Added ffi-buffer scaffolding functions

These are alternate versions of the current scaffolding functions that
use u64 buffers to handle their input/output.

The main use case is the gecko-js bindings used in Firefox.  This allows
us to replace the generated C++ code with static code, since the
scaffolding functions now always have the exact same signature.

This commit generates FFI buffer versions for scaffolding functions
defined for user functions/methods.  It does not generate them for
functions with known/static signatures like the rust buffer FFI
functions, or the object clone/delete functions.  If the signature is
always the same, then there isn't a problem calling the normal
scaffolding functions.

examples/arithmetic/Cargo.toml

@@ -11,11 +11,12 @@ crate-type = ["lib", "cdylib"]
 name = "arithmetical"
 
 [dependencies]
-uniffi = { workspace = true }
+uniffi = { workspace = true, features=["scaffolding-ffi-buffer-fns"] }
 thiserror = "1.0"
 
 [build-dependencies]
-uniffi = { workspace = true, features = ["build"] }
+# Add the "scaffolding-ffi-buffer-fns" feature to make sure things can build correctly
+uniffi = { workspace = true, features = ["build", "scaffolding-ffi-buffer-fns"] }
 
 [dev-dependencies]
 uniffi = { workspace = true, features = ["bindgen-tests"] }

fixtures/callbacks/Cargo.toml

@@ -11,7 +11,8 @@ crate-type = ["lib", "cdylib"]
 name = "uniffi_fixture_callbacks"
 
 [dependencies]
-uniffi = { workspace = true }
+# Add the "scaffolding-ffi-buffer-fns" feature to make sure things can build correctly
+uniffi = { workspace = true, features=["scaffolding-ffi-buffer-fns"] }
 thiserror = "1.0"
 
 [build-dependencies]

fixtures/coverall/Cargo.toml

@@ -11,7 +11,8 @@ crate-type = ["lib", "cdylib"]
 name = "uniffi_coverall"
 
 [dependencies]
-uniffi = { workspace = true }
+# Add the "scaffolding-ffi-buffer-fns" feature to make sure things can build correctly
+uniffi = { workspace = true, features=["scaffolding-ffi-buffer-fns"]}
 once_cell = "1.12"
 thiserror = "1.0"
 
@@ -20,3 +21,4 @@ uniffi = { workspace = true, features = ["build"] }
 
 [dev-dependencies]
 uniffi = { workspace = true, features = ["bindgen-tests"] }
+uniffi_meta = { path = "../../uniffi_meta/" }

fixtures/coverall/src/coverall.udl

@@ -30,8 +30,12 @@ namespace coverall {
 
     void try_input_return_only_dict(ReturnOnlyDict d);
 
+    [Throws=ComplexError]
+    f32 divide_by_text(f32 value, string value_as_text);
+
     Getters test_round_trip_through_rust(Getters getters);
     void test_round_trip_through_foreign(Getters getters);
+
 };
 
 dictionary SimpleDict {

fixtures/coverall/src/ffi_buffer_scaffolding_test.rs

@@ -0,0 +1,61 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::ComplexError;
+use uniffi::{
+    ffi_buffer_size, FfiBufferElement, FfiSerialize, LiftReturn, Lower, RustBuffer, RustCallStatus,
+};
+
+/// Test the FFI-buffer version of our scaffolding functions by manually calling one.
+///
+/// We use the `get_complex_error` version, since it's one of more complex cases:
+///    - It inputs multiple arguments
+///    - The Rust function returns a Result<> type, which means the ffi-buffer scaffolding function
+///      needs to deserialize the `RustCallStatus` out pointer, pass it to the regular scaffolding
+///      function, and everything needs to be put back together in the end.
+#[test]
+fn test_ffi_buffer_scaffolding() {
+    // Call the ffi-buffer version of the scaffolding function for `divide_by_text`
+    //
+    // This simulates the work that happens on the foreign side.
+    fn call_ffi_buffer_divide_by_text(
+        value: f32,
+        value_as_text: String,
+    ) -> Result<f32, ComplexError> {
+        // Get buffers ready to store the arguments/return values
+        let mut args_ffi_buffer = [FfiBufferElement::default(); ffi_buffer_size!(f32, RustBuffer)];
+        let mut return_ffi_buffer =
+            [FfiBufferElement::default(); ffi_buffer_size!(f32, RustCallStatus)];
+        // Lower the arguments
+        let value_lowered = <f32 as Lower<crate::UniFfiTag>>::lower(value);
+        let value_as_text_lowered = <String as Lower<crate::UniFfiTag>>::lower(value_as_text);
+        // Serialize the lowered arguments plus the RustCallStatus into the argument buffer
+        let args_cursor = &mut args_ffi_buffer.as_mut_slice();
+        <f32 as FfiSerialize>::write(args_cursor, value_lowered);
+        <RustBuffer as FfiSerialize>::write(args_cursor, value_as_text_lowered);
+        // Call the ffi-buffer version of the scaffolding function
+        unsafe {
+            crate::uniffi_ffibuffer_uniffi_coverall_fn_func_divide_by_text(
+                args_ffi_buffer.as_mut_ptr(),
+                return_ffi_buffer.as_mut_ptr(),
+            );
+        }
+        // Deserialize the return and the RustCallStatus from the return buffer
+        let return_cursor = &mut return_ffi_buffer.as_slice();
+        let return_value = <f32 as FfiSerialize>::read(return_cursor);
+        let rust_call_status = <RustCallStatus as FfiSerialize>::read(return_cursor);
+        // Lift the return from the deserialized value.
+        <Result<f32, ComplexError> as LiftReturn<crate::UniFfiTag>>::lift_foreign_return(
+            return_value,
+            rust_call_status,
+        )
+    }
+
+    assert_eq!(call_ffi_buffer_divide_by_text(1.0, "2".into()), Ok(0.5));
+    assert_eq!(call_ffi_buffer_divide_by_text(5.0, "2.5".into()), Ok(2.0));
+    assert_eq!(
+        call_ffi_buffer_divide_by_text(1.0, "two".into()),
+        Err(ComplexError::UnknownError)
+    );
+}

fixtures/coverall/src/lib.rs

@@ -9,6 +9,9 @@ use std::time::SystemTime;
 
 use once_cell::sync::Lazy;
 
+#[cfg(test)]
+mod ffi_buffer_scaffolding_test;
+
 mod traits;
 pub use traits::{
     ancestor_names, get_string_util_traits, get_traits, make_rust_getters, test_getters,
@@ -232,6 +235,13 @@ fn try_input_return_only_dict(_d: ReturnOnlyDict) {
     // FIXME: should be a compile-time error rather than a runtime error (#1850)
 }
 
+pub fn divide_by_text(value: f32, value_as_text: String) -> Result<f32, ComplexError> {
+    match value_as_text.parse::<f32>() {
+        Ok(divisor) if divisor != 0.0 => Ok(value / divisor),
+        _ => Err(ComplexError::UnknownError),
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct DictWithDefaults {
     name: String,

fixtures/ext-types/lib/Cargo.toml

@@ -22,6 +22,7 @@ name = "uniffi_ext_types_lib"
 [dependencies]
 anyhow = "1"
 bytes = "1.3"
+# Add the "scaffolding-ffi-buffer-fns" feature to make sure things can build correctly
 uniffi = { workspace = true }
 
 uniffi-fixture-ext-types-external-crate = {path = "../external-crate"}

fixtures/futures/Cargo.toml

@@ -15,15 +15,16 @@ name = "uniffi-fixtures-futures"
 path = "src/bin.rs"
 
 [dependencies]
-uniffi = { workspace = true, features = ["tokio", "cli"] }
+# Add the "scaffolding-ffi-buffer-fns" feature to make sure things can build correctly
+uniffi = { workspace = true, features = ["tokio", "cli", "scaffolding-ffi-buffer-fns"] }
 async-trait = "0.1"
 futures = "0.3"
 thiserror = "1.0"
 tokio = { version = "1.24.1", features = ["time", "sync"] }
 once_cell = "1.18.0"
 
 [build-dependencies]
-uniffi = { workspace = true, features = ["build"] }
+uniffi = { workspace = true, features = ["build", "scaffolding-ffi-buffer-fns"] }
 
 [dev-dependencies]
 uniffi = { workspace = true, features = ["bindgen-tests"] }

fixtures/proc-macro/Cargo.toml

@@ -11,12 +11,13 @@ name = "uniffi_proc_macro"
 crate-type = ["lib", "cdylib"]
 
 [dependencies]
-uniffi = { workspace = true }
+# Add the "scaffolding-ffi-buffer-fns" feature to make sure things can build correctly
+uniffi = { workspace = true, features = ["scaffolding-ffi-buffer-fns"] }
 thiserror = "1.0"
 lazy_static = "1.4"
 
 [build-dependencies]
-uniffi = { workspace = true, features = ["build"] }
+uniffi = { workspace = true, features = ["build", "scaffolding-ffi-buffer-fns"] }
 
 [dev-dependencies]
 uniffi = { workspace = true, features = ["bindgen-tests"] }

uniffi/Cargo.toml

@@ -43,3 +43,6 @@ bindgen-tests = [ "dep:uniffi_bindgen" ]
 # Enable support for Tokio's futures.
 # This must still be opted into on a per-function basis using `#[uniffi::export(async_runtime = "tokio")]`.
 tokio = ["uniffi_core/tokio"]
+# Generate extra scaffolding functions that use FfiBuffer to pass arguments and return values
+# This is needed for the gecko-js bindings.
+scaffolding-ffi-buffer-fns = ["uniffi_core/scaffolding-ffi-buffer-fns", "uniffi_macros/scaffolding-ffi-buffer-fns"]

uniffi_bindgen/src/interface/ffi.rs

@@ -218,6 +218,12 @@ impl FfiFunction {
         &self.name
     }
 
+    /// Name of the FFI buffer version of this function that's generated when the
+    /// `scaffolding-ffi-buffer-fns` feature is enabled.
+    pub fn ffi_buffer_fn_name(&self) -> String {
+        uniffi_meta::ffi_buffer_symbol_name(&self.name)
+    }
+
     pub fn is_async(&self) -> bool {
         self.is_async
     }

uniffi_core/Cargo.toml

@@ -33,3 +33,6 @@ default = []
 # Enable support for Tokio's futures.
 # This must still be opted into on a per-function basis using `#[uniffi::export(async_runtime = "tokio")]`.
 tokio = ["dep:async-compat"]
+
+# Enable support for the ffi buffer scaffolding functions
+scaffolding-ffi-buffer-fns = []