Skip to content

Improve formatting of "Transmutes" chapter #242

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 1 commit into from
Nov 1, 2020
Merged
Changes from all commits
Commits
File filter

Filter by extension

Filter by extension

Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
14 changes: 9 additions & 5 deletions src/transmutes.md
Original file line number Diff line number Diff line change
Expand Up @@ -15,20 +15,24 @@ boggling.
is going to cause arbitrary chaos that can't really be predicted. Do not
transmute `3` to `bool`. Even if you never *do* anything with the `bool`. Just
don't.

* Transmute has an overloaded return type. If you do not specify the return type
it may produce a surprising type to satisfy inference.
* Transmuting an & to &mut is UB.
* Transmuting an & to &mut is *always* UB.

* Transmuting an `&` to `&mut` is UB.
* Transmuting an `&` to `&mut` is *always* UB.
* No you can't do it.
* No you're not special.

* Transmuting to a reference without an explicitly provided lifetime
produces an [unbounded lifetime]
produces an [unbounded lifetime].

* When transmuting between different compound types, you have to make sure they
are laid out the same way! If layouts differ, the wrong fields are going to
get filled with the wrong data, which will make you unhappy and can also be UB
(see above).

So how do you know if the layouts are the same? For `repr(C)` types and
So how do you know if the layouts are the same? For `repr(C)` types and
`repr(transparent)` types, layout is precisely defined. But for your
run-of-the-mill `repr(Rust)`, it is not. Even different instances of the same
generic type can have wildly different layout. `Vec<i32>` and `Vec<u32>`
Expand All @@ -47,7 +51,7 @@ pointer casts or `union`s, but without any of the lints or other basic sanity
checks. Raw pointer casts and `union`s do not magically avoid the above rules.


[unbounded lifetime]: unbounded-lifetimes.html
[unbounded lifetime]: ./unbounded-lifetimes.md
[transmute]: ../std/mem/fn.transmute.html
[transmute_copy]: ../std/mem/fn.transmute_copy.html
[ucg-layout]: https://rust-lang.github.io/unsafe-code-guidelines/layout.html