Assume that any external function might return a type alias

[samueltardieu]

We might not have the declaration around if extern crate is used, or if rust-src is not installed for the standard library. Better err on the safe side rather than having a behavior depending on what happens to be available at compile time.

Fixes #14625

changelog: [unnecessary_cast]: do not warn on casts of external function return type

[samueltardieu]

Draft for the time being, in order to get a lintcheck run.

[github-actions]

Lintcheck changes for 759c2bf

Lint	Added	Removed	Changed
clippy::unnecessary_cast	4	0	1

This comment will be updated if you push new changes

[samueltardieu]

Well, lintcheck shows that we detect more cases by doing it more cleanly. We could also exclude code behind rust-src if we have a way to detect this easily, but this might be overkill.

Undrafting.

[rustbot]

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[Jarcho]

The lint emission in libc is wrong. cmsg_len is defined as size_t.

[samueltardieu]

The lint emission in libc is wrong. cmsg_len is defined as size_t.

It is just a manifestation of an existing shortcoming which already exists and was masked in libc at just this line for whatever reason. Other casts in this file were already detected the same way. There are no attempts to detect type aliases in struct fields or tuple definitions.

The following program exhibits this shortcoming both before and after this PR:

#![allow(unused)]
#![deny(clippy::unnecessary_cast)]

type I = i32;
struct S {
    i: I,
}

fn main() {
    let s = S { i: 42 };
    let i = s.i as i32;
}

$ clippy-driver t.rs
error: casting to the same type is unnecessary (`i32` -> `i32`)
  --> t.rs:11:13
   |
11 |     let i = s.i as i32;
   |             ^^^^^^^^^^ help: try: `s.i`