Unable to use select macro in #![forbid(unsafe_code)] environments

[karroffel]

Description

I am using #![forbid(unsafe_code)] in my crate and for a while I only had a single recv() inside of a select macro and everything was compiling fine.

When I added a second recv() arm it failed to compile.

I switched to using the Select struct for now and it compiles and works properly.

I expected the select! macro to only use the safe API of Select, not emitting unsafe code inside the macro.

Example code

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=efee4be440204c5c160827d5fa1de5fb

[taiki-e]

if the macro is completely safe, I think we should remove #[allow(unsafe_code)].

crossbeam/crossbeam-channel/src/select_macro.rs

Line 925 in ae148de

#[allow(unsafe_code)]

On the other hand, if the macro is not safe, it seems reasonable to use #[allow(unsafe_code)] (prevents unsafe macros from being used on the crate that uses #![forbid(unsafe_code)]).

(The current behavior of lint is a little odd, see rust-lang/pin-utils#2 for related discussion.)

[extrawurst]

I stumbled on this aswell. what can be done about it? I end up using Select manually instead of the macro because that works fine with #![forbid(unsafe_code)] - the least would be to document this.