What causes "expected named struct fields" in leptos-rs/leptos?

Convert the struct to use named fields: struct Login { username: String, password: String }

expected named struct fields

Error message

expected named struct fields

What it means

The #[server] (params extraction) macro's params_impl iterates over struct fields and calls field.ident.as_ref().expect("expected named struct fields"). The macro only supports structs with named fields; a unit struct or tuple struct passed to the params derive causes this panic at macro-expansion time.

Source

Thrown at leptos_macro/src/params.rs:19

use quote::{quote, quote_spanned};
use syn::spanned::Spanned;

pub fn params_impl(ast: &syn::DeriveInput) -> proc_macro::TokenStream {
    let name = &ast.ident;

    let fields = if let syn::Data::Struct(syn::DataStruct {
        fields: syn::Fields::Named(ref fields),
        ..
    }) = ast.data
    {
        fields
            .named
            .iter()
            .map(|field| {
				let field_name_string = &field
                    .ident
                    .as_ref()
                    .expect("expected named struct fields")
                    .to_string()
                    .trim_start_matches("r#")
                    .to_owned();
				let ident = &field.ident;
				let ty = &field.ty;
				let span = field.span();

				quote_spanned! {
					span=> #ident: ::leptos_router::params::macro_helpers::Wrapper::<#ty>::__into_param(
                        map.get_str(#field_name_string),
                        #field_name_string
                    )?
				}
			})
            .collect()
    } else {
        vec![]
    };

View on GitHub (pinned to 32d20f6c9d)

Solutions

Convert the struct to use named fields: struct Login { username: String, password: String }
If you need a newtype wrapper, extract its inner value into a named struct before passing to the macro
Use a different deserialization approach (manual FromRequestParts) if tuple structs are required

Example fix

// before
struct Login(String, String);
// after
struct Login {
    username: String,
    password: String,
}

Defensive patterns

Strategy: validation

Validate before calling

// Compile-time guard: named-field structs satisfy the macro; audit types before deriving
const _: () = {
    // unit/tuple structs will fail macro expansion — keep server param structs named-field only
};

Prevention

Use only named-field structs with the server params derive
Avoid newtype/tuple structs in server function argument positions
Review refactors that change struct field shape used by server functions
Let the compiler catch it: the panic occurs at macro expansion, so building immediately reveals the offending struct

When it happens

Trigger: Applying the params extraction derive/macro to a tuple struct (struct Login(String, String);) or unit struct (struct Login;) instead of a named-field struct.

Common situations: Refactoring a named struct into a newtype/tuple struct for convenience; copy-pasting a tuple struct into a server function signature; auto-generated types from schema tools that emit tuple structs.

Related errors

AI-assisted analysis of leptos-rs/leptos@32d20f6c9d (2026-09-01). Data as JSON: /api/errors/347061131dc28213. Report an issue: GitHub.