Code
I tried this code:
fn foo(b: bool) -> impl IntoIterator<Item = ()> {
if b {
return vec![()];
}
[()].into_iter().collect()
}I expected to see this happen: The code compiles (as it does on stable and beta). The compiler could infer the return type from the first return statement (Vec<()>), and successfully collect the iterator into it in the second return.
Instead, this happened:
The compiler seems to have trouble to infer the type of the second return, and gives confusing messages. Instead of explaining that it cannot infer the type (which can be understandable given some update in the library), it gives some serious complaint that the given type doesn't support this kind of operation.
error[[E0277]](https://doc.rust-lang.org/nightly/error-index.html#E0277): a value of type `impl IntoIterator<Item = ()>` cannot be built from an iterator over elements of type `()`
--> src/lib.rs:5:22
|
5 | [()].into_iter().collect()
| ^^^^^^^ value of type `impl IntoIterator<Item = ()>` cannot be built from `std::iter::Iterator<Item=()>`
|
= help: the trait `FromIterator<()>` is not implemented for `impl IntoIterator<Item = ()>`
note: required by a bound in `collect`
For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` due to previous error
Version it worked on
It most recently worked on: Stable version: 1.58.1, 1.59.0-beta.6(2022-02-02 0426998)
Version with regression
rustc --version --verbose:
rustc 1.60.0-nightly (0c292c966 2022-02-08)
binary: rustc
commit-hash: 0c292c9667f1b202a9150d58bdd2e89e3e803996
commit-date: 2022-02-08
host: x86_64-pc-windows-msvc
release: 1.60.0-nightly
LLVM version: 13.0.0
@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged
Code
I tried this code:
I expected to see this happen: The code compiles (as it does on stable and beta). The compiler could infer the return type from the first return statement (
Vec<()>), and successfully collect the iterator into it in the second return.Instead, this happened:
The compiler seems to have trouble to infer the type of the second return, and gives confusing messages. Instead of explaining that it cannot infer the type (which can be understandable given some update in the library), it gives some serious complaint that the given type doesn't support this kind of operation.
Version it worked on
It most recently worked on: Stable version: 1.58.1, 1.59.0-beta.6(2022-02-02 0426998)
Version with regression
rustc --version --verbose:@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged