rust-lang/rust · critical

Can't have a type error relating to itself

Error message

Can't have a type error relating to itself

What it means

This fires inside Polonius's `record_live_region_variance`, which extracts the variance (co/contra/invariant) of each region inside a type by relating the type with itself (`extractor.relate(value, value)`). Relating a type with itself should never produce a type error — identical types are always compatible. If it does, the compiler considers this a logic error in how the type or its regions were constructed.

Source

Thrown at compiler/rustc_borrowck/src/polonius/liveness_constraints.rs:26

use super::{ConstraintDirection, PoloniusContext};
use crate::universal_regions::UniversalRegions;

impl PoloniusContext {
    /// Record the variance of each region contained within the given value.
    pub(crate) fn record_live_region_variance<'tcx>(
        &mut self,
        tcx: TyCtxt<'tcx>,
        universal_regions: &UniversalRegions<'tcx>,
        value: impl TypeVisitable<TyCtxt<'tcx>> + Relate<TyCtxt<'tcx>>,
    ) {
        let mut extractor = VarianceExtractor {
            tcx,
            ambient_variance: ty::Variance::Covariant,
            directions: &mut self.live_region_variances,
            universal_regions,
        };
        extractor.relate(value, value).expect("Can't have a type error relating to itself");
    }
}

/// Extracts variances for regions contained within types. Follows the same structure as
/// `rustc_infer`'s `Generalizer`: we try to relate a type with itself to track and extract the
/// variances of regions.
struct VarianceExtractor<'a, 'tcx> {
    tcx: TyCtxt<'tcx>,
    ambient_variance: ty::Variance,
    directions: &'a mut BTreeMap<RegionVid, ConstraintDirection>,
    universal_regions: &'a UniversalRegions<'tcx>,
}

impl<'tcx> VarianceExtractor<'_, 'tcx> {
    fn record_variance(&mut self, region: ty::Region<'tcx>, variance: ty::Variance) {
        // We're only interested in the variance of vars and free regions.
        //
        // Note: even if we currently bail for two cases of unexpected region kinds here, missing

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Remove `-Zpolonius=next` and use the default NLL borrow checker; this code path is exclusive to polonius.
2. Minimize the reproduction and file a bug with the type that triggers the self-relation error.
3. Update to the latest nightly; polonius variance extraction is under active development.
4. If contributing to rustc, inspect the `VarianceExtractor` in `liveness_constraints.rs` — the `relate` call may need to handle additional region kinds that are currently unexpected.

Example fix

// before (complex GAT + opaque, triggers polonius variance ICE)
#![feature(polonius)]
trait L { type Item<T>; }
fn f<T>(x: <() as L>::Item<T>) { /* ... */ }

// after (avoid polonius for this crate; remove the flag)
// RUSTFLAGS='' cargo build  (uses default NLL)

// This is a compiler-internal invariant. No user-level validation possible.
// The fallback is to not use -Zpolonius=next:
// unset RUSTFLAGS or ensure -Zpolonius=next is not present

When it happens

Trigger: During Polonius analysis (`-Zpolonius=next`), when the variance extractor encounters a type that cannot be related to itself. This implies a malformed or internally inconsistent type — e.g., a type with opaque/hidden regions, inference variables that escaped their scope, or a type-rewrite bug in the MIR type-check pass feeding bad data to polonius.

Common situations: Nightly Rust with `-Zpolonius=next` on code using complex generics, associated types, opaque types, or GATs where a type's internal structure is subtly inconsistent. This is a compiler soundness-adjacent invariant; the panic indicates the variance data would be wrong, so the compiler aborts rather than emit incorrect borrow-check results.

Related errors

• missing polonius context with `-Zpolonius=next`
• Polonius output is unavailable despite `-Z polonius`
• Accessing live loans requires `-Zpolonius=next`
• coroutine lowered from async gen fn should be in fn
• simd {}

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/f639becd91528a2b. Report an issue: GitHub.