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Add lint against ptr-to-int transmutes in consts #87525

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RalfJung opened this issue Jul 27, 2021 · 7 comments · Fixed by #130540
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Add lint against ptr-to-int transmutes in consts #87525

RalfJung opened this issue Jul 27, 2021 · 7 comments · Fixed by #130540
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@veera-sivarajan
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A-const-eval Area: Constant evaluation (MIR interpretation)

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@RalfJung
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RalfJung commented Jul 27, 2021 •
edited by rustbot
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We do not allow ptr-to-int casts in const context, so people might be tempted to do a transmute instead. This will not work, and if they ever actually test their code during CTFE they will get an error, but it might still be good to add an extra layer of defense here (since this is a rather odd corner of the language) and have a lint for this.

@jhpratt started working on this, here's a link.
@RalfJung
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Cc @rust-lang/wg-const-eval

@jhpratt do you want to work on this, or leave this issue open for anyone to pick up?

@jhpratt
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jhpratt commented Jul 27, 2021

Someone else can pick this up, as I've got other priorities for the near future. As I mentioned feel free to use the code linked.

It's worth noting that a lint would ideally also catch transmutes that use a union, though that would be quite a bit more difficult I imagine.

@RalfJung
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It's worth noting that a lint would ideally also catch transmutes that use a union, though that would be quite a bit more difficult I imagine.

That'll require a lot of complexity, for a case that I assume will be exceedingly rare. So I don't think that is effort, lines of code, and compile time well spent.

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jhpratt commented Jul 27, 2021

Agreed. Just throwing it out there.

@oli-obk
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oli-obk commented Jul 28, 2021

More context on the lint can be found in the discussion starting at #85769 (comment)

@clubby789
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@rustbot label +A-const-eval

@rustbot rustbot added the A-const-eval Area: Constant evaluation (MIR interpretation) label Mar 30, 2023
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@rustbot claim

@veera-sivarajan veera-sivarajan mentioned this issue Sep 19, 2024
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@bors bors closed this as completed in daebce4 Oct 6, 2024
flip1995 pushed a commit to flip1995/rust that referenced this issue Oct 18, 2024
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Auto merge of rust-lang#130540 - veera-sivarajan:fix-87525, r=estebank
d050730 
Add a Lint for Pointer to Integer Transmutes in Consts

Fixes rust-lang#87525

This PR adds a MirLint for pointer to integer transmutes in const functions and associated consts. The implementation closely follows this comment: rust-lang#85769 (comment). More details about the implementation can be found in the comments.

Note: This could break some sound code as mentioned by RalfJung in rust-lang#85769 (comment):

> ... technically const-code could transmute/cast an int to a ptr and then transmute it back and that would be correct -- so the lint will deny some sound code. Does not seem terribly likely though.

References:
1. https://doc.rust-lang.org/std/mem/fn.transmute.html
2. https://doc.rust-lang.org/reference/items/associated-items.html#associated-constants
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Add a Lint for Pointer to Integer Transmutes in Consts veera-sivarajan/rust
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@RalfJung @oli-obk @jhpratt @clubby789 @veera-sivarajan @rustbot