`Self` of negative impl is not required to be well-formed

feature(negative_impls) tracking issue: https://github.com/rust-lang/rust/issues/68318

I'm not sure if this is intended behavior or not.

I tried this code:

#![feature(negative_impls)]

struct Foo<T: Sized>(T);

trait MyTrait {}

impl !MyTrait for Foo<str> {} // Foo requires T: Sized, but str: !Sized

I expected to see this happen: error[E0277]: the size for values of type `str` cannot be known at compilation time

Instead, this happened: successful compilation (playground)

Meta

rustc --version --verbose:

rustc 1.95.0-nightly (a423f68a0 2026-02-13)
binary: rustc
commit-hash: a423f68a0d75656eabab4982a06bed9f316f7109
commit-date: 2026-02-13
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 22.1.0

no backtrace

@rustbot label F-negative_impls requires-nightly

Start by compiling the provided negative_impls reproducer on the reported nightly and compare its behavior with the linked negative_impls tracking issue. Trace how the compiler checks the Self type in impl !MyTrait for Foo<str>; done means the invalid str argument is rejected with the expected well-formedness error.