync`
    = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
note: required because it appears within the type `table_editor_classes::ClassEditorMethodTable`
   --> /home/user/git/afgrust/rust/target/debug/build/godot-ffi-ef451ebf4bfe53c8/out/table_editor_classes.rs:7:12
    |
7   | pub struct ClassEditorMethodTable {
    |            ^^^^^^^^^^^^^^^^^^^^^^
note: required for `manual_init_cell::ManualInitCell<table_editor_classes::ClassEditorMethodTable>` to implement `Sync`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/toolbox.rs:529:33
    |
529 |     unsafe impl<T: Send + Sync> Sync for ManualInitCell<T> {}
    |                           ----  ^^^^     ^^^^^^^^^^^^^^^^^
    |                           |
    |                           unsatisfied trait bound introduced here
note: required because it appears within the type `binding::GodotBinding`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/mod.rs:32:19
    |
32  | pub(crate) struct GodotBinding {
    |                   ^^^^^^^^^^^^
note: required for `manual_init_cell::ManualInitCell<binding::GodotBinding>` to implement `Sync`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/toolbox.rs:529:33
    |
529 |     unsafe impl<T: Send + Sync> Sync for ManualInitCell<T> {}
    |                           ----  ^^^^     ^^^^^^^^^^^^^^^^^
    |                           |
    |                           unsatisfied trait bound introduced here
note: required because it appears within the type `BindingStorage`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/multi_threaded.rs:19:19
    |
19  | pub(super) struct BindingStorage {
    |                   ^^^^^^^^^^^^^^
    = note: shared static variables must have a type that implements `Sync`

error[E0277]: `RefCell<table_builtins::InnerTable>` cannot be shared between threads safely
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/multi_threaded.rs:27:25
    |
27  |         static BINDING: BindingStorage = BindingStorage {
    |                         ^^^^^^^^^^^^^^ `RefCell<table_builtins::InnerTable>` cannot be shared between threads safely
    |
    = help: within `table_builtins::BuiltinMethodTable`, the trait `Sync` is not implemented for `RefCell<table_builtins::InnerTable>`, which is required by `BindingStorage: Sync`
    = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
note: required because it appears within the type `table_builtins::BuiltinMethodTable`
   --> /home/user/git/afgrust/rust/target/debug/build/godot-ffi-ef451ebf4bfe53c8/out/table_builtins.rs:7:12
    |
7   | pub struct BuiltinMethodTable {
    |            ^^^^^^^^^^^^^^^^^^
note: required for `manual_init_cell::ManualInitCell<table_builtins::BuiltinMethodTable>` to implement `Sync`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/toolbox.rs:529:33
    |
529 |     unsafe impl<T: Send + Sync> Sync for ManualInitCell<T> {}
    |                           ----  ^^^^     ^^^^^^^^^^^^^^^^^
    |                           |
    |                           unsatisfied trait bound introduced here
note: required because it appears within the type `binding::GodotBinding`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/mod.rs:32:19
    |
32  | pub(crate) struct GodotBinding {
    |                   ^^^^^^^^^^^^
note: required for `manual_init_cell::ManualInitCell<binding::GodotBinding>` to implement `Sync`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/toolbox.rs:529:33
    |
529 |     unsafe impl<T: Send + Sync> Sync for ManualInitCell<T> {}
    |                           ----  ^^^^     ^^^^^^^^^^^^^^^^^
    |                           |
    |                           unsatisfied trait bound introduced here
note: required because it appears within the type `BindingStorage`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/multi_threaded.rs:19:19
    |
19  | pub(super) struct BindingStorage {
    |                   ^^^^^^^^^^^^^^
    = note: shared static variables must have a type that implements `Sync`

error[E0277]: `*const u8` cannot be sent between threads safely
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/multi_threaded.rs:27:25
    |
27  |         static BINDING: BindingStorage = BindingStorage {
    |                         ^^^^^^^^^^^^^^ `*const u8` cannot be sent between threads safely
    |
    = help: within `Opaque<8>`, the trait `Send` is not implemented for `*const u8`, which is required by `BindingStorage: Sync`
note: required because it appears within the type `PhantomData<*const u8>`
   --> /home/user/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/marker.rs:753:12
    |
753 | pub struct PhantomData<T: ?Sized>;
    |            ^^^^^^^^^^^
note: required because it appears within the type `Opaque<8>`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/opaque.rs:25:12
    |
25  | pub struct Opaque<const N: usize> {
    |            ^^^^^^
    = note: required for `Unique<Opaque<8>>` to implement `Send`
note: required because it appears within the type `Box<Opaque<8>>`
   --> /home/user/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:234:12
    |
234 | pub struct Box<
    |            ^^^
    = note: required because it appears within the type `(&'static str, Box<Opaque<8>>)`
    = note: required for `hashbrown::raw::RawTable<(&'static str, Box<Opaque<8>>)>` to implement `Send`
note: required because it appears within the type `hashbrown::map::HashMap<&'static str, Box<Opaque<8>>, RandomState>`
   --> /rust/deps/hashbrown-0.15.0/src/map.rs:185:12
note: required because it appears within the type `HashMap<&'static str, Box<Opaque<8>>>`
   --> /home/user/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/collections/hash/map.rs:211:12
    |
211 | pub struct HashMap<K, V, S = RandomState> {
    |            ^^^^^^^
note: required because it appears within the type `StringCache<'static>`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/string_cache.rs:15:12
    |
15  | pub struct StringCache<'a> {
    |            ^^^^^^^^^^^
note: required because it appears within the type `table_servers_classes::InnerTable`
   --> /home/user/git/afgrust/rust/target/debug/build/godot-ffi-ef451ebf4bfe53c8/out/table_servers_classes.rs:4:8
    |
4   | struct InnerTable {
    |        ^^^^^^^^^^
    = note: required for `RefCell<table_servers_classes::InnerTable>` to implement `Send`
note: required because it appears within the type `table_servers_classes::ClassServersMethodTable`
   --> /home/user/git/afgrust/rust/target/debug/build/godot-ffi-ef451ebf4bfe53c8/out/table_servers_classes.rs:7:12
    |
7   | pub struct ClassServersMethodTable {
    |            ^^^^^^^^^^^^^^^^^^^^^^^
note: required for `manual_init_cell::ManualInitCell<table_servers_classes::ClassServersMethodTable>` to implement `Send`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/toolbox.rs:531:26
    |
531 |     unsafe impl<T: Send> Send for ManualInitCell<T> {}
    |                    ----  ^^^^     ^^^^^^^^^^^^^^^^^
    |                    |
    |                    unsatisfied trait bound introduced here
note: required because it appears within the type `binding::GodotBinding`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/mod.rs:32:19
    |
32  | pub(crate) struct GodotBinding {
    |                   ^^^^^^^^^^^^
note: required for `manual_init_cell::ManualInitCell<binding::GodotBinding>` to implement `Sync`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/toolbox.rs:529:33
    |
529 |     unsafe impl<T: Send + Sync> Sync for ManualInitCell<T> {}
    |                    ----         ^^^^     ^^^^^^^^^^^^^^^^^
    |                    |
    |                    unsatisfied trait bound introduced here
note: required because it appears within the type `BindingStorage`
   --> /home/user/.cargo/git/checkouts/gdext-76630c89719e160c/7461251/godot-ffi/src/binding/multi_threaded.rs:19:19
    |
19  | pub(super) struct BindingStorage {
    |                   ^^^^^^^^^^^^^^
    = note: shared static variables must have a type that implements `Sync`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `godot-ffi` (lib) due to 5 previous errors
warning: build failed, waiting for other jobs to finish...

I've tracked it down to these two features being incompatible...

[Bromeon]

This combination is explicitly not supported:

gdext/godot/src/lib.rs

Lines 146 to 147 in 97b66ec

	#[cfg(all(feature = "lazy-function-tables", feature = "experimental-threads"))]
	compile_error!("Thread safety for lazy function pointers is not yet implemented.");

But it looks like this check is happening too late (godot crate), because your error happens already in godot-ffi. So we should fix that.