Confusing error message: the trait std::ops::Fn<(char,)> is not implemented for T
6838fae
Opened by toddWannaCode at
Hey I am a newbie trying to learn rust following th rust-book(2nd edition). So, I was trying to implement the greps project and I am stuck at the search function.
fn search<'a, T>(query: &T, contents: &'a str) -> Vec<&'a str> {
let mut results = Vec::new();
for line in contents.lines() {
if line.contains(query) {
results.push(line);
}
}
results
}
I am getting this error. :
rustc 1.18.0 (03fc9d622 2017-06-06)
error[E0277]: the trait bound `T: std::ops::Fn<(char,)>` is not satisfied
--> <anon>:39:17
|
39 | if line.contains(query) {
| ^^^^^^^^ the trait `std::ops::Fn<(char,)>` is not implemented for `T`
|
= help: consider adding a `where T: std::ops::Fn<(char,)>` bound
= note: required because of the requirements on the impl of `std::ops::FnOnce<(char,)>` for `&T`
= note: required because of the requirements on the impl of `std::str::pattern::Pattern<'_>` for `&T`
error[E0277]: the trait bound `T: std::ops::FnOnce<(char,)>` is not satisfied
--> <anon>:39:17
|
39 | if line.contains(query) {
| ^^^^^^^^ the trait `std::ops::FnOnce<(char,)>` is not implemented for `T`
|
= help: consider adding a `where T: std::ops::FnOnce<(char,)>` bound
= note: required because of the requirements on the impl of `std::str::pattern::Pattern<'_>` for `&T`
error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
--> <anon>:57:40
|
57 | assert_eq!(vec!["KAMEHAMEHA"], search(query, contents));
| ^^^^^^ the trait `std::marker::Sized` is not implemented for `str`
|
= note: `str` does not have a constant size known at compile-time
= note: required by `search`
error: aborting due to 3 previous errors
Why do I need the Fn trait? And also adding that trait isn't solving my problem. I am using generics, and I know I don't really need generics here but I am trying to understand the topic. Rust-playground gist: https://play.rust-lang.org/?gist=ca11eb8e51982f63a61ff05c542ec0db&version=stable&backtrace=0
For reference, it looks like a cross-posting of an SO question.
Masaki Hara at 2017-07-02 14:08:30
Current output removes the duplicates, but nothing more:
error[E0277]: the trait bound `T: std::ops::Fn<(char,)>` is not satisfied --> src/main.rs:5:17 | 5 | if line.contains(query) { | ^^^^^^^^ the trait `std::ops::Fn<(char,)>` is not implemented for `T` | = help: consider adding a `where T: std::ops::Fn<(char,)>` bound = note: required because of the requirements on the impl of `std::ops::FnOnce<(char,)>` for `&T` = note: required because of the requirements on the impl of `std::str::pattern::Pattern<'_>` for `&T`Esteban Kuber at 2018-02-10 03:01:27
Current output:
error[E0277]: expected a `std::ops::Fn<(char,)>` closure, found `T` --> src/lib.rs:7:26 | 7 | if line.contains(query) { | ^^^^^ expected an `Fn<(char,)>` closure, found `T` | = help: the trait `std::ops::Fn<(char,)>` is not implemented for `T` = note: required because of the requirements on the impl of `std::ops::FnOnce<(char,)>` for `&T` = note: required because of the requirements on the impl of `std::str::pattern::Pattern<'_>` for `&T` help: consider restricting type parameter `T` | 3 | fn search<'a, T: std::ops::Fn<(char,)>>(query: &T, contents: &'a str) -> Vec<&'a str> { | ^^^^^^^^^^^^^^^^^^^^^^^Applying the suggestion gives us the wrong closure return type:
error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change --> src/lib.rs:3:18 | 3 | fn search<'a, T: Fn<(char,)>>(query: &T, contents: &'a str) -> Vec<&'a str> { | ^^ help: use parenthetical notation instead: `Fn(char,) -> ()` | = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more informationwhich leads to
error[E0271]: type mismatch resolving `<&T as std::ops::FnOnce<(char,)>>::Output == bool` --> src/lib.rs:7:17 | 7 | if line.contains(query) { | ^^^^^^^^ expected `()`, found `bool` | = note: required because of the requirements on the impl of `std::str::pattern::Pattern<'_>` for `&T`The fix is to write
T: Fn(char) -> boolinstead ofT: Fn(char) -> ().This is not necessarily the correct fix for this case.
Esteban Kuber at 2020-06-11 23:08:36
Triage: a few cosmetic changes, but otherwise it's the same.
Esteban Kuber at 2022-06-08 22:17:59
Current output:
error[E0277]: the trait bound `&T: Pattern` is not satisfied --> src/lib.rs:39:26 | 39 | if line.contains(query) { | -------- ^^^^^ the trait `Fn(char)` is not implemented for `T`, which is required by `&T: Pattern` | | | required by a bound introduced by this call | = note: required for `&T` to implement `FnOnce(char)` = note: required for `&T` to implement `Pattern` note: required by a bound in `core::str::<impl str>::contains` --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/str/mod.rs:1124:24 | 1124 | pub fn contains<P: Pattern>(&self, pat: P) -> bool { | ^^^^^^^ required by this bound in `core::str::<impl str>::contains` help: consider restricting type parameter `T` | 35 | fn search<'a, T: Fn(char)>(query: &T, contents: &'a str) -> Vec<&'a str> { | ++++++++++ error[E0277]: the size for values of type `str` cannot be known at compilation time --> src/lib.rs:57:47 | 57 | assert_eq!(vec!["KAMEHAMEHA"], search(query, contents)); | ------ ^^^^^ doesn't have a size known at compile-time | | | required by a bound introduced by this call | = help: the trait `Sized` is not implemented for `str` note: required by an implicit `Sized` bound in `search` --> src/lib.rs:35:15 | 35 | fn search<'a, T>(query: &T, contents: &'a str) -> Vec<&'a str> { | ^ required by the implicit `Sized` requirement on this type parameter in `search` help: consider relaxing the implicit `Sized` restriction | 35 | fn search<'a, T: ?Sized>(query: &T, contents: &'a str) -> Vec<&'a str> { | ++++++++Esteban Kuber at 2024-10-04 23:38:55