The trait `FnMut<(char,)>` is not implemented for `String` when trying to split a string

Question

I need to split a String (not &str) by another String:

use std::str::Split; fn main() { let x = "".to_string().split("".to_string()); }

Why do I get this error and how to avoid it if I already have to operate on strings?

error[E0277]: the trait bound `std::string::String: std::ops::FnMut<(char,)>` is not satisfied --> src/main.rs:4:32 | 4 | let x = "".to_string().split("".to_string()); | ^^^^^ the trait `std::ops::FnMut<(char,)>` is not implemented for `std::string::String` | = note: required because of the requirements on the impl of `std::str::pattern::Pattern<'_>` for `std::string::String`

According to the #rust-beginners IRC channel, this might be an example of Deref failing in 1.20.0-nightly. How to split a string in Rust? doesn't address the problem of splitting by String, not &str.

Yup, passing a reference to String instead of the String itself does solve the error message - you could post that as a separate answer. In order to reproduce the error, remove & from your code. — d33tah
– d33tah, Commented Jul 7, 2017 at 15:23
@Boiethios: only if you know that in this particular case String won't get derefed. — d33tah
– d33tah, Commented Jul 7, 2017 at 15:26

Shepmaster · Accepted Answer · 2017-07-07 15:30:17Z

6

All is in the documentation. You can provide one of:

A &str,
A char,
A closure,

Those three types implement the Pattern trait. You are giving a String to split instead of a &str.

Example:

fn main() { let x = "".to_string(); let split = x.split(""); }

edited Jul 7, 2017 at 15:30

Shepmaster

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answered Jul 7, 2017 at 15:18

Boiethios

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3 Comments

Stargateur Over a year ago

please don't answer to duplicate.

Boiethios Over a year ago

@Stargateur Sorry, I answered before.

d33tah Over a year ago

It's not a duplicate - the assumption here is that I don't have an &str, but a String. I explained this in an edit.

Boiethios · Accepted Answer · 2017-07-07 15:32:31Z

I talked about this with #rust-beginners IRC channel and heard the following:

15:12:15 achird | d33tah: split accepts a Pattern, where Pattern can be &str or char, but you're passing a String (no idea why deref is not working) 15:13:01 d33tah | achird: thanks! how can I convert string to str? 15:13:03 achird | i think a simple split(&delimiter2) should fix the problem 15:16:26 calops | why isn't deref working though? 15:21:33 @mbrubeck | calops, d33tah: Deref coercions only work if one exact "expected" type is known. For a generic type like <P: Pattern>, coercion doesn't kick in. 15:24:11 @mbrubeck | d33tah: The error should definitely be improved... It should complain that `String` doesn't impl `Pattern`, instead of jumping straight to `FnMut(char)`

So basically, the solution is to add & before the delimiter string, like this:

fn main() { let s1 = "".to_string(); let s2 = "".to_string(); let x = s1.split(&s2); }

I clearly agree that the error is misleading. Should definitively be improved.

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