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Rust mutable union allows write to field w/o needing unsafe block in stable ( Documentation issue? ) #2095

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@DanielJoyce

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@DanielJoyce

https://play.rust-lang.org/?gist=beaba7cc10b93f3e6ac1d97717099f26&version=undefined

The code produces a warning:


   Compiling playground v0.0.1 (file:///playground)
warning: unnecessary `unsafe` block
 --> src/main.rs:9:5
  |
9 |     unsafe { u.f1 = 5 };
  |     ^^^^^^^^^^^^^^^^^^^ unnecessary `unsafe` block
  |
  = note: #[warn(unused_unsafe)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.55 secs
     Running `target/debug/playground`

Saying the unsafe block is not needed for a write.

But the rfc says writes to union mutable fields require a unsafe block

https://github.com/rust-lang/rfcs/blob/master/text/1444-union.md#writing-fields

Found here:

https://www.reddit.com/r/rust/comments/6rcegm/unsafe_field_accesses_for_unions/

On second reading, the RFC says "May" which is not the same as "Shall". Which seems weird. Do you HAVE to use unsafe code to write mutable union fields? Is it optional? When is it needed/not needed?

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