Unused format string literals (string interpolation)

[alice-i-cecile]

What it does

When the user attempts to use {}, {:?} or similar invocations, we should expect them to actually have a value. Warn the user when no value is provided and "{}" or the like would actually be printed.

Inspired by bevyengine/bevy#16228

Advantage

These errors are likely an oversight, and they should either be removed or the correct value added.

Drawbacks

It will make it harder to draw strange ASCII faces in your error messages.

Example

            .expect("Component id: {:?}, already has an on_add hook")

Could be written as:

            .expect("Component already has an on_add hook")

[Piturnah]

The problem here, as I understand it, is that expect takes a &str, not a format string. So you actually couldn't supply a value.

In my opinion, a lint should be added for &str literals which include format-string syntax (like {:?}) to make sure the user actually intended the literal string "{:?}" or whether they thought they were in a format string and forgot a value.

Hopefully it's clear what I mean...

Edit: consider -- if a value was supposed to be provided, then it would be a compile fail, rather than something for the linter