Add unwrap_or_else_default lint

This will catch `unwrap_or_else(Default::default)` on Result and Option
and suggest `unwrap_or_default()` instead.

CHANGELOG.md

@@ -2999,6 +2999,7 @@ Released 2018-09-13
 [`unused_unit`]: https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit
 [`unusual_byte_groupings`]: https://rust-lang.github.io/rust-clippy/master/index.html#unusual_byte_groupings
 [`unwrap_in_result`]: https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_in_result
+[`unwrap_or_else_default`]: https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_or_else_default
 [`unwrap_used`]: https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_used
 [`upper_case_acronyms`]: https://rust-lang.github.io/rust-clippy/master/index.html#upper_case_acronyms
 [`use_debug`]: https://rust-lang.github.io/rust-clippy/master/index.html#use_debug

clippy_lints/src/lib.rs

@@ -797,6 +797,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         methods::UNNECESSARY_FILTER_MAP,
         methods::UNNECESSARY_FOLD,
         methods::UNNECESSARY_LAZY_EVALUATIONS,
+        methods::UNWRAP_OR_ELSE_DEFAULT,
         methods::UNWRAP_USED,
         methods::USELESS_ASREF,
         methods::WRONG_SELF_CONVENTION,
@@ -1341,6 +1342,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(methods::UNNECESSARY_FILTER_MAP),
         LintId::of(methods::UNNECESSARY_FOLD),
         LintId::of(methods::UNNECESSARY_LAZY_EVALUATIONS),
+        LintId::of(methods::UNWRAP_OR_ELSE_DEFAULT),
         LintId::of(methods::USELESS_ASREF),
         LintId::of(methods::WRONG_SELF_CONVENTION),
         LintId::of(methods::ZST_OFFSET),
@@ -1535,6 +1537,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(methods::STRING_EXTEND_CHARS),
         LintId::of(methods::UNNECESSARY_FOLD),
         LintId::of(methods::UNNECESSARY_LAZY_EVALUATIONS),
+        LintId::of(methods::UNWRAP_OR_ELSE_DEFAULT),
         LintId::of(methods::WRONG_SELF_CONVENTION),
         LintId::of(misc::TOPLEVEL_REF_ARG),
         LintId::of(misc::ZERO_PTR),

clippy_lints/src/methods/mod.rs

@@ -56,6 +56,7 @@ mod uninit_assumed_init;
 mod unnecessary_filter_map;
 mod unnecessary_fold;
 mod unnecessary_lazy_eval;
+mod unwrap_or_else_default;
 mod unwrap_used;
 mod useless_asref;
 mod utils;
@@ -310,6 +311,31 @@ declare_clippy_lint! {
     "using `ok().expect()`, which gives worse error messages than calling `expect` directly on the Result"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for usages of `_.unwrap_or_else(Default::default)` on Option and
+    /// Result values.
+    ///
+    /// ### Why is this bad?
+    /// Readability, these can be written as `option.unwrap_or_default` or
+    /// `result.unwrap_or_default`.
+    ///
+    /// ### Examples
+    /// ```rust
+    /// # let x = Some(1);
+    ///
+    /// // Bad
+    /// x.unwrap_or_else(Default::default);
+    /// x.unwrap_or_else(u32::default);
+    ///
+    /// // Good
+    /// x.unwrap_or_default();
+    /// ```
+    pub UNWRAP_OR_ELSE_DEFAULT,
+    style,
+    "using `.unwrap_or_else(Default::default)`, which is more succinctly expressed as `.unwrap_or_default()`"
+}
+
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for usage of `option.map(_).unwrap_or(_)` or `option.map(_).unwrap_or_else(_)` or
@@ -1766,6 +1792,7 @@ impl_lint_pass!(Methods => [
     SHOULD_IMPLEMENT_TRAIT,
     WRONG_SELF_CONVENTION,
     OK_EXPECT,
+    UNWRAP_OR_ELSE_DEFAULT,
     MAP_UNWRAP_OR,
     RESULT_MAP_OR_INTO_OPTION,
     OPTION_MAP_OR_NONE,
@@ -2172,7 +2199,10 @@ fn check_methods<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, msrv: Optio
             },
             ("unwrap_or_else", [u_arg]) => match method_call!(recv) {
                 Some(("map", [recv, map_arg], _)) if map_unwrap_or::check(cx, expr, recv, map_arg, u_arg, msrv) => {},
-                _ => unnecessary_lazy_eval::check(cx, expr, recv, u_arg, "unwrap_or"),
+                _ => {
+                    unwrap_or_else_default::check(cx, expr, recv, u_arg);
+                    unnecessary_lazy_eval::check(cx, expr, recv, u_arg, "unwrap_or");
+                },
             },
             _ => {},
         }

clippy_lints/src/methods/or_fun_call.rs

@@ -1,7 +1,8 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::eager_or_lazy::is_lazyness_candidate;
 use clippy_utils::source::{snippet, snippet_with_applicability, snippet_with_macro_callsite};
-use clippy_utils::ty::{implements_trait, is_type_diagnostic_item, match_type};
+use clippy_utils::ty::{implements_trait, qpath_target_trait};
+use clippy_utils::ty::{is_type_diagnostic_item, match_type};
 use clippy_utils::{contains_return, last_path_segment, paths};
 use if_chain::if_chain;
 use rustc_errors::Applicability;
@@ -34,15 +35,25 @@ pub(super) fn check<'tcx>(
         or_has_args: bool,
         span: Span,
     ) -> bool {
+        let is_default_default = |qpath, default_trait_id| {
+            qpath_target_trait(cx, qpath, fun.hir_id).map_or(false, |target_trait| target_trait == default_trait_id)
+        };
+
+        let implements_default = |arg, default_trait_id| {
+            let arg_ty = cx.typeck_results().expr_ty(arg);
+            implements_trait(cx, arg_ty, default_trait_id, &[])
+        };
+
         if_chain! {
             if !or_has_args;
             if name == "unwrap_or";
             if let hir::ExprKind::Path(ref qpath) = fun.kind;
-            let path = last_path_segment(qpath).ident.name;
-            if matches!(path, kw::Default | sym::new);
-            let arg_ty = cx.typeck_results().expr_ty(arg);
             if let Some(default_trait_id) = cx.tcx.get_diagnostic_item(sym::Default);
-            if implements_trait(cx, arg_ty, default_trait_id, &[]);
+            let path = last_path_segment(qpath).ident.name;
+            // needs to target Default::default in particular or be *::new and have a Default impl
+            // available
+            if (matches!(path, kw::Default) && is_default_default(qpath, default_trait_id))
+                || (matches!(path, sym::new) && implements_default(arg, default_trait_id));
 
             then {
                 let mut applicability = Applicability::MachineApplicable;

clippy_lints/src/methods/unwrap_or_else_default.rs

@@ -0,0 +1,50 @@
+//! Lint for `some_result_or_option.unwrap_or_else(Default::default)`
+
+use super::UNWRAP_OR_ELSE_DEFAULT;
+use clippy_utils::{
+    diagnostics::span_lint_and_sugg,
+    source::snippet_with_applicability,
+    ty::{is_type_diagnostic_item, qpath_target_trait},
+};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx hir::Expr<'_>,
+    recv: &'tcx hir::Expr<'_>,
+    u_arg: &'tcx hir::Expr<'_>,
+) {
+    // something.unwrap_or_else(Default::default)
+    // ^^^^^^^^^- recv          ^^^^^^^^^^^^^^^^- u_arg
+    // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- expr
+    let recv_ty = cx.typeck_results().expr_ty(recv);
+    let is_option = is_type_diagnostic_item(cx, recv_ty, sym::option_type);
+    let is_result = is_type_diagnostic_item(cx, recv_ty, sym::result_type);
+
+    if_chain! {
+        if is_option || is_result;
+        if let hir::ExprKind::Path(ref qpath) = u_arg.kind;
+        if let Some(default_trait_id) = cx.tcx.get_diagnostic_item(sym::Default);
+        if let Some(target_trait) = qpath_target_trait(cx, qpath, u_arg.hir_id);
+        if target_trait == default_trait_id;
+        then {
+            let mut applicability = Applicability::MachineApplicable;
+
+            span_lint_and_sugg(
+                cx,
+                UNWRAP_OR_ELSE_DEFAULT,
+                expr.span,
+                "use of `.unwrap_or_else(..)` to construct default value",
+                "try",
+                format!(
+                    "{}.unwrap_or_default()",
+                    snippet_with_applicability(cx, recv.span, "..", &mut applicability)
+                ),
+                applicability,
+            );
+        }
+    }
+}

clippy_utils/src/source.rs

@@ -168,7 +168,7 @@ pub fn snippet<'a, T: LintContext>(cx: &T, span: Span, default: &'a str) -> Cow<
     snippet_opt(cx, span).map_or_else(|| Cow::Borrowed(default), From::from)
 }
 
-/// Same as `snippet`, but it adapts the applicability level by following rules:
+/// Same as [`snippet`], but it adapts the applicability level by following rules:
 ///
 /// - Applicability level `Unspecified` will never be changed.
 /// - If the span is inside a macro, change the applicability level to `MaybeIncorrect`.

clippy_utils/src/ty.rs

@@ -2,6 +2,7 @@
 
 #![allow(clippy::module_name_repetitions)]
 
+use hir::{HirId, QPath};
 use rustc_ast::ast::Mutability;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir as hir;
@@ -114,7 +115,7 @@ pub fn has_iter_method(cx: &LateContext<'_>, probably_ref_ty: Ty<'_>) -> Option<
 
 /// Checks whether a type implements a trait.
 /// The function returns false in case the type contains an inference variable.
-/// See also `get_trait_def_id`.
+/// See also [`get_trait_def_id`].
 pub fn implements_trait<'tcx>(
     cx: &LateContext<'tcx>,
     ty: Ty<'tcx>,
@@ -136,6 +137,21 @@ pub fn implements_trait<'tcx>(
     })
 }
 
+/// Gets the trait that a path targets. For example `<SomeTy as Trait>::a` would return the
+/// [`DefId`] for `Trait`.
+///
+/// `cx` must be in a body.
+pub fn qpath_target_trait<'tcx>(cx: &LateContext<'tcx>, qpath: &QPath<'_>, expr_id: HirId) -> Option<DefId> {
+    let method_res = cx.typeck_results().qpath_res(qpath, expr_id);
+    let method_id = match method_res {
+        hir::def::Res::Def(_kind, id) => Some(id),
+        _ => None,
+    };
+    let method_id = method_id?;
+
+    cx.tcx.trait_of_item(method_id)
+}
+
 /// Checks whether this type implements `Drop`.
 pub fn has_drop<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
     match ty.ty_adt_def() {

tests/ui/or_fun_call.fixed

@@ -18,6 +18,19 @@ fn or_fun_call() {
         }
     }
 
+    struct FakeDefault;
+    impl FakeDefault {
+        fn default() -> Self {
+            FakeDefault
+        }
+    }
+
+    impl Default for FakeDefault {
+        fn default() -> Self {
+            FakeDefault
+        }
+    }
+
     enum Enum {
         A(i32),
     }
@@ -53,6 +66,12 @@ fn or_fun_call() {
     let with_default_type = Some(1);
     with_default_type.unwrap_or_default();
 
+    let self_default = None::<FakeDefault>;
+    self_default.unwrap_or_else(<FakeDefault>::default);
+
+    let real_default = None::<FakeDefault>;
+    real_default.unwrap_or_default();
+
     let with_vec = Some(vec![1]);
     with_vec.unwrap_or_default();
 

tests/ui/or_fun_call.rs

@@ -18,6 +18,19 @@ fn or_fun_call() {
         }
     }
 
+    struct FakeDefault;
+    impl FakeDefault {
+        fn default() -> Self {
+            FakeDefault
+        }
+    }
+
+    impl Default for FakeDefault {
+        fn default() -> Self {
+            FakeDefault
+        }
+    }
+
     enum Enum {
         A(i32),
     }
@@ -53,6 +66,12 @@ fn or_fun_call() {
     let with_default_type = Some(1);
     with_default_type.unwrap_or(u64::default());
 
+    let self_default = None::<FakeDefault>;
+    self_default.unwrap_or(<FakeDefault>::default());
+
+    let real_default = None::<FakeDefault>;
+    real_default.unwrap_or(<FakeDefault as Default>::default());
+
     let with_vec = Some(vec![1]);
     with_vec.unwrap_or(vec![]);
 

tests/ui/or_fun_call.stderr

@@ -1,5 +1,5 @@
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:33:19
+  --> $DIR/or_fun_call.rs:46:19
    |
 LL |     with_const_fn.unwrap_or(Duration::from_secs(5));
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| Duration::from_secs(5))`
@@ -7,130 +7,142 @@ LL |     with_const_fn.unwrap_or(Duration::from_secs(5));
    = note: `-D clippy::or-fun-call` implied by `-D warnings`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:36:22
+  --> $DIR/or_fun_call.rs:49:22
    |
 LL |     with_constructor.unwrap_or(make());
    |                      ^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(make)`
 
 error: use of `unwrap_or` followed by a call to `new`
-  --> $DIR/or_fun_call.rs:39:5
+  --> $DIR/or_fun_call.rs:52:5
    |
 LL |     with_new.unwrap_or(Vec::new());
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `with_new.unwrap_or_default()`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:42:21
+  --> $DIR/or_fun_call.rs:55:21
    |
 LL |     with_const_args.unwrap_or(Vec::with_capacity(12));
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| Vec::with_capacity(12))`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:45:14
+  --> $DIR/or_fun_call.rs:58:14
    |
 LL |     with_err.unwrap_or(make());
    |              ^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|_| make())`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:48:19
+  --> $DIR/or_fun_call.rs:61:19
    |
 LL |     with_err_args.unwrap_or(Vec::with_capacity(12));
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|_| Vec::with_capacity(12))`
 
 error: use of `unwrap_or` followed by a call to `default`
-  --> $DIR/or_fun_call.rs:51:5
+  --> $DIR/or_fun_call.rs:64:5
    |
 LL |     with_default_trait.unwrap_or(Default::default());
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `with_default_trait.unwrap_or_default()`
 
 error: use of `unwrap_or` followed by a call to `default`
-  --> $DIR/or_fun_call.rs:54:5
+  --> $DIR/or_fun_call.rs:67:5
    |
 LL |     with_default_type.unwrap_or(u64::default());
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `with_default_type.unwrap_or_default()`
 
+error: use of `unwrap_or` followed by a function call
+  --> $DIR/or_fun_call.rs:70:18
+   |
+LL |     self_default.unwrap_or(<FakeDefault>::default());
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(<FakeDefault>::default)`
+
+error: use of `unwrap_or` followed by a call to `default`
+  --> $DIR/or_fun_call.rs:73:5
+   |
+LL |     real_default.unwrap_or(<FakeDefault as Default>::default());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `real_default.unwrap_or_default()`
+
 error: use of `unwrap_or` followed by a call to `new`
-  --> $DIR/or_fun_call.rs:57:5
+  --> $DIR/or_fun_call.rs:76:5
    |
 LL |     with_vec.unwrap_or(vec![]);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `with_vec.unwrap_or_default()`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:60:21
+  --> $DIR/or_fun_call.rs:79:21
    |
 LL |     without_default.unwrap_or(Foo::new());
    |                     ^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(Foo::new)`
 
 error: use of `or_insert` followed by a function call
-  --> $DIR/or_fun_call.rs:63:19
+  --> $DIR/or_fun_call.rs:82:19
    |
 LL |     map.entry(42).or_insert(String::new());
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `or_insert_with(String::new)`
 
 error: use of `or_insert` followed by a function call
-  --> $DIR/or_fun_call.rs:66:23
+  --> $DIR/or_fun_call.rs:85:23
    |
 LL |     map_vec.entry(42).or_insert(vec![]);
    |                       ^^^^^^^^^^^^^^^^^ help: try this: `or_insert_with(Vec::new)`
 
 error: use of `or_insert` followed by a function call
-  --> $DIR/or_fun_call.rs:69:21
+  --> $DIR/or_fun_call.rs:88:21
    |
 LL |     btree.entry(42).or_insert(String::new());
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `or_insert_with(String::new)`
 
 error: use of `or_insert` followed by a function call
-  --> $DIR/or_fun_call.rs:72:25
+  --> $DIR/or_fun_call.rs:91:25
    |
 LL |     btree_vec.entry(42).or_insert(vec![]);
    |                         ^^^^^^^^^^^^^^^^^ help: try this: `or_insert_with(Vec::new)`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:75:21
+  --> $DIR/or_fun_call.rs:94:21
    |
 LL |     let _ = stringy.unwrap_or("".to_owned());
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| "".to_owned())`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:83:21
+  --> $DIR/or_fun_call.rs:102:21
    |
 LL |     let _ = Some(1).unwrap_or(map[&1]);
    |                     ^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| map[&1])`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:85:21
+  --> $DIR/or_fun_call.rs:104:21
    |
 LL |     let _ = Some(1).unwrap_or(map[&1]);
    |                     ^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| map[&1])`
 
 error: use of `or` followed by a function call
-  --> $DIR/or_fun_call.rs:109:35
+  --> $DIR/or_fun_call.rs:128:35
    |
 LL |     let _ = Some("a".to_string()).or(Some("b".to_string()));
    |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `or_else(|| Some("b".to_string()))`
 
 error: use of `or` followed by a function call
-  --> $DIR/or_fun_call.rs:113:10
+  --> $DIR/or_fun_call.rs:132:10
    |
 LL |         .or(Some(Bar(b, Duration::from_secs(2))));
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `or_else(|| Some(Bar(b, Duration::from_secs(2))))`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:141:14
+  --> $DIR/or_fun_call.rs:160:14
    |
 LL |         None.unwrap_or(s.as_mut_vec());
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| s.as_mut_vec())`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:146:14
+  --> $DIR/or_fun_call.rs:165:14
    |
 LL |         None.unwrap_or(unsafe { s.as_mut_vec() });
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| unsafe { s.as_mut_vec() })`
 
 error: use of `unwrap_or` followed by a function call
-  --> $DIR/or_fun_call.rs:148:14
+  --> $DIR/or_fun_call.rs:167:14
    |
 LL |         None.unwrap_or( unsafe { s.as_mut_vec() }    );
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `unwrap_or_else(|| unsafe { s.as_mut_vec() })`
 
-error: aborting due to 22 previous errors
+error: aborting due to 24 previous errors
 

tests/ui/unwrap_or_else_default.fixed

@@ -0,0 +1,71 @@
+// run-rustfix
+
+#![warn(clippy::unwrap_or_else_default)]
+#![allow(dead_code)]
+#![allow(clippy::unnecessary_wraps)]
+
+/// Checks implementation of the `UNWRAP_OR_ELSE_DEFAULT` lint.
+fn unwrap_or_else_default() {
+    struct Foo;
+
+    impl Foo {
+        fn new() -> Foo {
+            Foo
+        }
+
+        // fake default, we should not trigger on this
+        fn default() -> Foo {
+            Foo
+        }
+    }
+
+    struct HasDefaultAndDuplicate;
+
+    impl HasDefaultAndDuplicate {
+        fn default() -> Self {
+            HasDefaultAndDuplicate
+        }
+    }
+
+    impl Default for HasDefaultAndDuplicate {
+        fn default() -> Self {
+            HasDefaultAndDuplicate
+        }
+    }
+
+    enum Enum {
+        A(),
+    }
+
+    fn make<T, V>(_: V) -> T {
+        unimplemented!();
+    }
+
+    let with_enum = Some(Enum::A());
+    with_enum.unwrap_or_else(Enum::A);
+
+    let with_new = Some(vec![1]);
+    with_new.unwrap_or_else(Vec::new);
+
+    let with_err: Result<_, ()> = Ok(vec![1]);
+    with_err.unwrap_or_else(make);
+
+    // should not be changed
+    let with_fake_default = None::<Foo>;
+    with_fake_default.unwrap_or_else(Foo::default);
+
+    // should not be changed
+    let with_fake_default2 = None::<HasDefaultAndDuplicate>;
+    with_fake_default2.unwrap_or_else(<HasDefaultAndDuplicate>::default);
+
+    let with_real_default = None::<HasDefaultAndDuplicate>;
+    with_real_default.unwrap_or_default();
+
+    let with_default_trait = Some(1);
+    with_default_trait.unwrap_or_default();
+
+    let with_default_type = Some(1);
+    with_default_type.unwrap_or_default();
+}
+
+fn main() {}

tests/ui/unwrap_or_else_default.rs

@@ -0,0 +1,71 @@
+// run-rustfix
+
+#![warn(clippy::unwrap_or_else_default)]
+#![allow(dead_code)]
+#![allow(clippy::unnecessary_wraps)]
+
+/// Checks implementation of the `UNWRAP_OR_ELSE_DEFAULT` lint.
+fn unwrap_or_else_default() {
+    struct Foo;
+
+    impl Foo {
+        fn new() -> Foo {
+            Foo
+        }
+
+        // fake default, we should not trigger on this
+        fn default() -> Foo {
+            Foo
+        }
+    }
+
+    struct HasDefaultAndDuplicate;
+
+    impl HasDefaultAndDuplicate {
+        fn default() -> Self {
+            HasDefaultAndDuplicate
+        }
+    }
+
+    impl Default for HasDefaultAndDuplicate {
+        fn default() -> Self {
+            HasDefaultAndDuplicate
+        }
+    }
+
+    enum Enum {
+        A(),
+    }
+
+    fn make<T, V>(_: V) -> T {
+        unimplemented!();
+    }
+
+    let with_enum = Some(Enum::A());
+    with_enum.unwrap_or_else(Enum::A);
+
+    let with_new = Some(vec![1]);
+    with_new.unwrap_or_else(Vec::new);
+
+    let with_err: Result<_, ()> = Ok(vec![1]);
+    with_err.unwrap_or_else(make);
+
+    // should not be changed
+    let with_fake_default = None::<Foo>;
+    with_fake_default.unwrap_or_else(Foo::default);
+
+    // should not be changed
+    let with_fake_default2 = None::<HasDefaultAndDuplicate>;
+    with_fake_default2.unwrap_or_else(<HasDefaultAndDuplicate>::default);
+
+    let with_real_default = None::<HasDefaultAndDuplicate>;
+    with_real_default.unwrap_or_else(<HasDefaultAndDuplicate as Default>::default);
+
+    let with_default_trait = Some(1);
+    with_default_trait.unwrap_or_else(Default::default);
+
+    let with_default_type = Some(1);
+    with_default_type.unwrap_or_else(u64::default);
+}
+
+fn main() {}

tests/ui/unwrap_or_else_default.stderr

@@ -0,0 +1,22 @@
+error: use of `.unwrap_or_else(..)` to construct default value
+  --> $DIR/unwrap_or_else_default.rs:62:5
+   |
+LL |     with_real_default.unwrap_or_else(<HasDefaultAndDuplicate as Default>::default);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `with_real_default.unwrap_or_default()`
+   |
+   = note: `-D clippy::unwrap-or-else-default` implied by `-D warnings`
+
+error: use of `.unwrap_or_else(..)` to construct default value
+  --> $DIR/unwrap_or_else_default.rs:65:5
+   |
+LL |     with_default_trait.unwrap_or_else(Default::default);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `with_default_trait.unwrap_or_default()`
+
+error: use of `.unwrap_or_else(..)` to construct default value
+  --> $DIR/unwrap_or_else_default.rs:68:5
+   |
+LL |     with_default_type.unwrap_or_else(u64::default);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `with_default_type.unwrap_or_default()`
+
+error: aborting due to 3 previous errors
+