Add lint against function pointer comparisons

[Urgau]

This is kind of a follow-up to #117758 where we added a lint against wide pointer comparisons for being ambiguous and unreliable; well function pointer comparisons are also unreliable. We should IMO follow a similar logic and warn people about it.

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unpredictable_function_pointer_comparisons

warn-by-default

The unpredictable_function_pointer_comparisons lint checks comparison of function pointer as the operands.

Example

fn foo() {}
let a = foo as fn();

let _ = a == foo;

Explanation

Function pointers comparisons do not produce meaningful result since they are never guaranteed to be unique and could vary between different code generation units. Furthermore different function could have the same address after being merged together.

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This PR also uplift the very similar clippy::fn_address_comparisons lint, which only linted on if one of the operand was an ty::FnDef while this PR lints proposes to lint on all ty::FnPtr and ty::FnDef.

@rustbot labels +I-lang-nominated

Edit: Blocked on rust-lang/libs-team#323 being accepted and it's follow-up pr

[rustbot]

r? @cjgillot

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[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

The Miri subtree was changed

cc @rust-lang/miri

[Noratrieb]

The lint name is very long, it would be nice if we could come up with a shorter name. I don't have a suggestion though.

[Urgau]

The lint name is very long, it would be nice if we could come up with a shorter name. I don't have a suggestion though.

Sure, but I don't think it matter that much, I'm not expecting anyone to write the lint name by hand (since it's warn-by-default, people can just copy/paste in the diagnostic output). We also already have some pretty long lint name, like unknown_or_malformed_diagnostic_attributes.
I also think it's better to have a long and descriptive lint name than a short one that is less descriptive.

But if someone comes up with a name as descriptive but shorter I can integrate it, I'm just not sure it's worth anyone time.

[compiler-errors]

If it's not too much work, could we do a crater run here to find out what kind of places are using this kind of comparison in the wild?

[Urgau]

If it's not too much work, could we do a crater run here to find out what kind of places are using this kind of comparison in the wild?

Sure, I just pushed a commit making the lint deny-by-default. I will let you do the bors+craterbot commands.

[compiler-errors]

@bors try

[bors]

⌛ Trying commit 6c5c546 with merge 2959657...

[bors]

☀️ Try build successful - checks-actions
Build commit: 2959657 (29596578cb5035fa2b7ac7fbdfea23fab38836e7)

[traviscross]

@rustbot labels -I-lang-nominated

We discussed this in the T-lang meeting today. There was some support for this, but people were concerned about whether there may be legitimate use cases and what we would be suggesting that those people do instead. For wide pointer comparisons, we suggest that people use ptr::addr_eq even though that has the same problems as comparison with == as it at least makes the intent of the user clear. What would be the comparable thing we would do here?

The consensus was that seeing use cases would help, and that a crater run would help to find those use cases. That's now happening in the comments above. Once the crater run comes back and has been analyzed, please renominate for T-lang discussion.

For the analysis, we're looking to find 1) use cases of this that are correct in the sense that they rely only on the actual semantics, and 2) the prevalence of bugs where people are using this in ways that rely on semantics that do not actually hold.

[RalfJung]

What would be the comparable thing we would do here?

There's no such thing, comparing Rust functions for equality is in general just inherently meaningless. That doesn't make code using == any less buggy though. If we want to support that usecase we need a fundamental language change, likely removing the LLVM flag which tells the backend that function addresses do not matter.

The flip side of this is that the standard library itself actually relies on such a comparison in the format-args machinery... it's for a special case (a monomorphic function) where the current implementation AFAIK does not generate duplicates (but I don't know how multi-CGU handling works so I am not sure). It's certainly not guaranteed by the language.

[compiler-errors]

@craterbot check

[craterbot]

👌 Experiment pr-118833 created and queued.
🤖 Automatically detected try build 2959657
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[traviscross]

What would be the comparable thing we would do here?

There's no such thing, comparing Rust functions for equality is in general just inherently meaningless.

One thought expressed in the meeting was as follows:

Comparing functions for equality via pointers may yield false negatives but not false positives. The fact that two functions may compare unequal (based on pointers to them) but in fact do the same thing is a rather fundamental property of not just Rust, but of any language. In general, it's impossible to know whether two different functions may in fact do the same thing.

In this light, maybe it's OK that these comparisons produce false negatives, and maybe there exist valid use cases that only rely on the property that we will not return false positives.

[RalfJung]

Comparing functions for equality via pointers may yield false negatives but not false positives.

That's not true. There are both false negatives and false positives. That's exactly why I wanted the lint description to be clear about this.

False positives arise when LLVM merges two functions because they optimized to the same code.

[nikomatsakis]

@rfcbot reviewed

[tmandry]

rfcbot doesn't seem to have registered my checkbox, so

@rfcbot reviewed

[cjgillot]

A few nits and r=me

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[Urgau]

Hooray 🎉

This is now just waiting on the FCP for ptr::fn_addr_eq to complete.

@rustbot label -S-waiting-on-author +S-blocked

[nikomatsakis]

@rfcbot reviewed