Warn on stdout output in a Debug or Display implementation

[meithecatte]

What it does

Detect uses of print (instead of write) and similar inside of implementations of formatting traits (Debug, Display, LowerHex, etc.)

Categories (optional)

• Kind: correctness

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

Using print will appear to work correctly until one tries to print somewhere other than stdout (a file, string buffer, ...)

Drawbacks

None.

Example

impl fmt::Display for Test {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        println!("Hello");
        writeln!(f, "there!")
    }
}

Should be written as:

impl fmt::Display for Test {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        writeln!(f, "Hello")?;
        writeln!(f, "there!")
    }
}

[xFrednet]

@rustbot label +A-lint

[unvalley]

@rustbot claim

[unvalley]

@NieDzejkob (cc: @xFrednet )
This may already be implemented.

ref:

• https://rust-lang.github.io/rust-clippy/master/#print_in_format_impl
• Add print_in_format_impl lint #8253

Please tell me if I'm incorrect.

[xFrednet]

The lint looked familiar. Guess this was a duplicate issue. Thank you for pointing that out @unvalley! 🙃