Do not use as to convert reference to pointer

[SoniEx2]

• I have checked the latest main branch to see if this has already been fixed
• I have searched existing issues and pull requests for duplicates

URL to the section(s) of the book with this problem:

https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html#dereferencing-a-raw-pointer

Description of the problem:

This uses as to convert the reference to a pointer. We've ran into issues in the past with as, e.g. RustyYato/cell-project#4. Specifically, it breaks inference in a bad way.

Suggested fix:

Use type ascription instead of as, e.g. let r1: *const i32.

[SoniEx2]

we can't remember what we were doing where it broke inference... we're struggling to figure out a test case which demonstrates it tho. sorry about that.

[SoniEx2]

found it: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=4a7780954f610b8c4adff2d19925b824

(ps: we believe this example to be unsound, specifically due to the Drop impl, but we digress. the main thing we're trying to point out is the difference between as and type ascription and the Drop impl doesn't actually matter for that.)

[chriskrycho]

Following up on this as we’re prepping for the 2024 Edition update to the book—I honestly am having a hard time figuring out from that code exactly what the issue is—I don’t see type inference issues when I try to build it. Do you have some code which exercises it which does show the issue?

[SoniEx2]

as is bad because it does too many things, particularly in the context of macros.

as can be used to transmute, to coerce, and to convert. (and one of those is not like the others...)

deprecating as would solve issues.

[chriskrycho]

Those may all be true, but it’s not clear to me from merely those assertions how a reader of the book should think about them in Rust today, where as does exist.

[SoniEx2]

ah, yeah...

personally we'd rather pretend as if as doesn't exist but something tells us you'd rather not do that. maybe you can acknowledge it as a footgun somehow?