Unnecessary cast for non-literal expressions

What it does

There's an unnecessary_cast lint for literals. Unnecessary casting can also happen for expressions. For example, I've refactored a codebase to remove uses of c_int, which left me with lots of as _ conversions that were no longer doing anything.

I'm proposing this as a new lint, not an extension to the unnecessary_cast, because users may want to enable/disable this lint separately.

Lint Name

unnecessary_cast_expression

Category

complexity

Advantage

Clarifies intent of the code. Removes doubt whether the cast is intentional, or written because of confusion about expression's type.

Drawbacks

as _ is easier to read and write than try_into().unwrap(), and code that works with non-generic types that are changed by conditional compilation may prefer the terser syntax.

#[cfg(foo)]
type Int = u32;

#[cfg(not(foo))]
type Int = usize;

fn main() { takes_usize(my_int as _); }

Example

fn takes_usize(_x: usize) {}

fn main() {
    takes_usize([1,2,3].len() as _);
}

Start by examining the existing unnecessary_cast lint and how Clippy defines lints in the complexity category. Determine how to identify casts on expressions that have no effect, while preserving the separate unnecessary_cast_expression name and the conditional-compilation example; done means the proposed cases are detected without collapsing the two lints.