What causes "unexpected end of input, expected parentheses" in diesel-rs/diesel?

Add the key columns: `#[diesel(primary_key(id))]` or `#[diesel(primary_key(a, b))]` for composite keys

unexpected end of input, expected parentheses

Error message

unexpected end of input, expected parentheses

What it means

The deprecated `primary_key` attribute requires a parenthesized list of column identifiers; the attribute was given with empty or no parentheses, so the parser errors with `unexpected end of input, expected parentheses`.

Solutions

Add the key columns: `#[diesel(primary_key(id))]` or `#[diesel(primary_key(a, b))]` for composite keys
Remove the attribute to fall back to the default `id` primary key

Example fix

// before
#[diesel(primary_key)]
// after
#[diesel(primary_key(id))]

Defensive patterns

Strategy: validation

Validate before calling

// compile-time: #[diesel(primary_key(id))] or #[diesel(primary_key(a, b))]

Prevention

List the key columns explicitly whenever overriding the primary key
Rely on the default `id` key when no override is needed

When it happens

Trigger: Writing `#[diesel(primary_key)]` without `(col1, col2)` on a table/struct in `QueryableByName`/`Identifiable` derives.

Common situations: Declaring a custom primary key but forgetting the column list, or deleting the list during refactoring.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

AI-assisted analysis of diesel-rs/diesel@6fa6ed01b2 (2026-09-07). Data as JSON: /api/errors/6133f4b04ad44968. Report an issue: GitHub.

Appendix: source

Thrown at diesel_attribute_parser/src/deprecated/primary_key.rs:8

use syn::parse::{Parse, ParseStream, Result};
use syn::punctuated::Punctuated;
use syn::token::Comma;
use syn::{Ident, parenthesized};

pub fn parse_primary_key(name: Ident, input: ParseStream) -> Result<Punctuated<Ident, Comma>> {
    if input.is_empty() {
        return Err(syn::Error::new(
            name.span(),
            "unexpected end of input, expected parentheses",
        ));
    }

    let content;
    parenthesized!(content in input);

    content.parse_terminated(Ident::parse, syn::Token![,])
}

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