rust-lang / rust-lang/rust

Confusing diagnostic when doing a recursive call in a const block in a const fn: "cycle detected when elaborating drops"

Open
#125,713 0 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

A-const-eval A-diagnostics D-confusing T-compiler
Dominant language
Rust
Stars
119k
Forks
16.1k
PR merge metrics
PR metrics pending

Description

Code
const fn foo() {
    const { foo() };
}
Current output
Compiling playground v0.0.1 (/playground)
error[E0391]: cycle detected when elaborating drops for `foo`
 --> src/lib.rs:2:5
  |
2 |     const { foo() };
  |     ^^^^^^^^^^^^^^^
  |
note: ...which requires simplifying constant for the type system `foo::{constant#0}`...
 --> src/lib.rs:2:5
  |
2 |     const { foo() };
  |     ^^^^^^^^^^^^^^^
note: ...which requires const-evaluating + checking `foo::{constant#0}`...
 --> src/lib.rs:2:5
  |
2 |     const { foo() };
  |     ^^^^^^^^^^^^^^^
note: ...which requires caching mir of `foo` for CTFE...
 --> src/lib.rs:1:1
  |
1 | const fn foo() {
  | ^^^^^^^^^^^^^^
  = note: ...which again requires elaborating drops for `foo`, completing the cycle
  = note: cycle used when running analysis passes on this crate
  = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

For more information about this error, try `rustc --explain E0391`.
error: could not compile `playground` (lib) due to 1 previous error
Desired output
Something about constant evaluation failing.
Rationale and extra context

No response

Other cases

No response

Rust Version
Nightly rustc on the playground
1.80.0-nightly (2024-05-28 da159eb331b27df52818)
Anything else?

No response

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by compiling the provided recursive const-block example with the referenced nightly Rust version and inspect how the diagnostic is produced. Trace the relevant compiler diagnostic path for the cycle and make the result describe constant-evaluation failure rather than an opaque drop-elaboration cycle; add or update a regression test if the relevant test location is found.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.