Add test for issue-40231

src/test/ui/issues/issue-40231-1.rs

@@ -0,0 +1,54 @@
+// check-pass
+
+#![allow(dead_code)]
+
+trait Structure<E>: Sized where E: Encoding {
+    type RefTarget: ?Sized;
+    type FfiPtr;
+    unsafe fn borrow_from_ffi_ptr<'a>(ptr: Self::FfiPtr) -> Option<&'a Self::RefTarget>;
+}
+
+enum Slice {}
+
+impl<E> Structure<E> for Slice where E: Encoding {
+    type RefTarget = [E::Unit];
+    type FfiPtr = (*const E::FfiUnit, usize);
+    unsafe fn borrow_from_ffi_ptr<'a>(_ptr: Self::FfiPtr) -> Option<&'a Self::RefTarget> {
+        panic!()
+    }
+}
+
+trait Encoding {
+    type Unit: Unit;
+    type FfiUnit;
+}
+
+trait Unit {}
+
+enum Utf16 {}
+
+impl Encoding for Utf16 {
+    type Unit = Utf16Unit;
+    type FfiUnit = u16;
+}
+
+struct Utf16Unit(pub u16);
+
+impl Unit for Utf16Unit {}
+
+type SUtf16Str = SeStr<Slice, Utf16>;
+
+struct SeStr<S, E> where S: Structure<E>, E: Encoding {
+    _data: S::RefTarget,
+}
+
+impl<S, E> SeStr<S, E> where S: Structure<E>, E: Encoding {
+    pub unsafe fn from_ptr<'a>(_ptr: S::FfiPtr) -> Option<&'a Self> {
+        panic!()
+    }
+}
+
+fn main() {
+    const TEXT_U16: &'static [u16] = &[];
+    let _ = unsafe { SUtf16Str::from_ptr((TEXT_U16.as_ptr(), TEXT_U16.len())).unwrap() };
+}

src/test/ui/issues/issue-40231-2.rs

@@ -0,0 +1,54 @@
+// check-pass
+
+#![allow(dead_code)]
+
+trait Structure<E>: Sized where E: Encoding {
+    type RefTarget: ?Sized;
+    type FfiPtr;
+    unsafe fn borrow_from_ffi_ptr<'a>(ptr: Self::FfiPtr) -> Option<&'a Self::RefTarget>;
+}
+
+enum Slice {}
+
+impl<E> Structure<E> for Slice where E: Encoding {
+    type RefTarget = [E::Unit];
+    type FfiPtr = (*const E::FfiUnit, usize);
+    unsafe fn borrow_from_ffi_ptr<'a>(_ptr: Self::FfiPtr) -> Option<&'a Self::RefTarget> {
+        panic!()
+    }
+}
+
+trait Encoding {
+    type Unit: Unit;
+    type FfiUnit;
+}
+
+trait Unit {}
+
+enum Utf16 {}
+
+impl Encoding for Utf16 {
+    type Unit = Utf16Unit;
+    type FfiUnit = u16;
+}
+
+struct Utf16Unit(pub u16);
+
+impl Unit for Utf16Unit {}
+
+struct SUtf16Str {
+    _data: <Slice as Structure<Utf16>>::RefTarget,
+}
+
+impl SUtf16Str {
+    pub unsafe fn from_ptr<'a>(ptr: <Slice as Structure<Utf16>>::FfiPtr)
+    -> Option<&'a Self> {
+        std::mem::transmute::<Option<&[<Utf16 as Encoding>::Unit]>, _>(
+            <Slice as Structure<Utf16>>::borrow_from_ffi_ptr(ptr))
+    }
+}
+
+fn main() {
+    const TEXT_U16: &'static [u16] = &[];
+    let _ = unsafe { SUtf16Str::from_ptr((TEXT_U16.as_ptr(), TEXT_U16.len())).unwrap() };
+}