rust-lang/rust · error

alt layout should always work

Error message

alt layout should always work

What it means

Internal layout invariant in rustc_abi's niche calculator (layout.rs:300). After a regular univariant layout with a niche is computed successfully, the compiler recomputes an end-biased (NicheBias::End) layout and expects it to also succeed via .expect("alt layout should always work"). The assumption: if the default field ordering yields a valid layout, the end-biased ordering must too. A panic here is a compiler bug (ICE), not a user error.

Solutions

1. Treat this as an ICE: file a rustc issue with the minimized repro (the panic message is the anchor).
2. Reduce the offending type: strip niched fields (Option, NonZero*, bool) and remove #[repr(...)] attributes until the panic disappears.
3. Bisect with `cargo bisect-rustc` to find the regression, and try the same code on stable vs nightly.
4. As a workaround, reorder fields or change the repr so the biased-niche recomputation path is not taken.

When it happens

Trigger: Lowering a multi-field struct/enum that has a niche (e.g. an Option/NonZero/bool field), head_space != 0, and tail_space > 0, so the biased-recomputation branch runs and univariant_biased returns Err.

Common situations: An ICE on nightly with unusual #[repr(...)] combinations or niche-bearing fields. This signals a bug in the layout algorithm, typically hit only by compiler developers or with exotic reprs.

Related errors

• alt layout should have a niche like the regular one
• expected pointee to have a layout
• expected `Session`
• missing QSelf for ::...
• must be at least one segment

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/49288306c628a8ab. Report an issue: GitHub.

Appendix: source

Thrown at compiler/rustc_abi/src/layout.rs:300

        // run and bias niches to the right and then check which one is closer to one of the
        // struct's edges.
        if let Ok(layout) = &layout {
            // Don't try to calculate an end-biased layout for unsizable structs,
            // otherwise we could end up with different layouts for
            // Foo<Type> and Foo<dyn Trait> which would break unsizing.
            if !matches!(kind, StructKind::MaybeUnsized) {
                if let Some(niche) = layout.largest_niche {
                    let head_space = niche.offset.bytes();
                    let niche_len = niche.value.size(dl).bytes();
                    let tail_space = layout.size.bytes() - head_space - niche_len;

                    // This may end up doing redundant work if the niche is already in the last
                    // field (e.g. a trailing bool) and there is tail padding. But it's non-trivial
                    // to get the unpadded size so we try anyway.
                    if fields.len() > 1 && head_space != 0 && tail_space > 0 {
                        let alt_layout = self
                            .univariant_biased(fields, repr, kind, NicheBias::End)
                            .expect("alt layout should always work");
                        let alt_niche = alt_layout
                            .largest_niche
                            .expect("alt layout should have a niche like the regular one");
                        let alt_head_space = alt_niche.offset.bytes();
                        let alt_niche_len = alt_niche.value.size(dl).bytes();
                        let alt_tail_space =
                            alt_layout.size.bytes() - alt_head_space - alt_niche_len;

                        debug_assert_eq!(layout.size.bytes(), alt_layout.size.bytes());

                        let prefer_alt_layout =
                            alt_head_space > head_space && alt_head_space > tail_space;

                        debug!(
                            "sz: {}, default_niche_at: {}+{}, default_tail_space: {}, alt_niche_at/head_space: {}+{}, alt_tail: {}, num_fields: {}, better: {}\n\
                            layout: {}\n\
                            alt_layout: {}\n",
                            layout.size.bytes(),

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