[rustc.git] / src / librustc_error_codes / error_codes / E0059.md

The built-in function traits are generic over a tuple of the function arguments.
If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses
(`Fn(T) -> U`) to denote the function trait, the type parameter should be a
tuple. Otherwise function call notation cannot be used and the trait will not be
implemented by closures.

The most likely source of this error is using angle-bracket notation without
wrapping the function argument type into a tuple, for example:

```compile_fail,E0059
#![feature(unboxed_closures)]

fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
```

It can be fixed by adjusting the trait bound like this:

```
#![feature(unboxed_closures)]

fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }
```

Note that `(T,)` always denotes the type of a 1-tuple containing an element of
type `T`. The comma is necessary for syntactic disambiguation.
Commit	Line	Data
60c5eb7d XL	1	The built-in function traits are generic over a tuple of the function arguments.
	2	If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses
	3	(`Fn(T) -> U`) to denote the function trait, the type parameter should be a
	4	tuple. Otherwise function call notation cannot be used and the trait will not be
	5	implemented by closures.
	6
	7	The most likely source of this error is using angle-bracket notation without
	8	wrapping the function argument type into a tuple, for example:
	9
	10	```compile_fail,E0059
	11	#![feature(unboxed_closures)]
	12
	13	fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
	14	```
	15
	16	It can be fixed by adjusting the trait bound like this:
	17
	18	```
	19	#![feature(unboxed_closures)]
	20
	21	fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }
	22	```
	23
	24	Note that `(T,)` always denotes the type of a 1-tuple containing an element of
	25	type `T`. The comma is necessary for syntactic disambiguation.