Support slice of trivial extern type alias

syntax/check.rs

@@ -266,7 +266,9 @@ fn check_type_ptr(cx: &mut Check, ty: &Ptr) {
 fn check_type_slice_ref(cx: &mut Check, ty: &SliceRef) {
     let supported = !is_unsized(cx, &ty.inner)
         || match &ty.inner {
-            Type::Ident(ident) => cx.types.rust.contains(&ident.rust),
+            Type::Ident(ident) => {
+                cx.types.rust.contains(&ident.rust) || cx.types.aliases.contains_key(&ident.rust)
+            }
             _ => false,
         };
 

syntax/trivial.rs

@@ -11,6 +11,7 @@ pub enum TrivialReason<'a> {
     FunctionReturn(&'a ExternFn),
     BoxTarget,
     VecElement,
+    SliceElement { mutable: bool },
     UnpinnedMut(&'a ExternFn),
 }
 
@@ -105,6 +106,14 @@ pub fn required_trivial_reasons<'a>(
                     insist_extern_types_are_trivial(ident, reason);
                 }
             }
+            Type::SliceRef(ty) => {
+                if let Type::Ident(ident) = &ty.inner {
+                    let reason = TrivialReason::SliceElement {
+                        mutable: ty.mutable,
+                    };
+                    insist_extern_types_are_trivial(ident, reason);
+                }
+            }
             _ => {}
         }
     }
@@ -128,6 +137,8 @@ pub fn as_what<'a>(name: &'a Pair, reasons: &'a [TrivialReason]) -> impl Display
             let mut return_of = Set::new();
             let mut box_target = false;
             let mut vec_element = false;
+            let mut slice_shared_element = false;
+            let mut slice_mut_element = false;
             let mut unpinned_mut = Set::new();
 
             for reason in self.reasons {
@@ -143,6 +154,13 @@ pub fn as_what<'a>(name: &'a Pair, reasons: &'a [TrivialReason]) -> impl Display
                     }
                     TrivialReason::BoxTarget => box_target = true,
                     TrivialReason::VecElement => vec_element = true,
+                    TrivialReason::SliceElement { mutable } => {
+                        if *mutable {
+                            slice_mut_element = true;
+                        } else {
+                            slice_shared_element = true;
+                        }
+                    }
                     TrivialReason::UnpinnedMut(efn) => {
                         unpinned_mut.insert(&efn.name.rust);
                     }
@@ -185,6 +203,15 @@ pub fn as_what<'a>(name: &'a Pair, reasons: &'a [TrivialReason]) -> impl Display
                     param: self.name,
                 });
             }
+            if slice_shared_element || slice_mut_element {
+                clauses.push(Clause::Slice {
+                    article: "a",
+                    desc: "slice element in",
+                    shared: slice_shared_element,
+                    mutable: slice_mut_element,
+                    param: self.name,
+                });
+            }
             if !unpinned_mut.is_empty() {
                 clauses.push(Clause::Set {
                     article: "a",
@@ -219,12 +246,21 @@ pub fn as_what<'a>(name: &'a Pair, reasons: &'a [TrivialReason]) -> impl Display
             desc: &'a str,
             param: &'a Pair,
         },
+        Slice {
+            article: &'a str,
+            desc: &'a str,
+            shared: bool,
+            mutable: bool,
+            param: &'a Pair,
+        },
     }
 
     impl<'a> Clause<'a> {
         fn article(&self) -> &'a str {
             match self {
-                Clause::Set { article, .. } | Clause::Ty1 { article, .. } => article,
+                Clause::Set { article, .. }
+                | Clause::Ty1 { article, .. }
+                | Clause::Slice { article, .. } => article,
             }
         }
 
@@ -249,6 +285,25 @@ pub fn as_what<'a>(name: &'a Pair, reasons: &'a [TrivialReason]) -> impl Display
                     desc,
                     param,
                 } => write!(f, "{}<{}>", desc, param.rust),
+                Clause::Slice {
+                    article: _,
+                    desc,
+                    shared,
+                    mutable,
+                    param,
+                } => {
+                    write!(f, "{} ", desc)?;
+                    if *shared {
+                        write!(f, "&[{}]", param.rust)?;
+                    }
+                    if *shared && *mutable {
+                        write!(f, " and ")?;
+                    }
+                    if *mutable {
+                        write!(f, "&mut [{}]", param.rust)?;
+                    }
+                    Ok(())
+                }
             }
         }
     }

tests/ui/slice_of_type_alias.rs

@@ -0,0 +1,30 @@
+use cxx::{type_id, ExternType};
+
+#[repr(C)]
+struct ElementTrivial(usize);
+
+#[repr(C)]
+struct ElementOpaque(usize);
+
+#[cxx::bridge]
+mod ffi {
+    unsafe extern "C++" {
+        type ElementTrivial = crate::ElementTrivial;
+        type ElementOpaque = crate::ElementOpaque;
+
+        fn f(slice: &mut [ElementTrivial]);
+        fn g(slice: &[ElementOpaque]);
+    }
+}
+
+unsafe impl ExternType for ElementTrivial {
+    type Id = type_id!("ElementTrivial");
+    type Kind = cxx::kind::Trivial;
+}
+
+unsafe impl ExternType for ElementOpaque {
+    type Id = type_id!("ElementOpaque");
+    type Kind = cxx::kind::Opaque;
+}
+
+fn main() {}

tests/ui/slice_of_type_alias.stderr

@@ -0,0 +1,10 @@
+error[E0271]: type mismatch resolving `<ElementOpaque as ExternType>::Kind == Trivial`
+   --> $DIR/slice_of_type_alias.rs:13:9
+    |
+13  |         type ElementOpaque = crate::ElementOpaque;
+    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected enum `Trivial`, found enum `cxx::kind::Opaque`
+    |
+   ::: $WORKSPACE/src/extern_type.rs
+    |
+    | pub fn verify_extern_kind<T: ExternType<Kind = Kind>, Kind: self::Kind>() {}
+    |                                         ----------- required by this bound in `verify_extern_kind`

tests/ui/slice_unsupported.stderr

@@ -3,3 +3,9 @@ error: unsupported &mut [T] element type: opaque C++ type is not supported yet
   |
 6 |         fn f(_: &mut [Opaque]);
   |                 ^^^^^^^^^^^^^
+
+error: needs a cxx::ExternType impl in order to be used as a slice element in &mut [Opaque]
+ --> $DIR/slice_unsupported.rs:4:9
+  |
+4 |         type Opaque;
+  |         ^^^^^^^^^^^