Implement default_mismatches_new lint

### What it does
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.

### Why is this bad?
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.

### Example
```rust
#[derive(Default)]
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
#[derive(Default)]
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

@@ -5549,6 +5549,7 @@ 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: &[&crate::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::DEFAULT_MISMATCHES_NEW_INFO,
     crate::new_without_default::NEW_WITHOUT_DEFAULT_INFO,
     crate::no_effect::NO_EFFECT_INFO,
     crate::no_effect::NO_EFFECT_UNDERSCORE_BINDING_INFO,

clippy_lints/src/default_constructed_unit_structs.rs

@@ -46,30 +46,23 @@ declare_clippy_lint! {
 }
 declare_lint_pass!(DefaultConstructedUnitStructs => [DEFAULT_CONSTRUCTED_UNIT_STRUCTS]);
 
-fn is_alias(ty: hir::Ty<'_>) -> bool {
-    if let hir::TyKind::Path(ref qpath) = ty.kind {
-        is_ty_alias(qpath)
-    } else {
-        false
-    }
-}
-
 impl LateLintPass<'_> for DefaultConstructedUnitStructs {
     fn check_expr<'tcx>(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
         if let ExprKind::Call(fn_expr, &[]) = expr.kind
             // make sure we have a call to `Default::default`
             && let ExprKind::Path(ref qpath @ hir::QPath::TypeRelative(base, _)) = fn_expr.kind
             // make sure this isn't a type alias:
             // `<Foo as Bar>::Assoc` cannot be used as a constructor
-            && !is_alias(*base)
+            && !matches!(base.kind, hir::TyKind::Path(ref qpath) if is_ty_alias(qpath))
             && let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id)
             && cx.tcx.is_diagnostic_item(sym::default_fn, def_id)
             // make sure we have a struct with no fields (unit struct)
             && let ty::Adt(def, ..) = cx.typeck_results().expr_ty(expr).kind()
             && def.is_struct()
             && let var @ ty::VariantDef { ctor: Some((hir::def::CtorKind::Const, _)), .. } = def.non_enum_variant()
             && !var.is_field_list_non_exhaustive()
-            && !expr.span.from_expansion() && !qpath.span().from_expansion()
+            && !expr.span.from_expansion()
+            && !qpath.span().from_expansion()
         {
             span_lint_and_sugg(
                 cx,

clippy_lints/src/new_without_default.rs

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

tests/ui/default_constructed_unit_structs.fixed

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

tests/ui/default_constructed_unit_structs.rs

@@ -0,0 +1,166 @@
+#![allow(clippy::needless_return, clippy::diverging_sub_expression)]
+#![warn(clippy::default_mismatches_new)]
+
+fn main() {}
+
+//
+// Nothing to change
+//
+struct ManualDefault(i32);
+impl ManualDefault {
+    fn new() -> Self {
+        Self(42)
+    }
+}
+impl Default for ManualDefault {
+    fn default() -> Self {
+        Self(42)
+    }
+}
+
+#[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();
+    }
+}
+
+#[derive(Default)]
+struct MakeResultSelf(i32);
+impl MakeResultSelf {
+    fn new() -> Result<Self, ()> {
+        Ok(Self(10))
+    }
+}
+
+#[derive(Default)]
+struct WithParams(i32);
+impl WithParams {
+    fn new(val: i32) -> Self {
+        Self(val)
+    }
+}
+
+#[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;
+    }
+}
+
+//
+// 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)
+    }
+}