`unnecessary_cast` false negative with `rust-src` installed

[Catamantaloedis]

Summary

Under certain circumstances, clippy does not detect unnecessary casts when rust-src is installed, whereas it does accurately detect them when that component is removed.

Lint Name

unnecessary_cast

Reproducer

I tried this code:

pub struct Foo(i32);

pub fn clippy_failure() {
    let a: Foo = std::convert::identity(Foo(0));
    let _ = a.0 as i32;
}

I expected to see this happen:

warning: casting to the same type is unnecessary (`i32` -> `i32`)
 --> src/lib.rs:5:13
  |
5 |     let _ = a.0 as i32;
  |             ^^^^^^^^^^ help: try: `a.0`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast
  = note: `#[warn(clippy::unnecessary_cast)]` on by default

Instead, this happened:
There is no such output when component rust-src is installed, but when not installed the output matches above.

Version

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-pc-windows-msvc
release: 1.86.0
LLVM version: 19.1.7

[Alexendoo]

The problem is in is_cast_from_ty_alias, it doesn't hit the issue unless the rust-src rustup component is installed