help clause points to adding where clause on impl when it should be on function #104089

OneRaynyDay · 2022-11-07T06:33:25Z

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e5d9c21342f0c0824ddba9445daf7cb1

struct A {
    x: String
}

struct B {
    x: String
}

impl From<A> for B {
    fn from(a: A) -> Self {
        B { x: a.x }
    }
}

impl B {
    pub fn from_many<T: Into<B> + Clone>(v: Vec<T>) -> Self {
        B { x: v.iter().map(|e| B::from(e.clone()).x).collect::<Vec<String>>().join(" ") }
    }
}

fn main() {
    let b: B = B { x: "foobar".to_string() };
    let a: A = A { x: "frob".to_string() };
    let ab: B = a.into();
    println!("Hello, {}!", ab.x);
}

The current output is:

error[[E0277]](https://doc.rust-lang.org/stable/error-index.html#E0277): the trait bound `B: From<T>` is not satisfied
  --> src/main.rs:17:41
   |
17 |         B { x: v.iter().map(|e| B::from(e.clone()).x).collect::<Vec<String>>().join(" ") }
   |                                 ------- ^^^^^^^^^ the trait `From<T>` is not implemented for `B`
   |                                 |
   |                                 required by a bound introduced by this call
   |
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
   |
15 | impl B where B: From<T> {
   |        ++++++++++++++++

Ideally the output should look like:

...
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
   |
pub fn from_many<T: Into<B> + Clone>(v: Vec<T>) -> Self
where B: From<T> {
...
}

link to stack overflow: https://stackoverflow.com/questions/74342563/convert-vector-of-type-a-to-type-b-where-a-is-convertible-to-b?noredirect=1#comment131245422_74342563

The text was updated successfully, but these errors were encountered:

When encountering a missing bound that involves a type parameter, on associated functions we look at the generics to see if the type parameter is present. If so, we suggest the bound on the associated function instead of on the impl/trait. At the impl/trait doesn't have another type parameter of that name, then we don't suggest the bound at that item level. ``` error[E0277]: the trait bound `B: From<T>` is not satisfied --> $DIR/suggest-restriction-involving-type-param.rs:20:33 | LL | B { x: v.iter().map(|e| B::from(e.clone()).x).collect::<Vec<String>>().join(" ") } | ^ the trait `From<T>` is not implemented for `B` | help: consider further restricting the type | LL | pub fn from_many<T: Into<B> + Clone>(v: Vec<T>) -> Self where B: From<T> { | ++++++++++++++++ ``` Fix rust-lang#104089.

OneRaynyDay added A-diagnostics Area: Messages for errors, warnings, and lints T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 7, 2022

estebank linked a pull request Dec 30, 2024 that will close this issue

Account for type parameters in bound suggestion #134937

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help clause points to adding where clause on impl when it should be on function #104089

help clause points to adding where clause on impl when it should be on function #104089

OneRaynyDay commented Nov 7, 2022 •

edited

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help clause points to adding where clause on impl when it should be on function #104089

help clause points to adding where clause on impl when it should be on function #104089

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OneRaynyDay commented Nov 7, 2022 • edited Loading

OneRaynyDay commented Nov 7, 2022 •

edited

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