Cycle detected when computing type, but only when function is const #112602
Description
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.
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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.
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Area: Constant evaluation, covers all const contexts (static, const fn, ...)Area: `impl Trait`. Universally / existentially quantified anonymous types with static dispatch.Category: This is a bug.Call for participation: An issue has been fixed and does not reproduce, but no test has been added.Issue: A query cycle occurred while none was expectedRelevant to the compiler team, which will review and decide on the PR/issue.