assert_has_field - a Rust macro for checking if a struct has a specific field

This macro is designed to be used in Rust code to assert that a struct has a specific field and, if necessary, that this field of specific type.

Usage

The macro offers three syntaxes for checking if a struct has a field

1. assert_has_field!(Struct, field); - checks if the struct has a field with the given name.
2. assert_has_field!(Struct, field: Type); - checks if the struct has a field with the given name and type.
3. assert_has_field!(Struct, field :~ Type); - checks if the struct has a field with the given name and type that can be coerced to the specified type Type.

Checking that a struct has a field

use assert_has_field::assert_has_field;

#[allow(dead_code)]
struct MyStruct {
    field1: i32,
    field2: String,
}

assert_has_field!(MyStruct, field1); // This will compile

Checking that a struct has a field of a specific type

use assert_has_field::assert_has_field;

#[allow(dead_code)]
struct MyStruct {
    field1: i32,
    field2: String,
}

assert_has_field!(MyStruct, field1: i32); // This will compile

Checking that a struct has a field of a specific type (failure case for a totally different type)

use assert_has_field::assert_has_field;

#[allow(dead_code)]
struct MyStruct {
    field1: i32,
    field2: String,
}

assert_has_field!(MyStruct, field1: String); // This will fail to compile

Checking that a struct has a field of a specific type (failure case for a type that can be coerced to)

struct Wrapper<T>(T);

impl core::ops::Deref for Wrapper<i32> {
    type Target = i32;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

#[allow(dead_code)]
struct MyStruct {
    field1: &'static Wrapper<i32>,
    field2: String,
}

assert_has_field!(MyStruct, field1: &'static i32); // This will fail to compile

Checking that a struct has a field of a type that can be coerced to another type

use assert_has_field::assert_has_field;

struct Wrapper<T>(T);

impl core::ops::Deref for Wrapper<i32> {
    type Target = i32;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

#[allow(dead_code)]
struct MyStruct {
    field1: &'static Wrapper<i32>,
    field2: String,
}

assert_has_field!(MyStruct, field1 :~ &'static i32);

Checking that a struct has a field of a type that can be coerced to another type (failure case)

use assert_has_field::assert_has_field;

struct Point {
    x: i32,
    y: i32,
}

assert_has_field!(Point, x :~ String); // This will not compile

How it works

#[macro_export]
macro_rules! assert_has_field {
    (@ASSERT $unreachable_obj:ident, $field:ident) => {
        // Here, it is only checked that the field exists.
        let _: _ = $unreachable_obj.$field;
    };
    (@ASSERT $unreachable_obj:ident, $field:ident : $field_ty:ty) => {
        // Here, the value on the right hand side must be the same type as the type on the left hand side
        // and the field must exist.
        let _ : $field_ty = type_equalities::coerce($unreachable_obj.$field, type_equalities::refl());
    };
    (@ASSERT $unreachable_obj:ident, $field:ident :~ $field_ty:ty) => {
        // Here, the value on the right hand side can be coerced to the type on the left hand side
        // and the field must exist.
        let _ : $field_ty = $unreachable_obj.$field;
    };
    (
        $struct:ty,
        $field:ident
            $($rest:tt)*
    ) => {
        // The const block forces the const evaluation.
        #[allow(
            unreachable_code,
            unused_variables,
            clippy::diverging_sub_expression,
        )]
        const _: () = {
            // `if false { ... }` ensures that the unreacahble! macro invokation is indeed unreachable.
            if false {
                // Rust performs the type-checking at compile time even if the code is unreachable.
                //
                // The return type of core::unreachable!() is never type,
                // which can be assigned to any type.
                let unreachable_obj: $struct = core::unreachable!();
                assert_has_field!(@ASSERT unreachable_obj, $field $($rest)*);
            }
        };
    };
}

On the real use-cases of this macro

Let's say that you're writing a backend server and have a DTO, which is meant to be used on the frontend. Assume that this DTO aggregates different kinds of data that pertains to a candidate. You may be in a situation where candidate_id is stored in one of the fields-structures. You can use assert_has_field to document that expectation and future-proof the type in case the field-structure that used to store candidate_id is removed entirely or modified in a way that moves or removes the candidate_id.