fix: Resolve projection types before checking casts

crates/hir-ty/src/infer/cast.rs

@@ -374,6 +374,7 @@ enum PointerKind {
 
 fn pointer_kind(ty: &Ty, table: &mut InferenceTable<'_>) -> Result<Option<PointerKind>, ()> {
     let ty = table.resolve_ty_shallow(ty);
+    let ty = table.normalize_associated_types_in(ty);
 
     if table.is_sized(&ty) {
         return Ok(Some(PointerKind::Thin));

crates/ide-diagnostics/src/handlers/invalid_cast.rs

@@ -1129,4 +1129,39 @@ fn main() {
         "#,
         );
     }
+
+    #[test]
+    fn regression_18682() {
+        check_diagnostics(
+            r#"
+//- minicore: coerce_unsized
+struct Flexible {
+    body: [u8],
+}
+
+trait Field {
+    type Type: ?Sized;
+}
+
+impl Field for Flexible {
+    type Type = [u8];
+}
+
+trait KnownLayout {
+    type MaybeUninit: ?Sized;
+}
+
+
+impl<T> KnownLayout for [T] {
+    type MaybeUninit = [T];
+}
+
+struct ZerocopyKnownLayoutMaybeUninit(<<Flexible as Field>::Type as KnownLayout>::MaybeUninit);
+
+fn test(ptr: *mut [u8]) -> *mut ZerocopyKnownLayoutMaybeUninit {
+    ptr as *mut _
+}
+"#,
+        );
+    }
 }