New system for interface metadata

- Added the `uniffi_core::metadata` module, which provides a system for
  building metadata buffers using const code.
- Added the `FfiConverter::TYPE_ID_META` const, which holds metadata
  to identify the type.
- Made the proc-macros generate use the new system to generate metadata
  consts, then export them using static variables.
- Removed the current code to generate/store metadata based on the syn
  parsing.
- Removed the type assertions and the requirement for a `uniffi_types`
  module.  We don't need them now that we`re getting the type ids from
  the type itself.
- Made the `FnMetadata.throws` field a Type rather than a String.
- Calculate module paths with the `module_path!()` macro.  This means we
  get accurate module paths without nightly.  Changed mod_path to be a
  String, rather than a Vec since this is what `module_path!()` outputs.
- Added code to strip the `r#` part out of raw idents before serializing
  it into the metadata.
- Replaced the `collect_params()` function with the `ScaffoldingBits`
  struct.  There was too much complexity for a single function -- for
  example unzip() only works with pairs, not 3-tuples.

In general, the new metadata system is more reliable doing everything in
the proc-macros.  Proc-macros can only see the type identifiers, but
they don't anything about the underlying type, since users can rename
types with type aliases.  It's also simpler since you don't have to walk
the AST so much.

One TODO is getting checksum working again.  One limitation of the const
code is that we can't use it to generate function identifiers.

CHANGELOG.md

@@ -22,6 +22,8 @@
 ### What's changed
 
 - The `include_scaffolding!()` macro must now either be called from your crate root or you must have `use the_mod_that_calls_include_scaffolding::*` in your crate root.  This was always the expectation, but wasn't required before.  This will now start failing with errors that say `crate::UniFfiTag` does not exist.
+- proc-macros now work with many more types including type aliases, type paths, etc.
+- The `uniffi_types` module is no longer needed when using proc-macros.
 
 ## v0.23.0 (backend crates: v0.23.0) - (_2023-01-27_)
 

Cargo.toml

@@ -34,6 +34,7 @@ members = [
   "fixtures/keywords/kotlin",
   "fixtures/keywords/rust",
   "fixtures/keywords/swift",
+  "fixtures/metadata",
   "fixtures/proc-macro",
   "fixtures/reexport-scaffolding-macro",
   "fixtures/regressions/enum-without-i32-helpers",

docs/manual/src/proc_macro/index.md

@@ -63,16 +63,6 @@ User-defined types are also supported in a limited manner: records (structs with
 their definition. Opaque objects (`interface` in UDL) can always be used regardless of whether they
 are defined in UDL and / or via derive macro; they just need to be put inside an `Arc` as always.
 
-User-defined types also have to be (re-)exported from a module called `uniffi_types` at the crate
-root. This is required to ensure that a given type name always means the same thing across all uses
-of `#[uniffi::export]` across the whole module tree.
-
-```rust
-mod uniffi_types {
-    pub(crate) use path::to::MyObject;
-}
-```
-
 ## The `uniffi::Record` derive
 
 The `Record` derive macro exposes a `struct` with named fields over FFI. All types that are

fixtures/futures/src/lib.rs

@@ -234,9 +234,3 @@ pub async fn broken_sleep(ms: u16, fail_after: u16) {
 }
 
 include!(concat!(env!("OUT_DIR"), "/uniffi_futures.uniffi.rs"));
-
-mod uniffi_types {
-    pub(crate) use super::Megaphone;
-    pub(crate) use super::MyError;
-    pub(crate) use super::MyRecord;
-}

fixtures/metadata/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "uniffi_fixture_metadata"
+version = "0.1.0"
+authors = ["Firefox Sync Team <sync-team@mozilla.com>"]
+edition = "2021"
+license = "MPL-2.0"
+publish = false
+
+[lib]
+name = "uniffi_fixture_metadata"
+
+[dependencies]
+uniffi = { path = "../../uniffi" }
+uniffi_meta = { path = "../../uniffi_meta" }
+uniffi_core = { path = "../../uniffi_core" }

fixtures/metadata/src/lib.rs

@@ -0,0 +1,11 @@
+/* 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/. */
+
+/// This entire crate is just a set of tests for metadata handling.  We use a separate crate
+/// for testing because the metadata handling is split up between several crates, and no crate
+/// on all the functionality.
+#[cfg(test)]
+mod tests;
+
+pub struct UniFfiTag;