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New lint to ensure Default::default calls T::new, rather than vice versa. #12662
Description
What it does
Detects cases where a parameterless new simply calls Default::default, and indicates that default should call new, rather than the opposite.
Advantage
new functions can potentially be const, while default cannot, and calling them in the opposite order can prohibit this.
Drawbacks
The lint is completely irrelevant if the new function is never intended to be const.
Example
pub struct Config;
impl Default for Config {
fn default() -> Self { Self }
}
impl Config {
pub fn new() -> Self {
Self::default()
}
}Could be written as:
pub struct Config;
impl Default for Config {
fn default() -> Self { Self::new() }
}
impl Config {
pub fn new() -> Self {
Self
}
}
Activity
Jarcho commentedon Apr 12, 2024
This is going to end up with way too many false-positives and be more work to write and maintain if
Defaultis derived. This would have to be limited toDefaultimpls that could be const if they weren't a trait function. Probably only for exported types as well.nyurik commentedon Jan 24, 2025
I would actually suggest the opposite IF default was derived. Case to the point is a bug I just caught in the core lib - rust-lang/rust#135977 -- where
fn newwould instantiate struct with non-zero values, whiledefault()was auto-derived.nyurik commentedon Jan 25, 2025
I created a new lint suggestion that detects non-matching
newanddefault- #14075nyurik commentedon Jan 31, 2025
After some more thinking, I think this would actually be a bad pattern, at least for the cases when the
Defaultimpl is auto derived. If Default is not autoderived, this same pattern should still be followed for consistency.nyurik commentedon Jan 31, 2025
@Skgland I see you disagreed - could you clarify why? Do you think
new() -> Selfshould have a different implementation than the auto-generatedDefaulttrait?Skgland commentedon Jan 31, 2025
Was the part I disagree with.
In case default is derived I would agree that it's reasonable to have new call default, but in the case where default is manually implemented having default call new so the later can be made const is in my opinion preferable (at least until const Traits are available).
nyurik commentedon Jan 31, 2025
ah, yes, that does make sense, thx. Hopefully rust-lang/rust#67792 will get stabilized at some point.