rust-lang/rust · critical

argument names not encoded for a function

Error message

argument names not encoded for a function

What it means

Fires in `get_fn_has_self_parameter` (decoder.rs:1380). The function looks up the `fn_arg_idents` table for a `DefIndex` and `.expect()`s an entry, because a function is assumed to have its argument names encoded. A missing entry means a non-function `DefIndex` was queried through this path, or the function's argument names were never serialized.

Source

Thrown at compiler/rustc_metadata/src/rmeta/decoder.rs:1380

            let children = self.root.tables.ambig_module_children.get(self, id);
            if !children.is_default() {
                for child in children.decode((self, tcx)) {
                    yield child;
                }
            }
        }
    }

    fn is_item_mir_available(&self, id: DefIndex) -> bool {
        self.root.tables.optimized_mir.get(self, id).is_some()
    }

    fn get_fn_has_self_parameter(&self, tcx: TyCtxt<'_>, id: DefIndex) -> bool {
        self.root
            .tables
            .fn_arg_idents
            .get(self, id)
            .expect("argument names not encoded for a function")
            .decode((self, tcx))
            .nth(0)
            .is_some_and(|ident| matches!(ident, Some(Ident { name: kw::SelfLower, .. })))
    }

    fn get_associated_item_or_field_def_ids(
        &self,
        tcx: TyCtxt<'_>,
        id: DefIndex,
    ) -> impl Iterator<Item = DefId> {
        self.root
            .tables
            .associated_item_or_field_def_ids
            .get(self, id)
            .unwrap_or_else(|| self.missing("associated_item_or_field_def_ids", id))
            .decode((self, tcx))
            .map(move |child_index| self.local_def_id(child_index))
    }

View on GitHub (pinned to 7088e4b63a)

Solutions

1. `cargo clean` and rebuild to regenerate `fn_arg_idents` for the current schema.
2. Ensure the crate providing the function and the crate querying it share one toolchain version.
3. If the function is from a prebuilt dependency, rebuild it with the active compiler instead of reusing the old rlib.
4. Reproduces on a clean single-toolchain build -> file an ICE; the query was likely routed to a non-function `DefIndex`.

# Regenerate function-arg metadata for the current schema
cargo clean
cargo build

cargo build || { cargo clean && cargo build; }

When it happens

Trigger: Querying `fn_has_self_parameter` on a `DefIndex` that is not a function (e.g. a const, static, or type alias routed here by a stale query cache); reading a function whose metadata was written without `fn_arg_idents` (older encoder, or an extern/imported fn without encoded args).

Common situations: Cross-version artifact reuse where the encoder stopped/started writing `fn_arg_idents`; incremental cache referencing a renumbered `DefIndex`; querying self-ness of a function from a proc-macro or older rlib.

Related errors

• no encoded ident for item
• variants are not encoded for an enum
• provided `DefIndex` must refer to a module-like item
• no parent for a constructor
• no encoded attributes for a structure or variant

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/ca0973f23cc1c3b7. Report an issue: GitHub.