FP: used_underscore_items lints on debatably legitimate use-cases

[mkrasnitski]

Summary

I feel the new used_underscore_items lint is not universally applicable and would best live in restriction rather than pedantic, as there are sometimes legitimate use-cases for prefixing item names with a _.

Lint Name

used_underscore_items

Reproducer

As motivation, I'll give two examples where prefixing method names with _ makes sense:

First, as a way to reduce the impact of monomorphization on binary size for functions that have large bodies:

pub fn my_method(&self, param: impl AsRef<str>) {
    self._my_method(param.as_ref())
}

fn _my_method(&self, param: &str) {
    // large function body here
}

Second, as a way to introduce non-breaking changes when adding a new argument to a function. If we have the following initial code:

pub fn my_method(&self, param1: u32) {
    // impl
}

...and we want to add support for a second parameter without making breaking changes, we could transform it in the following way:

pub fn my_method(&self, param1: u32) {
    self._my_method(param1, None)
}

pub fn my_method_2(&self, param1: u32, param2: u64) {
    self._my_method(param1, Some(param2))
}

fn _my_method(&self, param1: u32, param2: Option<u64>) {
    // impl
}

In both cases I feel that naming the private method with a prefixed underscore is a valid choice to make, in order to preserve the name and to not conflict with the public wrapper method. Granted, maybe this still means it should be a pedantic lint, but either way I feel these two cases qualify as false positives.

Version

rustc 1.83.0-nightly (ed04567ba 2024-09-28)
binary: rustc
commit-hash: ed04567ba1d5956d1080fb8121caa005ce059e12
commit-date: 2024-09-28
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 19.1.0

Additional Labels

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