Confusing error for missing From impls (and other traits with blanket impls)

Given the following code:

struct A;
struct B;

fn foo() {
    let a = A;
    let b = B::from(a);
}

The current output is:

error[[E0308]](https://doc.rust-lang.org/stable/error-index.html#E0308): mismatched types
 --> src/lib.rs:6:21
  |
6 |     let b = B::from(a);
  |             ------- ^ expected struct `B`, found struct `A`
  |             |
  |             arguments to this function are incorrect
  |
note: associated function defined here

Ideally the output should look like:

error: type `B` doesn't implement `From<A>` 
 --> src/lib.rs:6:21
  |
6 |     let b = B::from(a);
  |             ------- ^ expected struct `B`, found struct `A`
  |
note: type `B` has a blanket implementation
    impl<T> From<T> for T

I guess it makes sense to also leave a link to the URLO discussion where we’re coming from.

Do people think the following which is introduced by #164702 is enough?

error[E0308]: mismatched types
   --> tests/ui/fn_traits/point-at-impl-in-call-argument-type-error.rs:9:21
    |
  9 |     let b = B::from(a); //~ ERROR: mismatched types
    |             ------- ^ expected `B`, found `A`
    |             |
    |             arguments to this function are incorrect
    |
note: associated function defined here
   --> library/core/src/convert/mod.rs:594:8
    |
588 | pub const trait From<T>: Sized {
...
594 |     fn from(value: T) -> Self;
    |        ^^^^
note: associated function provided through this blanket `impl`
   --> library/core/src/convert/mod.rs:787:1
    |
787 | const impl<T> From<T> for T {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Rollup merge of rust-lang#164702 - estebank:point-at-blanket, r=mejrs

Unrolled build for #164702 in rollup 165002

Rollup merge of #164702 - estebank:point-at-blanket, r=mejrs

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Confusing error for missing From impls (and other traits with blanket impls) #104712

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Confusing error for missing From impls (and other traits with blanket impls) #104712

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steffahn commented on Nov 22, 2022

estebank commented on Oct 21, 2026

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