lint on more incorrect usages of core::ffi::c_void

[Jules-Bertholet]

core::ffi::c_void has size 1 and a highly non-trivial validity invariant. It should only ever be used behind a raw pointer. However, people mess this up all the time.

• c_void_references: Lint against & or &mut references to c_void. This is a widespread mistake, e.g. here in Miri, here across GitHub.
• c_void_statics: Lint against statics and consts of type c_void. Examples of this mistake across GitHub
• c_void_parameters: Lint against usage of c_void as a function parameter type. Examples of this mistake across GitHub
• c_void_values: Lint against usage of c_void by value in MIR.

Also fixes false negatives in the pre-exisiting c_void_returns. Like that lint, these new lints are warn-by-default, but should eventually be upgraded to deny.

See also #159935

@rustbot label T-lang needs-fcp A-FFI

[rustbot]

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

[rustbot]

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[RalfJung]

The lint should probably also complain about statics of type c_void.

[Jules-Bertholet]

The lint should probably also complain about statics of type c_void.

Added another lint for this

[Jules-Bertholet]

Added a third lint, against c_void function parameters.

[theemathas]

Idea (not sure if this is feasible): If we do anything that asks for the size of c_void, emit a warning unless it's for a pointer offset operation

[mejrs]

Idea (not sure if this is feasible): If we do anything that asks for the size of c_void, emit a warning unless it's for a pointer offset operation

We can emit a warning regardless, with a note to just use byte_add for offset operations. (Is that what people are using size_of for..?)

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[Jules-Bertholet]

Added a fourth lint to check for incorrect usage of c_void in MIR.