Parameters in generic associated types do not propagate if unused #144858
Description
I tried this code:
trait Cooker { type Cooked<T>; fn cook<T>(&self, ingredients: T) -> Self::Cooked<T>; fn eat<T>(&self, meal: Self::Cooked<T>); } struct MeaningCook; impl Cooker for MeaningCook { type Cooked<T> = u32; fn cook<T>(&self, _ingredients: T) -> Self::Cooked<T> { 42 } fn eat<T>(&self, _meal: Self::Cooked<T>) { todo!() } } fn cook_and_eat_generic<C: Cooker, T>(cooker: C, ingredients: T) { let meal = cooker.cook(ingredients); cooker.eat(meal); // Compiles } fn cook_and_eat_meaning<T>(cooker: MeaningCook, ingredients: T) { let meal = cooker.cook(ingredients); cooker.eat(meal); // Doesn't compile }I expected to see this happen: The code compiles
Instead, this happened: The non-generic function fails to compile, due to the unused generic parameter of the associated type. The only workaround that works is typing cooker.eat::<()>(meal); (or whatever other type).
Meta
rustc --version --verbose:
rustc 1.90.0-nightly (ba7e63b63 2025-07-29) binary: rustc commit-hash: ba7e63b63871a429533c189adbfb1d9a6337e000 commit-date: 2025-07-29 host: x86_64-unknown-linux-gnu release: 1.90.0-nightly LLVM version: 20.1.8 Backtrace
error[E0282]: type annotations needed --> src/main.rs:28:12 | 28 | cooker.eat(meal); // Doesn't compile | ^^^ cannot infer type of the type parameter `T` declared on the method `eat` | help: consider specifying the generic argument | 28 | cooker.eat::<T>(meal); // Doesn't compile | +++++ For more information about this error, try `rustc --explain E0282`.