diagnostics: use rustc_on_unimplemented to recommend `[].iter()` To make this work, the `#[rustc_on_unimplemented]` data needs to be used to report method resolution errors, which is most of what this commit does. Fixes #94581
diagnostics: use rustc_on_unimplemented to recommend `[].iter()` To make this work, the `#[rustc_on_unimplemented]` data needs to be used to report method resolution errors, which is most of what this commit does. Fixes #94581