Ensure structs and traits exposed via procmacros also staticallly ass…

…ert Send+Sync (mozilla#1727)

fixtures/uitests/tests/ui/interface_not_sync_and_send.rs

@@ -22,3 +22,23 @@ impl Counter {
         new_value
     }
 }
+
+#[derive(uniffi::Object)]
+pub struct ProcMacroCounter {
+    // This will fail to compile, because `Cell` is not `Sync`.
+    value: Cell<u32>,
+}
+
+#[uniffi::export]
+impl ProcMacroCounter {
+    #[uniffi::constructor]
+    pub fn new() -> std::sync::Arc<Self> {
+        std::sync::Arc::new(Self { value: Cell::new(0) })
+    }
+
+    pub fn increment(&self) -> u32 {
+        let new_value = self.value.get() + 1;
+        self.value.set(new_value);
+        new_value
+    }
+}

fixtures/uitests/tests/ui/interface_not_sync_and_send.stderr

@@ -1,8 +1,8 @@
 error[E0277]: `Cell<u32>` cannot be shared between threads safely
  --> $OUT_DIR[uniffi_uitests]/counter.uniffi.rs
   |
-  | uniffi::deps::static_assertions::assert_impl_all!(r#Counter: Sync, Send);
-  |                                                   ^^^^^^^^^ `Cell<u32>` cannot be shared between threads safely
+  | struct r#Counter { }
+  |        ^^^^^^^^^ `Cell<u32>` cannot be shared between threads safely
   |
   = help: within `Counter`, the trait `Sync` is not implemented for `Cell<u32>`
   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU32` instead
@@ -11,9 +11,29 @@ note: required because it appears within the type `Counter`
   |
 9 | pub struct Counter {
   |            ^^^^^^^
-note: required by a bound in `assert_impl_all`
+note: required by a bound in `_::{closure#0}::assert_impl_all`
  --> $OUT_DIR[uniffi_uitests]/counter.uniffi.rs
   |
-  | uniffi::deps::static_assertions::assert_impl_all!(r#Counter: Sync, Send);
-  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `assert_impl_all`
-  = note: this error originates in the macro `uniffi::deps::static_assertions::assert_impl_all` (in Nightly builds, run with -Z macro-backtrace for more info)
+  | #[::uniffi::expand_interface_support(tag = crate::UniFfiTag)]
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `assert_impl_all`
+  = note: this error originates in the macro `uniffi::deps::static_assertions::assert_impl_all` which comes from the expansion of the attribute macro `::uniffi::expand_interface_support` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: `Cell<u32>` cannot be shared between threads safely
+  --> tests/ui/interface_not_sync_and_send.rs:27:12
+   |
+27 | pub struct ProcMacroCounter {
+   |            ^^^^^^^^^^^^^^^^ `Cell<u32>` cannot be shared between threads safely
+   |
+   = help: within `ProcMacroCounter`, the trait `Sync` is not implemented for `Cell<u32>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` or `std::sync::atomic::AtomicU32` instead
+note: required because it appears within the type `ProcMacroCounter`
+  --> tests/ui/interface_not_sync_and_send.rs:27:12
+   |
+27 | pub struct ProcMacroCounter {
+   |            ^^^^^^^^^^^^^^^^
+note: required by a bound in `_::{closure#0}::assert_impl_all`
+  --> tests/ui/interface_not_sync_and_send.rs:26:10
+   |
+26 | #[derive(uniffi::Object)]
+   |          ^^^^^^^^^^^^^^ required by this bound in `assert_impl_all`
+   = note: this error originates in the macro `uniffi::deps::static_assertions::assert_impl_all` which comes from the expansion of the derive macro `uniffi::Object` (in Nightly builds, run with -Z macro-backtrace for more info)

fixtures/uitests/tests/ui/interface_trait_not_sync_and_send.rs

@@ -6,3 +6,8 @@ fn main() { /* empty main required by `trybuild` */}
 // This will fail to compile, because the trait is not explicit Send+Sync
 pub trait Trait {
 }
+
+// This will fail to compile, because the trait is not explicit Send+Sync
+#[uniffi::export]
+pub trait ProcMacroTrait {
+}

fixtures/uitests/tests/ui/interface_trait_not_sync_and_send.stderr

@@ -1,27 +1,55 @@
 error[E0277]: `dyn Trait` cannot be shared between threads safely
  --> $OUT_DIR[uniffi_uitests]/trait.uniffi.rs
   |
-  | uniffi::deps::static_assertions::assert_impl_all!(dyn r#Trait: Sync, Send);
-  |                                                   ^^^^^^^^^^^ `dyn Trait` cannot be shared between threads safely
+  | ::uniffi::expand_trait_interface_support!(r#Trait);
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn Trait` cannot be shared between threads safely
   |
   = help: the trait `Sync` is not implemented for `dyn Trait`
-note: required by a bound in `assert_impl_all`
+note: required by a bound in `_::{closure#0}::assert_impl_all`
  --> $OUT_DIR[uniffi_uitests]/trait.uniffi.rs
   |
-  | uniffi::deps::static_assertions::assert_impl_all!(dyn r#Trait: Sync, Send);
-  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `assert_impl_all`
-  = note: this error originates in the macro `uniffi::deps::static_assertions::assert_impl_all` (in Nightly builds, run with -Z macro-backtrace for more info)
+  | ::uniffi::expand_trait_interface_support!(r#Trait);
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `assert_impl_all`
+  = note: this error originates in the macro `::uniffi::expand_trait_interface_support` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: `dyn Trait` cannot be sent between threads safely
  --> $OUT_DIR[uniffi_uitests]/trait.uniffi.rs
   |
-  | uniffi::deps::static_assertions::assert_impl_all!(dyn r#Trait: Sync, Send);
-  |                                                   ^^^^^^^^^^^ `dyn Trait` cannot be sent between threads safely
+  | ::uniffi::expand_trait_interface_support!(r#Trait);
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `dyn Trait` cannot be sent between threads safely
   |
   = help: the trait `Send` is not implemented for `dyn Trait`
-note: required by a bound in `assert_impl_all`
+note: required by a bound in `_::{closure#0}::assert_impl_all`
  --> $OUT_DIR[uniffi_uitests]/trait.uniffi.rs
   |
-  | uniffi::deps::static_assertions::assert_impl_all!(dyn r#Trait: Sync, Send);
-  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `assert_impl_all`
-  = note: this error originates in the macro `uniffi::deps::static_assertions::assert_impl_all` (in Nightly builds, run with -Z macro-backtrace for more info)
+  | ::uniffi::expand_trait_interface_support!(r#Trait);
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `assert_impl_all`
+  = note: this error originates in the macro `::uniffi::expand_trait_interface_support` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: `dyn ProcMacroTrait` cannot be shared between threads safely
+  --> tests/ui/interface_trait_not_sync_and_send.rs:11:1
+   |
+11 | #[uniffi::export]
+   | ^^^^^^^^^^^^^^^^^ `dyn ProcMacroTrait` cannot be shared between threads safely
+   |
+   = help: the trait `Sync` is not implemented for `dyn ProcMacroTrait`
+note: required by a bound in `_::{closure#0}::assert_impl_all`
+  --> tests/ui/interface_trait_not_sync_and_send.rs:11:1
+   |
+11 | #[uniffi::export]
+   | ^^^^^^^^^^^^^^^^^ required by this bound in `assert_impl_all`
+   = note: this error originates in the attribute macro `uniffi::export` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: `dyn ProcMacroTrait` cannot be sent between threads safely
+  --> tests/ui/interface_trait_not_sync_and_send.rs:11:1
+   |
+11 | #[uniffi::export]
+   | ^^^^^^^^^^^^^^^^^ `dyn ProcMacroTrait` cannot be sent between threads safely
+   |
+   = help: the trait `Send` is not implemented for `dyn ProcMacroTrait`
+note: required by a bound in `_::{closure#0}::assert_impl_all`
+  --> tests/ui/interface_trait_not_sync_and_send.rs:11:1
+   |
+11 | #[uniffi::export]
+   | ^^^^^^^^^^^^^^^^^ required by this bound in `assert_impl_all`
+   = note: this error originates in the attribute macro `uniffi::export` (in Nightly builds, run with -Z macro-backtrace for more info)

uniffi_bindgen/src/scaffolding/templates/ObjectTemplate.rs

@@ -12,18 +12,12 @@
 
 {%- match obj.imp() -%}
 {%- when ObjectImpl::Trait %}
-::uniffi::scaffolding_ffi_converter_trait_interface!(r#{{ obj.name() }});
+::uniffi::expand_trait_interface_support!(r#{{ obj.name() }});
 {% else %}
-#[::uniffi::ffi_converter_interface(tag = crate::UniFfiTag)]
+#[::uniffi::expand_interface_support(tag = crate::UniFfiTag)]
 struct {{ obj.rust_name() }} { }
 {% endmatch %}
 
-// All Object structs must be `Sync + Send`. The generated scaffolding will fail to compile
-// if they are not, but unfortunately it fails with an unactionably obscure error message.
-// By asserting the requirement explicitly, we help Rust produce a more scrutable error message
-// and thus help the user debug why the requirement isn't being met.
-uniffi::deps::static_assertions::assert_impl_all!({{ obj.rust_name() }}: Sync, Send);
-
 {% let ffi_free = obj.ffi_object_free() -%}
 #[doc(hidden)]
 #[no_mangle]

uniffi_macros/src/export.rs

@@ -178,6 +178,12 @@ pub(crate) fn ffi_converter_trait_impl(trait_ident: &Ident, tag: Option<&Path>)
     };
 
     quote! {
+        // All traits must be `Sync + Send`. The generated scaffolding will fail to compile
+        // if they are not, but unfortunately it fails with an unactionably obscure error message.
+        // By asserting the requirement explicitly, we help Rust produce a more scrutable error message
+        // and thus help the user debug why the requirement isn't being met.
+        uniffi::deps::static_assertions::assert_impl_all!(dyn #trait_ident: Sync, Send);
+
         unsafe #impl_spec {
             type FfiType = *const ::std::os::raw::c_void;
             type ReturnType = Self::FfiType;

uniffi_macros/src/lib.rs

@@ -216,24 +216,26 @@ pub fn ffi_converter_error(attrs: TokenStream, input: TokenStream) -> TokenStrea
     .into()
 }
 
-/// Generate the FfiConverter implementation for an Interface
+/// Generate various support elements, including the FfiConverter implementation,
+/// for an Interface
 ///
 /// This is used by the Askama scaffolding code.  It this inputs an struct/enum definition, but
 /// only outputs the `FfiConverter` implementation, not the item.
 #[doc(hidden)]
 #[proc_macro_attribute]
-pub fn ffi_converter_interface(attrs: TokenStream, input: TokenStream) -> TokenStream {
-    object::expand_ffi_converter_interface(
+pub fn expand_interface_support(attrs: TokenStream, input: TokenStream) -> TokenStream {
+    object::expand_interface_support(
         syn::parse_macro_input!(attrs),
         syn::parse_macro_input!(input),
     )
     .into()
 }
 
-/// Generate the FfiConverter implementation for an trait interface for the scaffolding code
+/// Generate various support elements, including the FfiConverter implementation,
+/// for a trait interface for the scaffolding code
 #[doc(hidden)]
 #[proc_macro]
-pub fn scaffolding_ffi_converter_trait_interface(tokens: TokenStream) -> TokenStream {
+pub fn expand_trait_interface_support(tokens: TokenStream) -> TokenStream {
     export::ffi_converter_trait_impl(
         &syn::parse_macro_input!(tokens),
         Some(&syn::parse_quote!(crate::UniFfiTag)),

uniffi_macros/src/object.rs

@@ -44,7 +44,7 @@ pub fn expand_object(input: DeriveInput, module_path: String) -> TokenStream {
     }
 }
 
-pub(crate) fn expand_ffi_converter_interface(attr: CommonAttr, input: DeriveInput) -> TokenStream {
+pub(crate) fn expand_interface_support(attr: CommonAttr, input: DeriveInput) -> TokenStream {
     interface_impl(&input.ident, attr.tag.as_ref())
 }
 
@@ -57,6 +57,12 @@ pub(crate) fn interface_impl(ident: &Ident, tag: Option<&Path>) -> TokenStream {
     };
 
     quote! {
+        // All Object structs must be `Sync + Send`. The generated scaffolding will fail to compile
+        // if they are not, but unfortunately it fails with an unactionably obscure error message.
+        // By asserting the requirement explicitly, we help Rust produce a more scrutable error message
+        // and thus help the user debug why the requirement isn't being met.
+        uniffi::deps::static_assertions::assert_impl_all!(#ident: Sync, Send);
+
         #[doc(hidden)]
         #[automatically_derived]
         /// Support for passing reference-counted shared objects via the FFI.