add lint deref_nullptr detecting when a null ptr is dereferenced

[ABouttefeux]

fixes #83856
changelog: add lint that detect code like

unsafe {
      &*core::ptr::null::<i32>()
 };
unsafe {
     addr_of!(*std::ptr::null::<i32>())
};
let x: i32 = unsafe {*core::ptr::null()};
let x: i32 = unsafe {*core::ptr::null_mut()};
unsafe {*(0 as *const i32)};
unsafe {*(core::ptr::null() as *const i32)};

warning: Dereferencing a null pointer causes undefined behavior
 --> src\main.rs:5:26
  |
5 |     let x: i32 = unsafe {*core::ptr::null()};
  |                          ^^^^^^^^^^^^^^^^^^
  |                          |
  |                          a null pointer is dereferenced
  |                          this code causes undefined behavior when executed
  |
  = note: `#[warn(deref_nullptr)]` on by default

Limitation:
It does not detect code like

const ZERO: usize = 0;
unsafe {*(ZERO as *const i32)};

or code where 0 is not directly a literal

[rust-highfive]

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[jyn514]

I thought the plan was to uplift the existing clippy lint?

[jyn514]

r? @RalfJung

[RalfJung]

Thanks a lot. :)
It might take until the weekend that I have time to review this. However, one thing already: please add a test file in src/test/ui. See the docs for how to write a test.

The CI failure is caused by that file crossing the threshold of 3000 lines... I am not sure what to do about that though, so adding // ignore-tidy-filelength to the file seems okay to me. 🤷

[RalfJung]

I thought the plan was to uplift the existing clippy lint?

Yeah it is at least the easiest option I think... @ABouttefeux did you use the clippy lint as a basis or write this from scratch? It would be good to at least ensure that we catch all the same cases. Clippy will likely have tests that you can also use as inspiration.

[ABouttefeux]

I wrote it from scratch. I will look at the clippy lint.

[ABouttefeux]

Also from what I see the lint clippy::zero_ptr is different from what is described in #83856.
From clippy's documentation

What it does

Catch casts from 0 to some pointer type

Example

// Bad
let a = 0 as *const u32;

// Good
let a = std::ptr::null::<u32>();

[RalfJung]

@ABouttefeux good point, that lint is not what we are looking for. Thanks for checking. :)

[RalfJung]

changelog: add lint that detect code like

Would be good to also mention &*core::ptr::null() and addr_of!(core::ptr::null()) in your examples (and have them in the test), just to drive home the point that the lint fires even in place expression context.

[RalfJung]

This looks good to me. :)
I'll inquire if any further approval is needed to add a new lint.

[ABouttefeux]

Thank you for your help 🎉

[RalfJung]

Thanks for working on this. ❤️

[RalfJung]

Btw, does this also detect addr_of!((*ptr::null::<Struct>()).field)?

if not, that could be an interesting future extension of this lint. :) If yes, please add a test. :D

[ABouttefeux]

I think it should detect that. I will add a test

[apiraino]

@scottmcm which team the I-nominated is relevant to? Perhaps the T-compiler? (asking because in case, this issue can be mentioned tomorrow during the compiler team meeting). Specifically, what should be discussed? Whether this should be a clippy lint?

(thanks for the additional context)

[RalfJung]

I was told lints are lang team territory so setting the flag accordingly.

[RalfJung]

@scottmcm wrote on Zulip

Personally, I think it should just go in because it's not a one-way door -- we can always tweak it (behaviour, severity, or even remove it entirely) later if needed, so I don't see a strict need for an FCP. I'll motion that you can r+ it on my lang member "seems reasonable", the same way unstable library items can go in on a libs member "seems reasonable".

So, let's go with this then. :) @bors r+

[bors]

📌 Commit 7f0f83a has been approved by RalfJung

[bors]

⌛ Testing commit 7f0f83a with merge 7537b20...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing 7537b20 to master...

[rust-highfive]

📣 Toolstate changed by #83948!

Tested on commit 7537b20.
Direct link to PR: #83948

💔 miri on windows: test-pass → test-fail (cc @oli-obk @RalfJung @eddyb).
💔 miri on linux: test-pass → test-fail (cc @oli-obk @RalfJung @eddyb).

[WaffleLapkin]

PR description is a bit missleading:

unsafe {
     addr_of!(std::ptr::null::<i32>())
};

Doesn't trigger the lint (there is no dereference), moreover this doesn't compile (cannot take address of a temporary).

What does trigger the lint is

unsafe {
     addr_of!(*std::ptr::null::<i32>())
     //       ^ deref
};

[RalfJung]

@WaffleLapkin thanks for pointing that out, I fixed the PR description.