write_fmt instead of write_str

What it does

I propose to add a lint to detect uses of write_str(format!) and suggest to replace them by write_fmt(format_args!) or write!(fmt...).

This would be usefull in all implementation of Display.

This proposal is motivated from one of my first projects where I've implemented Display for all of my error types using write_str where I could have used write_fmt instead. I've seen in some other project of mine where I've used write_fmt where applicable and I could have a bit better performance on my CPU.

Categories (optional)

• Kind: perf, maybe as well complexity

What is the advantage of the recommended code over the original code

• format! has to allocate a String anyway to store the formatted string inside.
• The proposed code is less complicated (no need to take reference to formatted string can just pass the macro directly).

Drawbacks

• Longer to type.

For the second variant: I can't find any.

Example

fmt.write_str(&format!("Some string with pars {} {}", par1, par2))

Could be written as:

fmt.write_fmt(format_args!("Some string with pars {} {}", par1, par2))

or even better

write!(fmt, "Some string with pars {} {}", par1, par2)

Start from the issue's Rust examples and inspect how rust-clippy implements existing lints for formatting and macro calls. Define done as detecting write_str(format!(...)) in Display implementations and offering the stated write_fmt(format_args!(...)) or write! alternatives, with coverage for the example patterns.