Do not report as_conversions for coercions

[SOF3]

The use of as is discouraged when it is semantically unclear, such as conversion between numeric types. However, when it is used as a coercion, there is no better concise way to indicate the desired semantics.

For example, the following examples are legitimate:

fn identical_position<T>(a: &T, b: &T) -> bool {
    a as *const T == b as *const T
}

fn read(e: Enum) -> &mut dyn Read {
    match e {
        Enum::File(file) => file as &mut dyn Read,
        Enum::Cursor(cursor) => cursor as &mut dyn Read,
    }
}

I understand that as_conversions is set to allow for these reasons. However, i wish to set warn(as_conversions) at the crate-level to prevent future contributors from adding numeric casts to my project. It would be helpful to have a variant of as_conversions only for non-coercing casts, in particular, numeric casts.

[LunaBorowska]

The first example could be changed to use std::ptr::eq instead. Meanwhile, in the second example as &mut dyn Read shouldn't be necessary (as in, removal of as &mut dyn Read shouldn't make this code not compile).

[SOF3]

For the second example, there are some more complex cases where the compiler cannot infer it to be &mut dyn immediately. I cannot construct such an example by hand, but I think most people have run into this at least once.

[rail-rain]

@SOF3 Could you confirm that you need a single lint that warns every uses of as except coercions, not a bunch of lints that triggers on different situations; already existing as the cast_* family for numeric casts and bonus fn_to_numeric_cast* + char_lit_as_u8. If I understand correctly, as_conversions is for people who want a simple lint to detect every use of as; independent from other as related lints.

[SOF3]

@rail-rain you mean separate lints such as clippy::numeric_as_conversions, clippy::ptr_as_conversions, etc.?

[rail-rain]

@SOF3 Um…I'm afraid not; the bunch of lints for specific cases (e.g. numeric casts that may truncate large values) already exist. You can find these by searching on "cast" here (I couldn't find a way to set that as a query string). I wanted to check these lints do not fit your use case.

[SOF3]

oh yeah, I forgot those exist. should probably have used them instead.

[rail-rain]

In that case, do you still need this issue? Otherwise, could you close the issue please?

[rail-rain]

Thank you!