error[E0277]: the size for values of type `T` cannot be known at compilation time
  --> $DIR/unsized-fn-arg.rs:5:17
   |
LL | fn f<T: ?Sized>(t: T) {}
   |      -          ^ doesn't have a size known at compile-time
   |      |
   |      this type parameter needs to be `Sized`
   |
   = help: unsized fn params are gated as an unstable feature
help: consider removing the `?Sized` bound to make the type parameter `Sized`
   |
LL - fn f<T: ?Sized>(t: T) {}
LL + fn f<T>(t: T) {}
   |
help: function arguments must have a statically known size, borrowed types always have a known size
   |
LL | fn f<T: ?Sized>(t: &T) {}
   |                    +

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.