Unsized types in required trait methods

[ionicmc-rs]

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

[ionicmc-rs]

@rustbot label T-compiler

[ionicmc-rs]

i will make an rfc as the label suggests

[ionicmc-rs]

here is the rfc

[ionicmc-rs]

@rustbot label -needs-rfc