Implement default_mismatches_new lint

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()
  }
}
```

Author: nyurik

CHANGELOG.md

@@ -6727,6 +6727,7 @@ Released 2018-09-13
 [`declare_interior_mutable_const`]: https://rust-lang.github.io/rust-clippy/master/index.html#declare_interior_mutable_const
 [`default_constructed_unit_structs`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_constructed_unit_structs
 [`default_instead_of_iter_empty`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_instead_of_iter_empty
+[`default_mismatches_new`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_mismatches_new
 [`default_numeric_fallback`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_numeric_fallback
 [`default_trait_access`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_trait_access
 [`default_union_representation`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_union_representation

clippy_lints/src/declared_lints.rs

@@ -572,6 +572,7 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::needless_update::NEEDLESS_UPDATE_INFO,
     crate::neg_cmp_op_on_partial_ord::NEG_CMP_OP_ON_PARTIAL_ORD_INFO,
     crate::neg_multiply::NEG_MULTIPLY_INFO,
+    crate::new_vs_default::DEFAULT_MISMATCHES_NEW_INFO,
     crate::new_vs_default::NEW_WITHOUT_DEFAULT_INFO,
     crate::no_effect::NO_EFFECT_INFO,
     crate::no_effect::NO_EFFECT_UNDERSCORE_BINDING_INFO,

clippy_lints/src/new_vs_default.rs

@@ -1,3 +1,4 @@
+#![allow(clippy::default_mismatches_new)]
 #![warn(clippy::default_constructed_unit_structs)]
 use std::marker::PhantomData;
 

tests/ui/default_constructed_unit_structs.fixed

@@ -1,3 +1,4 @@
+#![allow(clippy::default_mismatches_new)]
 #![warn(clippy::default_constructed_unit_structs)]
 use std::marker::PhantomData;
 

tests/ui/default_constructed_unit_structs.rs

@@ -1,5 +1,5 @@
 error: use of `default` to create a unit struct
-  --> tests/ui/default_constructed_unit_structs.rs:10:9
+  --> tests/ui/default_constructed_unit_structs.rs:11:9
    |
 LL |         Self::default()
    |         ^^^^^^^^^^^^^^^
@@ -13,7 +13,7 @@ LL +         Self
    |
 
 error: use of `default` to create a unit struct
-  --> tests/ui/default_constructed_unit_structs.rs:53:20
+  --> tests/ui/default_constructed_unit_structs.rs:54:20
    |
 LL |             inner: PhantomData::default(),
    |                    ^^^^^^^^^^^^^^^^^^^^^^
@@ -25,7 +25,7 @@ LL +             inner: PhantomData,
    |
 
 error: use of `default` to create a unit struct
-  --> tests/ui/default_constructed_unit_structs.rs:127:13
+  --> tests/ui/default_constructed_unit_structs.rs:128:13
    |
 LL |     let _ = PhantomData::<usize>::default();
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -37,7 +37,7 @@ LL +     let _ = PhantomData::<usize>;
    |
 
 error: use of `default` to create a unit struct
-  --> tests/ui/default_constructed_unit_structs.rs:129:31
+  --> tests/ui/default_constructed_unit_structs.rs:130:31
    |
 LL |     let _: PhantomData<i32> = PhantomData::default();
    |                               ^^^^^^^^^^^^^^^^^^^^^^
@@ -49,7 +49,7 @@ LL +     let _: PhantomData<i32> = PhantomData;
    |
 
 error: use of `default` to create a unit struct
-  --> tests/ui/default_constructed_unit_structs.rs:131:31
+  --> tests/ui/default_constructed_unit_structs.rs:132:31
    |
 LL |     let _: PhantomData<i32> = std::marker::PhantomData::default();
    |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -61,7 +61,7 @@ LL +     let _: PhantomData<i32> = std::marker::PhantomData;
    |
 
 error: use of `default` to create a unit struct
-  --> tests/ui/default_constructed_unit_structs.rs:133:13
+  --> tests/ui/default_constructed_unit_structs.rs:134:13
    |
 LL |     let _ = UnitStruct::default();
    |             ^^^^^^^^^^^^^^^^^^^^^
@@ -73,7 +73,7 @@ LL +     let _ = UnitStruct;
    |
 
 error: use of `default` to create a unit struct
-  --> tests/ui/default_constructed_unit_structs.rs:171:7
+  --> tests/ui/default_constructed_unit_structs.rs:172:7
    |
 LL |     f(<G>::default());
    |       ^^^^^^^^^^^^^^

tests/ui/default_constructed_unit_structs.stderr

@@ -0,0 +1,177 @@
+#![allow(clippy::needless_return, clippy::diverging_sub_expression)]
+#![warn(clippy::default_mismatches_new)]
+
+fn main() {}
+
+// Don't warn: Default impl is manual
+struct ManualDefault(i32);
+impl ManualDefault {
+    fn new() -> Self {
+        Self(42)
+    }
+}
+impl Default for ManualDefault {
+    fn default() -> Self {
+        Self(42)
+    }
+}
+
+// Don't warn: new() calls automatic Default impl
+#[derive(Default)]
+struct CallToDefaultDefault(i32);
+impl CallToDefaultDefault {
+    fn new() -> Self {
+        Default::default()
+    }
+}
+
+#[derive(Default)]
+struct CallToSelfDefault(i32);
+impl CallToSelfDefault {
+    fn new() -> Self {
+        Self::default()
+    }
+}
+
+#[derive(Default)]
+struct CallToTypeDefault(i32);
+impl CallToTypeDefault {
+    fn new() -> Self {
+        CallToTypeDefault::default()
+    }
+}
+
+#[derive(Default)]
+struct CallToFullTypeDefault(i32);
+impl CallToFullTypeDefault {
+    fn new() -> Self {
+        crate::CallToFullTypeDefault::default()
+    }
+}
+
+#[derive(Default)]
+struct ReturnCallToSelfDefault(i32);
+impl ReturnCallToSelfDefault {
+    fn new() -> Self {
+        return Self::default();
+    }
+}
+
+// Don't warn: new() returns Result, so it can't return Self::default()
+#[derive(Default)]
+struct MakeResultSelf(i32);
+impl MakeResultSelf {
+    fn new() -> Result<Self, ()> {
+        Ok(Self(10))
+    }
+}
+
+// Don't warn: new() takes parameters, so it is semantically different from default()
+#[derive(Default)]
+struct WithParams(i32);
+impl WithParams {
+    fn new(val: i32) -> Self {
+        Self(val)
+    }
+}
+
+// Don't warn: new() is async
+#[derive(Default)]
+struct Async(i32);
+impl Async {
+    async fn new() -> Self {
+        Self(42)
+    }
+}
+
+#[derive(Default)]
+struct DeriveDefault;
+impl DeriveDefault {
+    fn new() -> Self {
+        // Adding ::default() would cause clippy::default_constructed_unit_structs
+        Self
+    }
+}
+
+#[derive(Default)]
+struct DeriveTypeDefault;
+impl DeriveTypeDefault {
+    fn new() -> Self {
+        // Adding ::default() would cause clippy::default_constructed_unit_structs
+        return crate::DeriveTypeDefault;
+    }
+}
+
+// Don't warn: new() is const - user may need const fn and derived Default isn't const (yet)
+#[derive(Default)]
+struct ConstNew(i32);
+impl ConstNew {
+    const fn new() -> Self {
+        Self(42)
+    }
+}
+
+//
+// Offer suggestions
+//
+
+#[derive(Default)]
+struct DeriveIntDefault {
+    value: i32,
+}
+impl DeriveIntDefault {
+    fn new() -> Self {
+        Self::default()
+    }
+}
+
+#[derive(Default)]
+struct DeriveTupleDefault(i32);
+impl DeriveTupleDefault {
+    fn new() -> Self {
+        Self::default()
+    }
+}
+
+#[derive(Default)]
+struct NonZeroDeriveDefault(i32);
+impl NonZeroDeriveDefault {
+    fn new() -> Self {
+        Self::default()
+    }
+}
+
+#[derive(Default)]
+struct ExtraBlockDefault(i32);
+impl ExtraBlockDefault {
+    fn new() -> Self {
+        Self::default()
+    }
+}
+
+#[derive(Default)]
+struct ExtraBlockRetDefault(i32);
+impl ExtraBlockRetDefault {
+    fn new() -> Self {
+        Self::default()
+    }
+}
+
+#[derive(Default)]
+struct MultiStatements(i32);
+impl MultiStatements {
+    fn new() -> Self {
+        Self::default()
+    }
+}
+
+//
+// TODO: Fix in the future
+//
+#[derive(Default)]
+struct OptionGeneric<T>(Option<T>);
+impl<T> OptionGeneric<T> {
+    fn new() -> Self {
+        OptionGeneric(None)
+    }
+}