error[E0308]: mismatched types --> $DIR/issue-90213-expected-boxfuture-self-ice.rs:9:19 | LL | Self::foo(None) | --------- ^^^^ expected `Box>`, found `Option<_>` | | | arguments to this function are incorrect | = note: expected struct `Box>` found enum `Option<_>` = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html note: associated function defined here --> $DIR/issue-90213-expected-boxfuture-self-ice.rs:7:8 | LL | fn foo(_: Box>) {} | ^^^ ----------------- help: store this in the heap by calling `Box::new` | LL | Self::foo(Box::new(None)) | +++++++++ + error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0308`.