Fix proc-macro constructor support and document it

docs/manual/src/proc_macro/index.md

@@ -43,12 +43,28 @@ struct MyObject {
 
 #[uniffi::export]
 impl MyObject {
-    // All methods must have a `self` argument
+    // Constructors need to be annotated as such.
+    // As of right now, they must return `Arc<Self>`, this might change.
+    // If the constructor is named `new`, it is treated as the primary
+    // constructor, so in most languages this is invoked with `MyObject()`.
+    #[uniffi::constructor]
+    fn new(argument: String) -> Arc<Self> {
+        // ...
+    }
+
+    // Constructors with different names are also supported, usually invoked
+    // as `MyObject.named()` (depending on the target language)
+    #[uniffi::constructor]
+    fn named() -> Arc<Self> {
+        // ...
+    }
+
+    // All functions that are not constructors must have a `self` argument
     fn method_a(&self) {
         // ...
     }
 
-    // Arc<Self> is also supported
+    // `Arc<Self>` is also supported
     fn method_b(self: Arc<Self>) {
         // ...
     }

fixtures/proc-macro/src/lib.rs

@@ -32,8 +32,14 @@ pub struct Object;
 #[uniffi::export]
 impl Object {
     #[uniffi::constructor]
-    fn new() -> Arc<Object> {
-        Arc::new(Object)
+    fn new() -> Arc<Self> {
+        Arc::new(Self)
+    }
+
+    #[uniffi::constructor]
+    fn named_ctor(arg: u32) -> Arc<Self> {
+        _ = arg;
+        Self::new()
     }
 
     fn is_heavy(&self) -> MaybeBool {

fixtures/proc-macro/tests/bindings/test_proc_macro.kts

@@ -10,7 +10,8 @@ assert(one.inner == 123)
 val two = Two("a", null)
 assert(takeTwo(two) == "a")
 
-val obj = Object()
+var obj = Object()
+obj = Object.namedCtor(1u)
 assert(obj.isHeavy() == MaybeBool.UNCERTAIN)
 
 assert(enumIdentity(MaybeBool.TRUE) == MaybeBool.TRUE)

fixtures/proc-macro/tests/bindings/test_proc_macro.py

@@ -11,6 +11,7 @@
 assert take_two(two) == "a"
 
 obj = Object()
+obj = Object.named_ctor(1)
 assert obj.is_heavy() == MaybeBool.UNCERTAIN
 
 assert enum_identity(MaybeBool.TRUE) == MaybeBool.TRUE

fixtures/proc-macro/tests/bindings/test_proc_macro.swift

@@ -10,7 +10,8 @@ assert(one.inner == 123)
 let two = Two(a: "a", b: nil)
 assert(takeTwo(two: two) == "a")
 
-let obj = Object()
+var obj = Object()
+obj = Object.namedCtor(arg: 1)
 assert(obj.isHeavy() == .uncertain)
 
 assert(enumIdentity(value: .true) == .true)

uniffi_macros/src/export/scaffolding.rs

@@ -55,7 +55,7 @@ pub(super) fn gen_constructor_scaffolding(
     const RECEIVER_ERROR: &str = "constructors must not have a self parameter";
 
     let mut bits = ScaffoldingBits::new();
-    bits.collect_params(sig.inputs.iter().skip(1), RECEIVER_ERROR);
+    bits.collect_params(&sig.inputs, RECEIVER_ERROR);
     bits.set_rust_fn_call(quote! { #self_ident::#ident });
 
     let metadata_var = bits.gen_constructor_meta_static_var(self_ident, &sig, mod_path);