I tried this code:
pub struct Parser<H>(H);
impl<H, T> Parser<H>
where
H: for<'a> Fn(&'a str) -> T,
{
pub const fn new(handler: H) -> Parser<H> {
Parser(handler)
}
pub const fn many<'s>(
&'s self,
) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
Parser::new(|_| unimplemented!())
}
}
fn main() {
println!("Hello, world!");
}Which causes the following error:
error[E0391]: cycle detected when computing type of `<impl at src/main.rs:3:1: 3:21>::many::{opaque#0}`
--> src/main.rs:13:17
|
13 | ) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
note: ...which requires borrow-checking `<impl at src/main.rs:3:1: 3:21>::many`...
--> src/main.rs:11:5
|
11 | / pub const fn many<'s>(
12 | | &'s self,
13 | | ) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
| |________________________________________________________^
note: ...which requires const checking `<impl at src/main.rs:3:1: 3:21>::many`...
--> src/main.rs:11:5
|
11 | / pub const fn many<'s>(
12 | | &'s self,
13 | | ) -> Parser<impl for<'a> Fn(&'a str) -> Vec<T> + 's> {
| |________________________________________________________^
= note: ...which requires computing whether `Parser<<impl at src/main.rs:3:1: 3:21>::many::{opaque#0}>` is freeze...
= note: ...which requires evaluating trait selection obligation `Parser<<impl at src/main.rs:3:1: 3:21>::many::{opaque#0}>: core::marker::Freeze`...
= note: ...which again requires computing type of `<impl at src/main.rs:3:1: 3:21>::many::{opaque#0}`, completing the cycle
note: cycle used when checking item types in top-level module
--> src/main.rs:1:1
|
1 | / pub struct Parser<H>(H);
2 | |
3 | | impl<H, T> Parser<H>
4 | | where
... |
19 | | println!("Hello, world!");
20 | | }
| |_^
For more information about this error, try `rustc --explain E0391`.
error: could not compile `miri-cycle` (bin "miri-cycle") due to previous error
However, what is peculiar is that if you remove the const keyword from the many function, compilation succeeds, with absolutely no warnings or errors.
Meta
rustc --version --verbose:
rustc 1.72.0-nightly (df77afbca 2023-06-12)
binary: rustc
commit-hash: df77afbcaf3365a32066a8ca4a00ae6fc9a69647
commit-date: 2023-06-12
host: x86_64-unknown-linux-gnu
release: 1.72.0-nightly
LLVM version: 16.0.5
No backtrace is emitted when I use RUST_BACKTRACE=1 cargo build.
I tried this code:
Which causes the following error:
However, what is peculiar is that if you remove the
constkeyword from themanyfunction, compilation succeeds, with absolutely no warnings or errors.Meta
rustc --version --verbose:No backtrace is emitted when I use
RUST_BACKTRACE=1 cargo build.