tauri-apps/tauri · error · syn::Error

default_runtime only supports `struct`, `enum`, `type`, or `

Error message

default_runtime only supports `struct`, `enum`, `type`, or `trait` definitions

What it means

#[default_runtime] (tauri-macros, re-exported by tauri; syntax like #[default_runtime(crate::Wry, wry)]) injects a default for the last generic parameter of an item so downstream users can omit the runtime type. The parser tries the input as DeriveInput (struct/enum), then ItemTrait, then ItemType; if all three fail — the item is a fn, impl, module, union, use — it reports 'default_runtime only supports `struct`, `enum`, `type`, or `trait` definitions'.

Source

Thrown at crates/tauri-macros/src/runtime.rs:27

  parse_quote, DeriveInput, Error, GenericParam, Ident, ItemTrait, ItemType, Token, Type, TypeParam,
};

#[derive(Clone)]
pub(crate) enum Input {
  Derive(DeriveInput),
  Trait(ItemTrait),
  Type(ItemType),
}

impl Parse for Input {
  fn parse(input: ParseStream) -> syn::Result<Self> {
    input
      .parse::<DeriveInput>()
      .map(Self::Derive)
      .or_else(|_| input.parse().map(Self::Trait))
      .or_else(|_| input.parse().map(Self::Type))
      .map_err(|_| {
        Error::new(
          input.span(),
          "default_runtime only supports `struct`, `enum`, `type`, or `trait` definitions",
        )
      })
  }
}

impl Input {
  fn last_param_mut(&mut self) -> Option<&mut GenericParam> {
    match self {
      Input::Derive(d) => d.generics.params.last_mut(),
      Input::Trait(t) => t.generics.params.last_mut(),
      Input::Type(t) => t.generics.params.last_mut(),
    }
  }
}

impl ToTokens for Input {

View on GitHub (pinned to 52e4b6e71d)

Solutions

1. Move the attribute onto the struct/enum definition, trait, or type alias that actually carries the runtime generic
2. Ensure that item has at least one generic parameter — the macro defaults the last one and asserts on its presence
3. For functions, spell the runtime parameter explicitly instead of relying on the macro

Example fix

// before
#[default_runtime(crate::Wry, wry)]
impl<R: Runtime> App<R> { /* ... */ }
// after - annotate the type that owns the generic
#[default_runtime(crate::Wry, wry)]
struct App<R: Runtime> { /* ... */ }

When it happens

Trigger: Annotating an impl block, fn, mod, union, or use item with #[default_runtime]; the attribute only makes sense on definitions that own the generic parameter list being defaulted.

Common situations: Trying to get a defaulted runtime on associated functions or impls instead of the type definition; the attribute left behind after refactoring a type into something else; pasting it at the wrong nesting level.

Related errors

• unexpected list input
• expected "camelCase" or "snake_case"
• expected string literal for rename
• unexpected input, expected one of `rename_all`, `rename`, `r
• unable to use self as a command function parameter

AI-assisted analysis of tauri-apps/tauri@52e4b6e71d (2026-08-20). Data as JSON: /api/errors/302d9e0a1311a219. Report an issue: GitHub.