new_without_default: small clean-up#17172
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Move is_unit_struct into clippy_utils to use it in the new lint default_mismatches_new
Mostly to simplify next commit's diff
If a type has an auto-derived `Default` trait and a `fn new() -> Self`,
this lint checks if the `new()` method performs custom logic rather
than simply calling the `default()` method.
Users expect the `new()` method to be equivalent to `default()`,
so if the `Default` trait is auto-derived, the `new()` method should
not perform custom logic. Otherwise, there is a risk of different
behavior between the two instantiation methods.
```rust
struct MyStruct(i32);
impl MyStruct {
fn new() -> Self {
Self(42)
}
}
```
Users are unlikely to notice that `MyStruct::new()` and `MyStruct::default()` would produce
different results. The `new()` method should use auto-derived `default()` instead to be consistent:
```rust
struct MyStruct(i32);
impl MyStruct {
fn new() -> Self {
Self::default()
}
}
```
Alternatively, if the `new()` method requires a non-default initialization, implement a custom `Default`.
This also allows you to mark the `new()` implementation as `const`:
```rust
struct MyStruct(i32);
impl MyStruct {
const fn new() -> Self {
Self(42)
}
}
impl Default for MyStruct {
fn default() -> Self {
Self::new()
}
}
```
made a separate commit just to simplify the diff
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r? @llogiq rustbot has assigned @llogiq. Use Why was this reviewer chosen?The reviewer was selected based on:
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Lintcheck changes for deb6159
This comment will be updated if you push new changes |
Collaborator
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☔ The latest upstream changes (possibly #17096) made this pull request unmergeable. Please resolve the merge conflicts. |
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changelog: none
Should be merged after #16531 to avoid the rebase churn, so for now:
@rustbot blocked