Rust errors / Types and traits
Rust compiler error · checked with rustc 1.99.0
Rust error E0059: type parameter to bare `FnOnce` trait must be a tuple
What it meansThe 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.
What rustc prints
error[E0059]: type parameter to bare `FnOnce` trait must be a tuple --> main.rs:3:29 | 3 | fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) } | ^^^^^^^^^ the nightly-only, unstable trait `std::marker::Tuple` is not implemented for `i32` | note: required by a bound in `FnOnce` --> C:\Users\Matthew\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library/core\src\ops\function.rs:242:30 | 242 | pub const trait FnOnce<Args: Tuple> { | ^^^^^ required by this bound in `FnOnce`
Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.
The compiler team's explanation
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:
#![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.
From rustc --explain E0059 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.
Related errors
- E0308mismatched types
- E0277the trait bound is not satisfied
- E0599no method named X found in the current scope
- E0433failed to resolve
- E0053method `foo` has an incompatible type for trait
- E0055reached the recursion limit while auto-dereferencing `Foo`
- E0069`return;` in a function whose return type is not `()`
- E0071expected struct, variant or union type, found `u32`
Up next
Often hit nextE0308: mismatched typesA related error from the same family.Read it next →Did this answer your question?