Code
pub trait Add {
fn add(&self, left: usize, right: usize) -> usize;
}
impl<F> Add for F
where
F: Fn(usize, usize) -> usize,
{
fn add(&self, left: usize, right: usize) -> usize {
self(left, right)
}
}
pub struct NotAdd;
pub fn needs_add<A: Add>(add: &A) -> usize {
add.add(1, 2)
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn it_works() {
assert_eq!(needs_add(&NotAdd), 3);
}
}Current output
error[E0277]: expected a `Fn<(usize, usize)>` closure, found `NotAdd`
--> src/lib.rs:26:30
|
26 | assert_eq!(needs_add(&NotAdd), 3);
| --------- ^^^^^^^ expected an `Fn<(usize, usize)>` closure, found `NotAdd`
| |
| required by a bound introduced by this call
|
= help: the trait `Fn<(usize, usize)>` is not implemented for `NotAdd`
note: required for `NotAdd` to implement `Add`
--> src/lib.rs:5:9
|
5 | impl<F> Add for F
| ^^^ ^
6 | where
7 | F: Fn(usize, usize) -> usize,
| ------------------------- unsatisfied trait bound introduced here
note: required by a bound in `needs_add`
--> src/lib.rs:16:21
|
16 | pub fn needs_add<A: Add>(add: &A) -> usize {
| ^^^ required by this bound in `needs_add`
For more information about this error, try `rustc --explain E0277`.
error: could not compile `fn-trait-error-test` (lib test) due to previous error
Desired output
error[E0277]: `NotAdd` doesn't implement `Add`
--> src/lib.rs:26:30
|
26 | assert_eq!(needs_add(&NotAdd), 3);
| --------- ^^^^^^^ `NotAdd` doesn't implement `Add`
| |
| required by a bound introduced by this call
|
= help: the trait `Fn<(usize, usize)>` is not implemented for `NotAdd`
note: required for `NotAdd` to implement `Add`
--> src/lib.rs:5:9
|
5 | impl<F> Add for F
| ^^^ ^
6 | where
7 | F: Fn(usize, usize) -> usize,
| ------------------------- unsatisfied trait bound introduced here
note: required by a bound in `needs_add`
--> src/lib.rs:16:21
|
16 | pub fn needs_add<A: Add>(add: &A) -> usize {
| ^^^ required by this bound in `needs_add`
For more information about this error, try `rustc --explain E0277`.
error: could not compile `fn-trait-error-test` (lib test) due to previous error
Rationale and extra context
The first thing the error should make extremely obvious is the Trait that is required.
Then the existing (very useful and clever!) other bounds required to satisfy a blanket impl should be noted.
The fact that a type is implemented by a Blanket impl is often not in the forefront of a user's mind when they encounter this kind of error. They first need to realise that the type they probably think ought to impl a trait, definitely does not in fact impl that trait.
This becomes more important the more traits and types implementing those traits there are in scope, and the more bounds that might not be met.
Other cases
No response
Anything else?
No response
Code
Current output
Desired output
Rationale and extra context
The first thing the error should make extremely obvious is the Trait that is required.
Then the existing (very useful and clever!) other bounds required to satisfy a blanket impl should be noted.
The fact that a type is implemented by a Blanket impl is often not in the forefront of a user's mind when they encounter this kind of error. They first need to realise that the type they probably think ought to impl a trait, definitely does not in fact impl that trait.
This becomes more important the more traits and types implementing those traits there are in scope, and the more bounds that might not be met.
Other cases
No response
Anything else?
No response