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

[niacdoial]

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]

(
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]

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]

unfortunately the lint needs to be gutted and rewritten.

[workingjubilee]

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]

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

[niacdoial]

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]

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

[niacdoial]

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]

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

[workingjubilee]

hmm.

[workingjubilee]

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]

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

[workingjubilee]

1. Because unsized function parameters are something we may want to support.
2. 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.

[niacdoial]

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?

1. should the nature of the lint change?
   (temptative answer: yes)
2. 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!

[niacdoial]

@workingjubilee ...?
I think I'll end up rebasing anyway if you don't tell me to wait.
(sorry if this seems too insistent or something)

[rustbot]

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]

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]

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.