lint ImproperCTypes: overhaul (take 2 of "better handling of indirections") #134697

niacdoial · 2024-12-23T17:59:31Z

This PR started as a try to re-add the changes of #131669 (reverted in #134064 after one (1) nightly)

It has since evolved in an overhaul of the ImproperCTypes lints, splitting the two lints into four:

one for "imported" items in extern "ABI" blocks
one for "exported" extern "ABI" functions
one for "exported" #[no_mangle]/1[export_name=_] static variables
one for extern "ABI" callbacks (ty::FnPtr arguments/return types/ADT fields)

It also allows for "detailed" lint messages, with successive notes that amount to "this is unsafe because of that, which is unsafe because of that, which is unsafe for this reason", and possible help messages at every step.

Hopefully the overall architecture of the lint is something less special-case-y, with better-separated abstractions.

the comment containing the most recent summary of the decisions and considerations is #134697 (comment)

oh, also, since this was already fixing it for the wrong reasons, I decided to add something that fixes it for the right ones:
Fixes #130310

(leaving this in because it was in the original version of this description:)
r? workingjubilee (because you reviewed the first attempt)

niacdoial · 2024-12-23T18:03:42Z

(
hi Jubilee, I'm back at this again!
I know this is not the best time of year to add PRs, so I'm fine with postponing this if you don't feel like tackling it these upcoming weeks.
In any case, have some nice end-of-year festivities, if you celebrate any!
)

niacdoial · 2024-12-23T18:08:24Z

ah, and before I forget: a small part of the new test file is commented out because it hits ICE #134587, but there should be more than decent coverage anyway

workingjubilee · 2024-12-24T09:16:24Z

unfortunately the lint needs to be gutted and rewritten.

workingjubilee · 2024-12-24T09:18:44Z

Also while I was possibly having a mild case of get-there-itis and thus mostly tried to just make sure things were coherent, I would prefer all new code for the lint be in compiler/rustc_lint/src/types/improper_ctypes.rs.

workingjubilee · 2025-01-04T18:24:23Z

The version cut happened so there will be less time pressure now.

niacdoial · 2025-01-05T00:29:14Z

I have a first version for compiler/rustc_lint/src/types/improper_ctypes.rs if you want.
it's less of a from-the-ground-up rewrite as it is scrapping the original for parts, if the analogy makes sense.

you probably have things to say about its architecture, even if the whole thing still have a bunch of TODO comments
the progress so far looks like this:

completely separate the type-checking and reporting systems
(part of the way there) remove some of the special cases and integrate them to the "main logic"
- check_for_opaque_types is still a "special case" part of the checking logic
- Cstr and Cstring are also somewhat special-cased because the advice for them depends on the type around them, if any
- the unit type is handled in multiple places, see if this can be fixed
(almost complete) compile, pass existing tests
- only failed tests are for Cstring, due to different error messages
- one unrelated test had to have a second "#[allow(improper_ctypes)]" added, but it makes more sense for it to need that anyway
better separation of the different checks in different visit_* methods of ImproperCTypesVisitor
better tracking of how the currently-checked type is used (static, function argument, function return's inner type, etc...)
- raises questions about the separation of improper_ctypes and improper_ctypes_definitions versus declared/defined functions, especially when FnPtr:s are involved
allow single argument check to emit multiple errors (for fnptr:s, structures with multiple FFI-unsafe fields, etc)
review what is considered FFI-safe or not (once everything else is complete)

If you want to take a look in this state, should I just commit it here? (possibly put the PR in draft mode while I'm at it?)
or send you the files in a different way?

bors · 2025-01-15T12:53:21Z

☔ The latest upstream changes (presumably #135525) made this pull request unmergeable. Please resolve the merge conflicts.

niacdoial · 2025-01-18T20:20:25Z

aaaand I think I have something that's "first draft" material! (should I put this pull in the "draft" state?)
There's still a bunch of ~~TODOs~~ (...well, they were changed to FIXMEs to be pushed here) for questions I didn't manage to answer (and a bunch of failing tests because I don't know if the error should be here), but yeah.

here's a list of some of my remaining questions and concerns:

visit_numeric seems too x86_64-specific
should we revisit the distinction between ImproperCTypes and ImproperCTypesDefinitions?
- part 1: the output ("external fn" or vs "external block" vs other possibilities)
- part 2: handling opaque types (there's a high correlation between ImproperCTypesDefinitions and places where we allow FFI-opaque types to be fully specified. do we want this correlation to be 1?)
more on FFI-opaque types: how do we handle that in the context of the "context switch" between functions and possible FnPtr arguments? The answer that seems correct currently prevents a stage1 compiler from being built
- should we introduce a std::ffi::FfiOpaquePtr type? (which would be a *const c_void and some phantomdata, on first approximation)
for indirections whose values may be supplied by non-rust code: do we only allow pointers (and Optionstd::ptr::NonNull), or do we also allow Option<&T> and Option<Box<T>>?
not sure if the new error messages are intelligible in all cases (especially if there's a type param like Self or <Self as ::std::ops::Add<Self>>::Output that gets resolved in the error message).
it feels like the current handling of CStr/Cstring and Option-like enums uses special casing, since those are tested for in multiple places.
if we deny references and boxes in defined functions, what of &self in methods? We don't allow *const Self, last I checked.

bors · 2025-01-23T18:22:31Z

☔ The latest upstream changes (presumably #135921) made this pull request unmergeable. Please resolve the merge conflicts.

workingjubilee · 2025-01-31T05:14:25Z

hmm.

workingjubilee · 2025-01-31T05:15:35Z

aaaand I think I have something that's "first draft" material! (should I put this pull in the "draft" state?)

Yes, it's a good marker for "I don't want this merged yet, even if it looks done".

workingjubilee · 2025-02-20T18:35:04Z

visit_numeric seems too x86_64-specific

It probably is.

should we revisit the distinction between ImproperCTypes and ImproperCTypesDefinitions?

Yes, but in particular, not to just repartition them between: I think breaking them into as many conceptually-smaller lints as possible is good, as long as each one is a distinct idea (no splitting just for the sake of splitting!).

more on FFI-opaque types: how do we handle that in the context of the "context switch" between functions and possible FnPtr arguments? The answer that seems correct currently prevents a stage1 compiler from being built

I'm not sure what you mean?

for indirections whose values may be supplied by non-rust code: do we only allow pointers (and Option<std::ptr::NonNull>), or do we also allow Option<&T> and Option<Box<T>>?

We must allow Rust code to declare a pointer in a C signature to be Option<&T> or a number of things about our FFI story fall apart.

it feels like the current handling of CStr/CString and Option-like enums uses special casing, since those are tested for in multiple places.

Yes, probably.

workingjubilee

some initial nits on a first pass

compiler/rustc_lint/src/types/improper_ctypes.rs

workingjubilee · 2025-02-20T21:45:48Z

compiler/rustc_lint/src/types/improper_ctypes.rs

+                        // but for some reason one can just go and write function *pointers* like that:
+                        // `type Foo = extern "C" fn(::std::ffi::CStr);`


Because unsized function parameters are something we may want to support.

The code may not be well-formed: as you may have noticed at some point, you get warnings even if you get errors (usually), and this is because we lint even on "bad" code. This is because rustc didn't use to, once upon a time, and it was a bad debugging experience.

compiler/rustc_lint/src/types/improper_ctypes.rs

niacdoial · 2025-02-27T22:12:58Z

alright, sorry for taking a while!

I'm currently planning what changes I'll do in terms of splitting the lint(s)
my current idea is to separate based on the nature of the thing being (presumably) propped up against a FFI boundary

improper_ctypes: what's definitely an interface to an outside library (extern statics, extern function declarations)
improper_ctypes_fn_definitions: functions written in rust intended to be exported
improper_ctypes_callbacks: FnPtr arguments, no matter in what function they are being used
improper_ctypes_ty_definitions: repr(C) structs/enums(/unions)?

more on FFI-opaque types: how do we handle that in the context of the "context switch" between functions and possible FnPtr arguments? The answer that seems correct currently prevents a stage1 compiler from being built

I'm not sure what you mean?

well, this is more or less answered in what I said before that, but my question was about how to deal with "switching" from checking arguments for, say, a function definition, to checking the arguments of a FnPtr argument?

should the nature of the lint change?
(temptative answer: yes)
how should FFI-Safe-pointers-to-FFI-Unsafe-pointees work in FnPtr arguments? Should it be the rules for extern fn declarations? (throw the lint because one should use *const c_void, an extern type declaration, etc...) or the rules for extern fn definitions? (allow that, the function's body needs the full type even if it's opaque to the other side of the FFI boundary)
(temptative answer: it should be the former, but parts of the rustc codebase doesn't follow this rule, so I can't get a stage1 compiler if I make that the rule)

// you would think that int-range pattern types that exclude 0 would have Option layout optimisation
// they don't (see tests/ui/type/pattern_types/range_patterns.stderr)
// so there's no need to allow Option<pattern_type!(u32 in 1..)>.

oh, I should fix that probably

I... maybe? I can't for the life of me find the link to that again but I think I saw a discussion about that and type covariance/contravariance,
where i32 is 1.. is a subtype of i32 (well that was under consideration), meaning fn(Option<i32>) is a type of fn(Option<i32 is 1..>) and it might have impacts on whether there should be an optimisation because of transmutation?

Though you'll definitely know more than me on all the moving parts.
Especially assuming you might have looked at this more in the past week.

As for the rest of your advice, I already took all this in!
thanks for shedding light on my code, one nit at a time!

visitation steps [does not pass tests]

- removed special-case logic for a few cases (including the unit type, which is now only checked for in one place) - moved a lot of type-checking code in their dedicated visit_* methods - reworked FfiResult type to handle multiple diagnostics per type (currently imperfect due to type caching) - reworked the messages around CStr and CString

…ed from non-rust code

- now the lint scans repr(C) struct/enum/union definitions - it now also scans method declarations in traits - many other changes in the underlying logic - some extra tests

- also fix a couple of thorny typos in/around types.rs::is_outer_optionlike_around_ty() and subsequently fix library and tests

- do not check ADT definitions themselves, it turns out `repr(C)` is not a strong enough signal to determine if something is designed for FFIs - however, start checking static variables with `#[no_mangle]` or `#[export_name=_]`, even if that's not a perfect signal due to the lack of specified ABI - some changes to the LLVM codegen so it can be seen as FFI-safe

for now, let's fully remove this lint. it might be reintroduced later as a way to make the lints ignore the inside of some ADT definitions

- make clippy happy about the changes in rust_codegen_llvm - split a test stderr into 32bit and 64bit - fix some documentation

rustbot · 2025-08-06T22:09:13Z

This PR modifies tests/ui/issues/. If this PR is adding new tests to tests/ui/issues/,
please refrain from doing so, and instead add it to more descriptive subdirectories.

compiler-builtins is developed in its own repository. If possible, consider making this change to rust-lang/compiler-builtins instead.

cc @tgross35

niacdoial · 2025-08-07T09:47:43Z

re: not making changes in compiler-builtins:
I'll be able to ensure that once rust-lang/compiler-builtins#1006 lands
(thanks @/tgross35 for handling it before I could try to!)

tgross35 · 2025-08-07T17:19:42Z

re: not making changes in compiler-builtins: I'll be able to ensure that once rust-lang/compiler-builtins#1006 lands (thanks @/tgross35 for handling it before I could try to!)

For future reference, it's fine for small changes like this to go through rust-lang/rust. I just was curious about the results since i128 got removed from improper_ctypes a while ago, and wanted a CI run in that repo.

it would be correct to lint on those, but this is deemed too steep a change for now, especially for projects that turn all warnings into errors

- relaxed the uninhabited types allowed on function return

bors · 2025-08-10T18:03:15Z

☔ The latest upstream changes (presumably #145210) made this pull request unmergeable. Please resolve the merge conflicts.

tgross35

A meta question that may be somewhere in this discussion: could the current names be kept around as groups? E.g. make improper_ctypes_definitions a group for improper_c_fn_definitions and improper_c_callbacks, to help avoid the ecosystem churn.

It would be very helpful if you could update the PR message to summarize the user-visible changes from this, mentioning the specific lint names and maybe tiny examples.

Also, does this happen to fix #132699? I think it might but didn't see a test or Fixes: tag.

question, @workingjubilee: Do I rebase this branch now? I have no idea if you (or someone else) is in the middle of a review, and I would rather avoid changing the code while you're looking at it. But at the same time, you might be waiting for me to rebase to do another round of review so... yeah.

Yeah that triagebot warning is far too noisy. I don't really think there's much need to rebase unless there are conflicts (not really tests) or if something about the code you're working on has changed.

However, you may want to re-split your commits / squash some so they each only contains active code. I started doing a breeze through review and since this is huge started to do it by commit, only to find out that a lot of the code from the first commits is completely gone 🙃

tgross35 · 2025-08-14T20:50:10Z

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

    pub(crate) fn LLVMRustBuildMinNum<'a>(
        B: &Builder<'a>,
        LHS: &'a Value,
-        RHS: &'a Value,
+        LHS: &'a Value,
    ) -> &'a Value;
    pub(crate) fn LLVMRustBuildMaxNum<'a>(
        B: &Builder<'a>,
        LHS: &'a Value,
-        RHS: &'a Value,
+        LHS: &'a Value,
    ) -> &'a Value;



What's going on with this change (two args named LHS)?

tgross35 · 2025-08-14T20:55:20Z

library/std/src/panicking.rs

-#[allow(improper_ctypes)]
-unsafe extern "C" {
+unsafe extern "Rust" {
    #[rustc_std_internal_symbol]
    fn __rust_panic_cleanup(payload: *mut u8) -> *mut (dyn Any + Send + 'static);
 }


Changing the ABI here is orthogonal to the lint change, could you drop this part?

tgross35 · 2025-08-14T20:56:04Z

src/tools/rust-analyzer/crates/ide-db/src/generated/lints.rs

AIUI, you don't need to do changes to this file by hand since they get autogenerated at sync time

tgross35 · 2025-08-14T20:59:02Z

tests/ui/extern/extern-C-non-FFI-safe-arg-ice-52334.rs

 extern "C" {
    fn meh(blah: Foo);
-    //~^ WARN `extern` block uses type
+    // ^ FIXME: the error isn't seen here but at least it's reported elsewhere


Nit: Always using something like FIXME(ctypes) makes it much easier to find related changes among the thousands of FIXMEs

tgross35 · 2025-08-14T21:00:06Z

tests/ui/lint/improper_ctypes/allow_improper_ctypes.rs

+//@ aux-build: outer_crate_types.rs
+//@ compile-flags:--extern outer_crate_types
+extern crate outer_crate_types as outer;


extern_crate_types maybe?

tgross35 · 2025-08-14T21:02:02Z

tests/ui/lint/improper_ctypes/lint-94223.rs

We're trying to get away from tests named by numbers, so make sure any new files get a descriptive name (f not also a one line description in the file). An // Issue: https://github.com/rust-lang/rust/issues/94223 comment is the loose convention for keeping things linked.

tgross35 · 2025-08-14T21:02:47Z

tests/ui/lint/improper_ctypes/lint-tykind-fuzz.rs

+#[repr(C)]
+#[derive(Clone,Copy)]
+struct TemplateStruct<T> where T: std::ops::Add+Copy {
+   one: T,
+   two: T,
+}
+impl<T: std::ops::Add+Copy> TemplateStruct<T> {
+    type Out = <T as std::ops::Add>::Output;
+}


Nit: run rustfmt on this file, looks like indentation is mixed

tgross35 · 2025-08-14T21:04:30Z

tests/ui/rfcs/rfc-2008-non-exhaustive/improper_ctypes/same_crate_proper.rs

-#[non_exhaustive]
-#[repr(C)]
-pub struct UnitStruct;
+//#[non_exhaustive]
+//#[repr(C)]
+//pub struct UnitStruct;


Why was this commented?

tgross35 · 2025-08-14T21:23:20Z

This adds more visitors so it should get a perf run at some point (requires a conflict-free merge to master)

rustbot assigned workingjubilee Dec 23, 2024

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niacdoial added 15 commits August 7, 2025 00:09

lint ImproperCTypes: fix TypeSizedness code

175b708

lint ImproperCTypes: add test to cover all ty_kinds

d5dbf9a

lint ImproperCtypes: move code to new place, prior to full rewrite

b2195b1

lint ImproperCTypes: properly separate the lint generation from the type

2dcfd51

visitation steps [does not pass tests]

lint ImproperCTypes: add considerations for which values can be sourc…

df85b22

…ed from non-rust code

lint ImproperCTypes: smoothing out some nits and un-tidy-ness

efe5695

lint ImproperCTypes: add recursion limit

a37e0c5

lint ImproperCTypes: split the two lints into four total [...]

cde6ad5

- now the lint scans repr(C) struct/enum/union definitions - it now also scans method declarations in traits - many other changes in the underlying logic - some extra tests

lint ImproperCTypes: deal with uninhabited types

7810cd2

lint ImproperCTypes: redo handling of pattern types [...]

d15dbb5

- also fix a couple of thorny typos in/around types.rs::is_outer_optionlike_around_ty() and subsequently fix library and tests

lint ImproperCTypes: rm improper_ctype_definitions [...]

9dedb0c

for now, let's fully remove this lint. it might be reintroduced later as a way to make the lints ignore the inside of some ADT definitions

lint ImproperCTypes: clean up exported static variables

c105896

lint ImproperCTypes: changes to pass CI [...]

06aeed6

- make clippy happy about the changes in rust_codegen_llvm - split a test stderr into 32bit and 64bit - fix some documentation

niacdoial force-pushed the linting-ptrdyn-ffi branch from 754eaee to f19f4da Compare August 6, 2025 22:09

rustbot added A-compiler-builtins Area: compiler-builtins (https://github.com/rust-lang/compiler-builtins) T-clippy Relevant to the Clippy team. labels Aug 6, 2025

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niacdoial added 2 commits August 8, 2025 00:44

lint ImproperCTypes: remove the parts about checking value assumptions

02d8c95

it would be correct to lint on those, but this is deemed too steep a change for now, especially for projects that turn all warnings into errors

lint ImproperCTypes: misc. changes due to review

0d80e12

- relaxed the uninhabited types allowed on function return

niacdoial force-pushed the linting-ptrdyn-ffi branch from c414d41 to 0d80e12 Compare August 7, 2025 22:44

tgross35 reviewed Aug 14, 2025

View reviewed changes

		// but for some reason one can just go and write function pointers like that:
		// `type Foo = extern "C" fn(::std::ffi::CStr);`

lint ImproperCTypes: overhaul (take 2 of "better handling of indirections") #134697

Are you sure you want to change the base?

lint ImproperCTypes: overhaul (take 2 of "better handling of indirections") #134697

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niacdoial commented Dec 23, 2024 •

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