Warn about unused named parameter in format strings

What it does

Very few beginners know that named parameters can be used in format strings. (https://doc.rust-lang.org/std/fmt/index.html#named-parameters) This can often lead to great confusion as it looks like you are setting the variable to that value.

Additionally since you can now capture identifiers in format strings, named parameters will become less and less common. As such this new lint warns if a named parameter goes unused.

Lint Name

unused_named_parameter

Category

suspicious

Advantage

• Removes redundant code
• Prevents named parameters from being confused with an assignment such as print!("{}", x = x+1)

Drawbacks

No response

Example

println!("{}", x = 10);

Could be written as:

println!("{}", 10);