error: at least one trait must be specified --> $DIR/issue-78720.rs:1:16 | LL | fn server() -> impl { | ^^^^ error[E0412]: cannot find type `F` in this scope --> $DIR/issue-78720.rs:13:12 | LL | _func: F, | ^ | ::: $SRC_DIR/core/src/ops/function.rs:LL:COL | LL | pub trait Fn: FnMut { | ------------------------------- similarly named trait `Fn` defined here | help: a trait with a similar name exists | LL | _func: Fn, | ~~ help: you might be missing a type parameter | LL | struct Map2 { | +++ error[E0308]: mismatched types --> $DIR/issue-78720.rs:7:39 | LL | fn map2(self, f: F) -> Map2 {} | ^^ expected struct `Map2`, found `()` | = note: expected struct `Map2` found unit type `()` error[E0277]: the size for values of type `Self` cannot be known at compilation time --> $DIR/issue-78720.rs:7:16 | LL | fn map2(self, f: F) -> Map2 {} | ^^^^ doesn't have a size known at compile-time | = help: unsized fn params are gated as an unstable feature help: consider further restricting `Self` | LL | fn map2(self, f: F) -> Map2 where Self: Sized {} | +++++++++++++++++ help: function arguments must have a statically known size, borrowed types always have a known size | LL | fn map2(&self, f: F) -> Map2 {} | + error: aborting due to 4 previous errors Some errors have detailed explanations: E0277, E0308, E0412. For more information about an error, try `rustc --explain E0277`.