Lint suggestion: nested format!

[jplatte]

What it does

Warn on format! within the arguments of another macro that does formatting such as format! itself, write! or println!.

Suggests replacing the inner format! call with format_args! or inlining it entirely (see example).

Categories (optional)

• Kind: style / perf

The recommended code is both shorter and avoids a temporary allocation.

Drawbacks

There could be situations when the outer macro is format_args! where lifetimes don't work out with the format! changed to format_args! or inlined entirely. I'm not sure how hard it would be to detect these cases.

Example

println!("error: {}", format!("something failed at {}", Location::caller()));

Could be written as:

println!("error: {}", format_args!("something failed at {}", Location::caller()));

or inlined entirely (when the argument is used exactly once like here):

println!("error: something failed at {}", Location::caller());

[matthiaskrgr]

Related issues: #3155 #6259

[camsteffen]

Lint name idea: format_in_format_args. Note that panic!(.., format!(..)) is expanded to ..format_args!(.., format!(..))... The same is true for all format_args-like macros.

To implement, I would start with FormatArgsExpn::parse, then try FormatExpn::parse for each value arg, then look at the outer macro expansion to look for panic!, assert!, write!, etc.

[smoelius]

@camsteffen If I were to implement this, would it be too invasive if I were to change FormatExpn to FormatLikeExpn so that it could handle things like println!?

Currently, it strictly handles format!:

rust-clippy/clippy_utils/src/higher.rs

Line 488 in fb17671

if let ExpnKind::Macro(_, sym::format) = expn_data.kind;

[camsteffen]

@smoelius I think I would add something like this, on top of what we already have:

enum FormatLikeExpn {
    Assert(AssertExpn),
    Format(FormatExpn),
    FormatArgs(FormatArgsExpn),
    Print(PrintExpn),
}

so we still have the option of parsing a specific macro.

[smoelius]

OK. Thanks for your response. I'm going to play with this and see what makes sense.