More accurate span for type parameter suggestion After: ``` error[E0229]: associated item constraints are not allowed here --> $DIR/impl-block-params-declared-in-wrong-spot-issue-113073.rs:3:10 | LL | impl Foo<T: Default> for String {} | ^^^^^^^^^^ associated item constraint not allowed here | help: declare the type parameter right after the `impl` keyword | LL - impl Foo<T: Default> for String {} LL + impl<T: Default> Foo<T> for String {} | ``` Before: ``` error[E0229]: associated item constraints are not allowed here --> $DIR/impl-block-params-declared-in-wrong-spot-issue-113073.rs:3:10 | LL | impl Foo<T: Default> for String {} | ^^^^^^^^^^ associated item constraint not allowed here | help: declare the type parameter right after the `impl` keyword | LL | impl<T: Default> Foo<T> for String {} | ++++++++++++ ~ ```
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.