new lint: missing_fields_in_debug

[y21]

Fixes #10429

This PR adds a new lint that looks for manual Debug implementations that do not "use" all of the fields.
This often happens when adding a new field to a struct.
It also acts as a style lint in case leaving out a field was intentional. In that case, it's preferred to use DebugStruct::finish_non_exhaustive, which indicates that there are more fields that were explicitly not shown.

changelog: [`missing_fields_in_debug`]: missing fields in manual `Debug` implementation

[rustbot]

r? @Alexendoo

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[bors]

☔ The latest upstream changes (presumably #10578) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #10647) made this pull request unmergeable. Please resolve the merge conflicts.

[y21]

Is there anything else or is it just waiting on a rebase?

[Alexendoo]

Sorry for the delay, can't remember if I had anything else in mind so I just read it over again this weekend 😄

A thought that came to me just now is maybe we don't actually need to lint enums, unlike structs where it's very easy to add a new field and forget to update the Debug impl using a match already has you consider enum variants fully. The author would have to explicitly ignore the fields with ../_ in order to miss them

[y21]

A thought that came to me just now is maybe we don't actually need to lint enums, unlike structs where it's very easy to add a new field and forget to update the Debug impl using a match already has you consider enum variants fully.

That's a very good point, I agree with this. I went ahead and removed the enum checking (and also addressed the other comments). That made me realize that one can do something similar with structs by destructuring self (you get a compiler error when a new field is added):

struct F { a: i32, b: i32 }

impl fmt::Debug for F {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    let Self { a } = self; // error, b not handled
    // ...
  }
}

Although a lint for this would still be nice 😅

[Alexendoo]

Thanks! Yeah that's a good way of catching it

With a rebase for the conflicts I'd say this is ready to merge

[Alexendoo]

👍

@bors r+

[bors]

📌 Commit a859b0e has been approved by Alexendoo

It is now in the queue for this repository.

[bors]

⌛ Testing commit a859b0e with merge 652b4c7...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Alexendoo
Pushing 652b4c7 to master...