trait-impl-incorrect-safety false-positive with unsafe fields

This requires the unstable #![feature(unsafe_fields)] (https://github.com/rust-lang/rust/issues/132922). (So feel free to close if this should wait until the feature is closer to stabilization).

Implementing Copy for a struct with an unsafe field requires an unsafe impl as of https://github.com/rust-lang/rust/pull/134008.

However, rust-analyzer reports an "unsafe impl for safe trait (trait-impl-incorrect-safety)" error for such an impl.

rust-analyzer version: rust-analyzer version: 0.4.2359-standalone (2e1ff255ae 2025-03-28)

rustc version: rustc 1.87.0-nightly (3f5502370 2025-03-27)

editor or extension: VSCode

code snippet to reproduce:

#![feature(unsafe_fields)]
#![allow(incomplete_features)]

#[repr(transparent)]
pub struct Alpha { unsafe a: u8 }

impl Clone for Alpha {
    fn clone(&self) -> Self { *self }
}

unsafe impl Copy for Alpha {}

Start by reproducing the reported trait-impl-incorrect-safety diagnostic with the unsafe_fields code snippet and the listed Rust nightly version. Trace how rust-analyzer classifies the unsafe impl Copy case, then verify that the valid impl no longer produces a false positive while the diagnostic still catches genuinely invalid unsafe impls.