Lang item `coerce_unsized` is not implemented.

[P-E-P]

#[lang = "coerce_unsized"]
pub trait CoerceUnsized<T: ?Sized> {
    // Empty.
}

Meta

https://github.com/Rust-GCC/gccrs/commit/611d0bdc55a858736bd5cfabfc1ed4fef800d81a

[P-E-P]

Just seen this is unstable in 1.49, low priority.