Unfixable `clippy::undocumented_unsafe_blocks` in macro output

[CraftSpider]

Summary

When a proc macro emits an unsafe block with a span somewhere inside the crate using it, it triggers the undocumented_unsafe_blocks lint, even if there's documentation on it in the using crate, or if using quote, on the original unsafe block

Lint Name

undocumented_unsafe_blocks

Reproducer

I tried this code:

foo-proc crate

use proc_macro::TokenStream;
use quote::quote_spanned;
use quote::spanned::Spanned;

#[proc_macro]
pub fn foo(ts: TokenStream) -> TokenStream {
    quote_spanned!(proc_macro2::TokenStream::from(ts).__span() => {
        // SAFETY: Foobar
        unsafe { ::core::mem::transmute::<i32, i32>(1) }
    }).into()
}

foo crate

#![warn(clippy::undocumented_unsafe_blocks)]

use foo_proc::foo;

fn main() {
    // SAFETY: Foo
    let _ = foo!(a b c);
}

I saw this happen:

warning: unsafe block missing a safety comment
 --> src/main.rs:7:5
  |
7 |     let _ = foo!(a b c);
  |     ^^^^^^^^^^^^^^^^^^^^
  |
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![warn(clippy::undocumented_unsafe_blocks)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#undocumented_unsafe_blocks
help: consider adding a safety comment
  |
7 ~     // SAFETY: ...
8 +     let _ = foo!(a b c);
  |

I expected to see this happen:

No output. Preferably, there should be no SAFETY comment required in the user's code, as I'd expect it to be the macro's responsibility to handle its safety requirements.

Version

rustc 1.59.0-nightly (8f3238f89 2022-01-02)
binary: rustc
commit-hash: 8f3238f898163f09726c3d2b2cc9bafb09da26f3
commit-date: 2022-01-02
host: x86_64-unknown-linux-gnu
release: 1.59.0-nightly
LLVM version: 13.0.0

Additional Labels

No response