Description
In the compiler message for type mismatching error of associated type, "expected A, found B" should be reversed ("expected B, found A").
struct X;
trait Foo {
type Item;
}
impl Foo for X {
type Item = i32;
}
fn foo(_: impl Foo<Item = f64>) {}
fn main() {
foo(X);
}The above example shows the error below:
error[E0271]: type mismatch resolving `<X as Foo>::Item == f64`
--> (filename):14:5
|
14 | foo(X);
| ^^^ expected i32, found f64
|
note: required by `foo`
--> (filename):11:1
|
11 | fn foo(_: impl Foo<Item = f64>) {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error
For more information about this error, try `rustc --explain E0271`.
Especially, compiler says "expected i32, found f64". However "expected f64, found i32" is more natural since the mismatching here is that the function foo() requires X::Item to be f64 but X::Item is actually i32.
Another example is below:
fn main() {
let _ = vec![1, 2, 3].into_iter().cloned();
}It causes an error like:
error[E0271]: type mismatch resolving `<std::vec::IntoIter<{integer}> as std::iter::Iterator>::Item == &_`
--> (filename):2:39
|
2 | let _ = vec![1, 2, 3].into_iter().cloned();
| ^^^^^^ expected integer, found reference
|
= note: expected type `{integer}`
found type `&_`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0271`.
The error here is that Iterator::cloned() requires Iterator::Item to be a reference but the supplied iterator here (vec![1, 2, 3].into_iter()) is a iterator generating integers. However error message is shown as if Iterator::cloned() required something to be an integer but that was reference. The message should be "expected reference, found integer".
Meta
rustc --version --verbose:
rustc 1.33.0-nightly (b43986184 2019-01-11)
binary: rustc
commit-hash: b43986184b8f4e0d633e8ae1704f0e19aec30cb2
commit-date: 2019-01-11
host: x86_64-pc-windows-msvc
release: 1.33.0-nightly
LLVM version: 8.0