Delegation: It's impossible to `reuse` foreign fns

[fmease]

Example:

#![feature(fn_delegation)]

extern "C" {
    fn f();
}

reuse f as g;
//~^ ERROR call to unsafe function `f` is unsafe and requires unsafe function or block

The synthesized fn g ought to be marked unsafe as f is implicitly unsafe¹ but that doesn't happen.

Footnotes

1. Unless marked safe under feature unsafe_extern_blocks, these are possible to reuse for obvious reasons. ↩

[fmease]

Tangentially related: We should probably start wrapping a delegation's target expr (→→ return expr) in an unsafe block to accommodate lint unsafe_op_in_unsafe_fn (allow-by-default in Rust <2024, warn-by-default in Rust 2024):

#![deny(unsafe_op_in_unsafe_fn)]
#![feature(fn_delegation)]

unsafe fn f() {}
reuse f as g; //~ ERROR call to unsafe function `f` is unsafe and requires unsafe block

Edit: Ah, not quite. Forgot about lint unused_unsafe for a sec. We're in a bit of pickle here unless we inspect the lint levels at that node which feels ... hacky?

Edit 2: Well, we could just skip delegation items in both lints.

[fmease]

Ah, furthermore we don't seem to "inherit" the parameters of foreign fns. So it seems like we don't handle foreign fns correctly at all (dropping params and header (safety, ...)).

[workingjubilee]

awesome.