Add encoding UI test for function pointers

objc2-encode/src/encode.rs

@@ -430,8 +430,7 @@ encode_pointer_impls!(
 ///
 /// Ideally we'd implement it for all function pointers, but due to coherence
 /// issues, see <https://github.com/rust-lang/rust/issues/56105>, function
-/// pointers that take arguments with "special lifetimes" (don't know the
-/// termonology) don't get implemented properly.
+/// pointers that are higher-ranked over lifetimes don't get implemented.
 ///
 /// We could fix it by adding those impls and allowing `coherence_leak_check`,
 /// but it would have to be done for _all_ references, `Option<&T>` and such as

objc2-foundation/src/array.rs

@@ -303,7 +303,10 @@ impl<T: Message, O: Ownership> NSMutableArray<T, O> {
             NSComparisonResult::from((*closure)(obj1, obj2))
         }
 
-        let f: extern "C" fn(_, _, _) -> _ = compare_with_closure::<T, F>;
+        // We can't name the actual lifetimes in use here, so use `_`.
+        // See also https://github.com/rust-lang/rust/issues/56105
+        let f: extern "C" fn(_, _, *mut c_void) -> NSComparisonResult =
+            compare_with_closure::<T, F>;
 
         // Grab a type-erased pointer to the closure (a pointer to stack).
         let mut closure = compare;

tests/ui/fn_ptr_reference_encode.rs

@@ -0,0 +1,23 @@
+//! Test that `Encode` is not implemented for function pointers that are
+//! higher-ranked over lifetimes.
+//!
+//! Ideally, they should be, but they can't be right now.
+//!
+//! (Also test that we can use `_` to work around this).
+use objc2::Encode;
+
+extern "C" fn my_fn(_x: &i32) {}
+
+fn e<T: Encode>(_x: T) {}
+
+fn main() {
+    // Works
+    e(my_fn as extern "C" fn(_));
+    // Can't be written:
+    // let encoding = <extern "C" fn(_) as Encode>::ENCODING;
+
+    // Fails
+    e(my_fn as extern "C" fn(&i32));
+    // Also fails, properly tested in `fn_ptr_reference_encode2`
+    let encoding = <extern "C" fn(&i32) as Encode>::ENCODING;
+}

tests/ui/fn_ptr_reference_encode.stderr

@@ -0,0 +1,8 @@
+error: implementation of `Encode` is not general enough
+  --> ui/fn_ptr_reference_encode.rs:20:5
+   |
+20 |     e(my_fn as extern "C" fn(&i32));
+   |     ^ implementation of `Encode` is not general enough
+   |
+   = note: `Encode` would have to be implemented for the type `for<'r> extern "C" fn(&'r i32)`
+   = note: ...but `Encode` is actually implemented for the type `extern "C" fn(&'0 i32)`, for some specific lifetime `'0`

tests/ui/fn_ptr_reference_encode2.rs

@@ -0,0 +1,7 @@
+//! Extra test for `fn_ptr_reference_encode`
+//! (They fail at different compilation passes).
+use objc2::Encode;
+
+fn main() {
+    let encoding = <extern "C" fn(&i32) as Encode>::ENCODING;
+}

tests/ui/fn_ptr_reference_encode2.stderr

@@ -0,0 +1,8 @@
+error: implementation of `Encode` is not general enough
+ --> ui/fn_ptr_reference_encode2.rs:6:20
+  |
+6 |     let encoding = <extern "C" fn(&i32) as Encode>::ENCODING;
+  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Encode` is not general enough
+  |
+  = note: `Encode` would have to be implemented for the type `for<'r> extern "C" fn(&'r i32)`
+  = note: ...but `Encode` is actually implemented for the type `extern "C" fn(&'0 i32)`, for some specific lifetime `'0`

tests/ui/fn_ptr_reference_method.rs

@@ -0,0 +1,27 @@
+//! Test how `MethodImplementation` is implemented regarding lifetimes in
+//! function pointers and whether they're higher-ranked over them.
+//!
+//! Ideally it should work for all of these, but it can't be right now.
+//!
+//! (`_` can be used to work around this, by letting the compiler choose an
+//! appropriate lifetime '0 that the trait is implemented for).
+use objc2::{class, sel};
+use objc2::declare::ClassBuilder;
+use objc2::runtime::{Object, Sel};
+
+extern "C" fn my_fn(_this: &Object, _cmd: Sel, _x: &Object) {}
+
+fn main() {
+    let builder = ClassBuilder::new("SomeTestClass", class!(NSObject)).unwrap();
+    unsafe {
+        // Works
+        builder.add_method(sel!(first:), my_fn as extern "C" fn(&Object, _, _));
+
+        // Fails
+        builder.add_method(sel!(none:), my_fn as extern "C" fn(_, _, _));
+        builder.add_method(sel!(third:), my_fn as extern "C" fn(_, _, &Object));
+
+        // Also fails, properly tested in `fn_ptr_reference_method2`
+        builder.add_method(sel!(both:), my_fn as extern "C" fn(&Object, _, &Object));
+    }
+}

tests/ui/fn_ptr_reference_method.stderr

@@ -0,0 +1,47 @@
+error[E0277]: the trait bound `extern "C" fn(_, _, _): MethodImplementation` is not satisfied
+   --> ui/fn_ptr_reference_method.rs:21:41
+    |
+21  |         builder.add_method(sel!(none:), my_fn as extern "C" fn(_, _, _));
+    |                 ----------              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `MethodImplementation` is not implemented for `extern "C" fn(_, _, _)`
+    |                 |
+    |                 required by a bound introduced by this call
+    |
+    = help: the following other types implement trait `MethodImplementation`:
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B, C) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B, C, D) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B, C, D, E) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B, C, D, E, F) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B, C, D, E, F, G) -> R
+            and 70 others
+note: required by a bound in `ClassBuilder::add_method`
+   --> $WORKSPACE/objc2/src/declare.rs
+    |
+    |         F: MethodImplementation<Callee = T>,
+    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `ClassBuilder::add_method`
+
+error[E0277]: the trait bound `for<'r> extern "C" fn(_, _, &'r objc2::runtime::Object): MethodImplementation` is not satisfied
+   --> ui/fn_ptr_reference_method.rs:22:42
+    |
+22  |         builder.add_method(sel!(third:), my_fn as extern "C" fn(_, _, &Object));
+    |                 ----------               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `MethodImplementation` is not implemented for `for<'r> extern "C" fn(_, _, &'r objc2::runtime::Object)`
+    |                 |
+    |                 required by a bound introduced by this call
+    |
+    = help: the following other types implement trait `MethodImplementation`:
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B, C) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B, C, D) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B, C, D, E) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B, C, D, E, F) -> R
+              for<'r> extern "C" fn(&'r T, objc2::runtime::Sel, A, B, C, D, E, F, G) -> R
+            and 70 others
+note: required by a bound in `ClassBuilder::add_method`
+   --> $WORKSPACE/objc2/src/declare.rs
+    |
+    |         F: MethodImplementation<Callee = T>,
+    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `ClassBuilder::add_method`

tests/ui/fn_ptr_reference_method2.rs

@@ -0,0 +1,14 @@
+//! Extra test for `fn_ptr_reference_method`
+//! (They fail at different compilation passes).
+use objc2::{class, sel};
+use objc2::declare::ClassBuilder;
+use objc2::runtime::{Object, Sel};
+
+extern "C" fn my_fn(_this: &Object, _cmd: Sel, _x: &Object) {}
+
+fn main() {
+    let builder = ClassBuilder::new("SomeTestClass", class!(NSObject)).unwrap();
+    unsafe {
+        builder.add_method(sel!(both:), my_fn as extern "C" fn(&Object, Sel, &Object));
+    }
+}

tests/ui/fn_ptr_reference_method2.stderr

@@ -0,0 +1,8 @@
+error: implementation of `MethodImplementation` is not general enough
+  --> ui/fn_ptr_reference_method2.rs:12:17
+   |
+12 |         builder.add_method(sel!(both:), my_fn as extern "C" fn(&Object, Sel, &Object));
+   |                 ^^^^^^^^^^ implementation of `MethodImplementation` is not general enough
+   |
+   = note: `MethodImplementation` would have to be implemented for the type `for<'r, 's> extern "C" fn(&'r objc2::runtime::Object, objc2::runtime::Sel, &'s objc2::runtime::Object)`
+   = note: ...but `MethodImplementation` is actually implemented for the type `for<'r> extern "C" fn(&'r objc2::runtime::Object, objc2::runtime::Sel, &'0 objc2::runtime::Object)`, for some specific lifetime `'0`