Lint against casts from the `size_of` function item

[Noratrieb]

When using size_of, it is a common mistake to forget to call the function and instead cast the function item to an integer, which has a nonsense result.

There should be a lint, likely deny-by-default against this pattern.

example:

let size = size_of::<u16> as u16;

This is a pretty common mistake: https://github.com/search?q=language%3Arust+%2Fsize_of%3A%3A%3C%5B%5E(%5D*%3E+as%2F&type=code

[Noratrieb]

Clippy already has a lint that catches this, but it's more general and warn-by-default: https://rust-lang.github.io/rust-clippy/master/index.html#fn_to_numeric_cast

This one, by being more specific, can be deny-by-default in rustc, which should catch all these bugs.

[Urgau]

There is already an opened PR about adding a general version of this proposed lint, #141470.

[jieyouxu]

There is already an opened PR about adding a general version of this proposed lint, #141470.

cf. rust-lang/libs-team#589 (comment)

[Jules-Bertholet]

Clippy already has a lint that catches this, but it's more general and warn-by-default: https://rust-lang.github.io/rust-clippy/master/index.html#fn_to_numeric_cast

It also exempts casts to usize