rust-lang / rust-lang/rust

Infinite recursion in type constraint resolution

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#126,296 8 comments 0 reactions 0 assignees View on GitHub

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A-inference A-trait-system C-bug fixed-by-next-solver T-compiler
Dominant language
Rust
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Description

I tried this code:

use std::borrow::Borrow;

trait A {
    const XXX: usize = 1;
}

struct Special<U>(U);

struct Wrapper<T, const N: usize>([T; N]);

impl<T, const N: usize> A for Special<Wrapper<T, N>>
where
    Special<T>: A
{}

impl<T: Borrow<str>> A for Special<T> {
}

fn main() {
    let _ = <Special<Wrapper<String, 1>> as A>::XXX;
}

I expected to see this happen: the code should compile

Instead, this happened:

error[E0275]: overflow evaluating the requirement `Special<Wrapper<_, _>>: A`
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`blah`)
note: required for `Special<Wrapper<Wrapper<_, _>, _>>` to implement `A`
  --> blah.rs:11:25
   |
11 | impl<T, const N: usize> A for Special<Wrapper<T, N>>
   |                         ^     ^^^^^^^^^^^^^^^^^^^^^^
12 | where
13 |         Special<T>: A
   |                     - unsatisfied trait bound introduced here
   = note: 126 redundant requirements hidden
   = note: required for `Special<Wrapper<Wrapper<Wrapper<Wrapper<Wrapper<..., _>, _>, _>, _>, _>>` to implement `A`
   = note: the full name for the type has been written to 'blah.long-type-12463274173695242483.txt'
   = note: consider using `--verbose` to print the full type name to the console

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0275`.
Meta

rustc --version --verbose:

rustc 1.78.0 (9b00956e5 2024-04-29)
binary: rustc
commit-hash: 9b00956e56009bab2aa15d7bff10916599e3d6d6
commit-date: 2024-04-29
host: x86_64-unknown-linux-gnu
release: 1.78.0
LLVM version: 18.1.2
Backtrace

error[E0275]: overflow evaluating the requirement `Special<Wrapper<_, _>>: A`
   |
   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`blah`)
note: required for `Special<Wrapper<Wrapper<_, _>, _>>` to implement `A`
  --> blah.rs:11:25
   |
11 | impl<T, const N: usize> A for Special<Wrapper<T, N>>
   |                         ^     ^^^^^^^^^^^^^^^^^^^^^^
12 | where
13 |         Special<T>: A
   |                     - unsatisfied trait bound introduced here
   = note: 126 redundant requirements hidden
   = note: required for `Special<Wrapper<Wrapper<Wrapper<Wrapper<Wrapper<..., _>, _>, _>, _>, _>>` to implement `A`
   = note: the full name for the type has been written to 'blah.long-type-12463274173695242483.txt'
   = note: consider using `--verbose` to print the full type name to the console

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0275`.

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 reproducer in blah.rs with the reported rustc 1.78.0 toolchain and confirm the E0275 overflow. Then investigate the compiler's type constraint resolution path; done means the reproducer no longer produces the unintended infinite recursion while preserving the expected trait behavior.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
25/100

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