Change:
<ins> error[E0412]: cannot find type `Idx` in this scope --> $DIR/ice-unsized-struct-arg-issue-121612.rs:5:30 | LL | struct MySlice<T: FnOnce(&T, Idx) -> Idx>(bool, T); |                              ^^^ not found in this scope error[E0412]: cannot find type `Idx` in this scope --> $DIR/ice-unsized-struct-arg-issue-121612.rs:5:38 | LL | struct MySlice<T: FnOnce(&T, Idx) -> Idx>(bool, T); |                                      ^^^ not found in this scope error[E0277]: the size for values of type `[bool]` cannot be known at compilation time --> $DIR/ice-unsized-struct-arg-issue-121612.rs:9:22 | LL | const MYSLICE_GOOD: &MySliceBool = &MySlice(true, [false]); |                      ^^^^^^^^^^^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `[bool]` note: required by an implicit `Sized` bound in `MySlice` --> $DIR/ice-unsized-struct-arg-issue-121612.rs:5:16 | LL | struct MySlice<T: FnOnce(&T, Idx) -> Idx>(bool, T); |                ^ required by the implicit `Sized` requirement on this type parameter in `MySlice` help: you could relax the implicit `Sized` bound on `T` if it were used through indirection like `&T` or `Box<T>` --> $DIR/ice-unsized-struct-arg-issue-121612.rs:5:16 | LL | struct MySlice<T: FnOnce(&T, Idx) -> Idx>(bool, T); |                ^                                - ...if indirection were used here: `Box<T>` |                | |                this could be changed to `T: ?Sized`... error[E0277]: the size for values of type `[bool]` cannot be known at compilation time --> $DIR/ice-unsized-struct-arg-issue-121612.rs:9:22 | LL | const MYSLICE_GOOD: &MySliceBool = &MySlice(true, [false]); |                      ^^^^^^^^^^^ doesn't have a size known at compile-time | = help: the trait `Sized` is not implemented for `[bool]` note: required by an implicit `Sized` bound in `MySlice` --> $DIR/ice-unsized-struct-arg-issue-121612.rs:5:16 | LL | struct MySlice<T: FnOnce(&T, Idx) -> Idx>(bool, T); |                ^ required by the implicit `Sized` requirement on this type parameter in `MySlice` help: you could relax the implicit `Sized` bound on `T` if it were used through indirection like `&T` or `Box<T>` --> $DIR/ice-unsized-struct-arg-issue-121612.rs:5:16 | LL | struct MySlice<T: FnOnce(&T, Idx) -> Idx>(bool, T); |                ^                                - ...if indirection were used here: `Box<T>` |                | |                this could be changed to `T: ?Sized`... = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no` error: aborting due to 4 previous errors Some errors have detailed explanations: E0277, E0412. For more information about an error, try `rustc --explain E0277`. </ins>

Discussion:
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