Unsized types in required trait methods

so this issue is a follow up for #134422 (closed as not planned) to suggest making a lint instead.

What the issue talked about

Rust does not check for T: Sized in required trait methods

trait Foo {
    fn bar(self: Self, x: str);
}

The above code compiles, even though Self and str are both ?Sized

this makes the trait unimplementable

impl Foo for [u8] {
    fn bar(self: Self, x: str) {}
}

Produces:

The size for values of type `[u8]` cannot be known at compile time
The size for values of type `str` cannot be known at compile time

For more information please see the RFC

Meta

for anyone who cares, here is rustc --version --verbose

rustc 1.85.0-nightly (21fe748be 2024-12-11)
binary: rustc
commit-hash: 21fe748be15271ea5804e0507cd699b675efe038
commit-date: 2024-12-11
host: x86_64-unknown-linux-gnu
release: 1.85.0-nightly
LLVM version: 19.1.5

Here is the RFC

Start by reading RFC 3745 and the discussion in issue #134422, then examine the reported examples of required trait methods with unsized Self and parameters. Define the lint's intended diagnostics and applicability, and add coverage demonstrating the unsized cases are reported without making valid methods fail.