RS.CLIPPY.MISSING_FIELDS_IN_DEBUG

What it does

Checks for manual core::fmt::Debug implementations that do not use all fields.

Why is this bad?

A common mistake is to forget to update manual Debug implementations when adding a new field to a struct or a new variant to an enum.

At the same time, it also acts as a style lint to suggest using core::fmt::DebugStruct::finish_non_exhaustive for the times when the user intentionally wants to leave out certain fields (e.g. to hide implementation details).

Known problems

This lint works based on the DebugStruct helper types provided by the Formatter, so this won't detect Debug impls that use the write! macro. Oftentimes there is more logic to a Debug impl if it uses write! macro, so it tries to be on the conservative side and not lint in those cases in an attempt to prevent false positives.

This lint also does not look through function calls, so calling a function does not consider fields used inside of that function as used by the Debug impl.

Lastly, it also ignores tuple structs as their DebugTuple formatter does not have a finish_non_exhaustive method, as well as enums because their exhaustiveness is already checked by the compiler when matching on the enum, making it much less likely to accidentally forget to update the Debug impl when adding a new variant.

Example

use std::fmt;
struct Foo {
    data: String,
    // implementation detail
    hidden_data: i32
}
impl fmt::Debug for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<\'_>) -> fmt::Result {
        formatter
            .debug_struct("Foo")
            .field("data", &self.data)
            .finish()
    }
}

Use instead:

use std::fmt;
struct Foo {
    data: String,
    // implementation detail
    hidden_data: i32
}
impl fmt::Debug for Foo {
    fn fmt(&self, formatter: &mut fmt::Formatter<\'_>) -> fmt::Result {
        formatter
            .debug_struct("Foo")
            .field("data", &self.data)
            .finish_non_exhaustive()
    }
}