Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::missing_fields_in_debug: manual `Debug` impl does not include all fields

What it meansChecks for manual core::fmt::Debug implementations that do not use all fields. It is in the pedantic group and off by default.

What you see

warning: manual `Debug` impl does not include all fields
  --> main.rs:9:1
   |
 9 | / impl fmt::Debug for Foo {
10 | |     fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
11 | |         formatter
12 | |             .debug_struct("Foo")
...  |
16 | | }
   | |_^
   |
note: this field is unused
  --> main.rs:7:5
   |
 7 |     hidden_data: i32
   |     ^^^^^^^^^^^^^^^^
   = help: consider including all fields in this `Debug` impl
   = help: consider calling `.finish_non_exhaustive()` if you intend to ignore fields
   = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#missing_fields_in_debug
   = note: `-W clippy::missing-fields-in-debug` implied by `-W clippy::pedantic`

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

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).

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

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()
    }
}

Write this 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()
    }
}

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
missing_fields_in_debug = "warn"

In code

#![warn(clippy::missing_fields_in_debug)]

One run

cargo clippy -- -W clippy::missing-fields-in-debug

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.

Pedanticgroup

allowdefault level

nofixed by cargo clippy --fix

1.70.0added in Rust

Clippy's own documentation

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()
    }
}

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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