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

@@ -6633,6 +6633,7 @@ 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

@@ -567,7 +567,8 @@ 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_without_default::NEW_WITHOUT_DEFAULT_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,
     crate::no_effect::UNNECESSARY_OPERATION_INFO,

clippy_lints/src/lib.rs

@@ -266,7 +266,7 @@ mod needless_question_mark;
 mod needless_update;
 mod neg_cmp_op_on_partial_ord;
 mod neg_multiply;
-mod new_without_default;
+mod new_vs_default;
 mod no_effect;
 mod no_mangle_with_rust_abi;
 mod non_canonical_impls;
@@ -616,7 +616,7 @@ pub fn register_lint_passes(store: &mut rustc_lint::LintStore, conf: &'static Co
         },
         Box::new(|_| Box::new(swap::Swap)),
         Box::new(|_| Box::new(panicking_overflow_checks::PanickingOverflowChecks)),
-        Box::new(|_| Box::<new_without_default::NewWithoutDefault>::default()),
+        Box::new(|_| Box::<new_vs_default::NewVsDefault>::default()),
         Box::new(move |_| Box::new(disallowed_names::DisallowedNames::new(conf))),
         Box::new(move |tcx| Box::new(functions::Functions::new(tcx, conf))),
         Box::new(move |_| Box::new(doc::Documentation::new(conf))),