rust-lang/rust · error

`Self` generic param is not found while expected

Error message

`Self` generic param is not found while expected

What it means

Internal delegation-feature invariant (delegation/generics.rs:219). find_self_param() searches the HIR generic params for one named SelfUpper and expects to find it. It only runs when the delegation resolver decided a Self param should be generated (free-context delegation into a trait). A panic means the expected uplifted Self param is missing from the parent's HIR generics.

Solutions

1. File a rustc ICE with the minimized delegation repro (this is an unstable feature).
2. Avoid the delegation syntax that targets trait methods from free context; delegate to inherent/method fns instead.
3. Try a different nightly; bisect with cargo bisect-rustc.
4. Remove explicit generic args on the delegation path to simplify resolution.

When it happens

Trigger: Using the unstable `reuse`/delegation syntax where a free-standing delegate targets a trait method and generate_self is true, but the lowered parent generics lack a Self param.

Common situations: Experimental nightly code using fn delegation; a compiler bug in the delegation lowering pass; mixing delegation with unusual generic contexts.

Related errors

• must be at least one segment
• must contain self type as `SelfTy` propagation kind is…
• alt layout should always work
• alt layout should have a niche like the regular one
• arg must exist for infer

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/c8ace451f9978498. Report an issue: GitHub.

Appendix: source

Thrown at compiler/rustc_ast_lowering/src/delegation/generics.rs:219

    pub(super) fn is_trait_impl(&self) -> bool {
        match self {
            HirOrTyGenerics::Ty(ty) => ty.trait_impl,
            HirOrTyGenerics::Hir(hir) => hir.trait_impl,
        }
    }

    pub(super) fn find_self_param(&self) -> &'hir hir::GenericParam<'hir> {
        match self {
            HirOrTyGenerics::Ty(_) => {
                bug!("accessed ty-level generics while searching for uplifted `Self` param")
            }
            HirOrTyGenerics::Hir(hir) => hir
                .data
                .params
                .iter()
                .find(|p| p.name.ident().name == kw::SelfUpper)
                .expect("`Self` generic param is not found while expected"),
        }
    }

    pub(crate) fn pos(&self) -> GenericsPosition {
        match self {
            HirOrTyGenerics::Ty(ty) => ty.pos,
            HirOrTyGenerics::Hir(hir) => hir.pos,
        }
    }
}

impl<'hir> GenericsGenerationResult<'hir> {
    fn new(generics: DelegationGenerics<TyGenerics<'hir>>) -> GenericsGenerationResult<'hir> {
        GenericsGenerationResult {
            generics: HirOrTyGenerics::Ty(generics),
            args_segment_id: HirId::INVALID,
            use_for_sig_inheritance: false,
        }

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