RFC: map_or_default in Option and Result

text/0000-option-result-map-or-default.md

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+- Feature Name: `option_result_map_or_default`
+- Start Date: 2021-07-14
+- RFC PR: [rust-lang/rfcs#3148](/~https://github.com/rust-lang/rfcs/pull/3148)
+- Rust Issue: [rust-lang/rust#0000](/~https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+`Option` has the methods `unwrap`, `unwrap_or`, `unwrap_or_else` and `unwrap_or_default`. It
+similarly has `map`, `map_or`, `map_or_else`, however `map_or_default` is missing. The exact same
+problem exists for `Result`. This RFC is a proposal to add this method to `Option` and `Result`.
+
+# Motivation
+[motivation]: #motivation
+
+As mentioned before, a user might reasonably expect this method to exist, based on the existence of
+other `or_default` methods such as `unwrap_or_default`. Furthermore, this is a very common usecase.
+Searching for `map_or_else` in `.rs` files in the official Rust repository it is incredibly common
+to see instances like these:
+
+```rust
+.map_or_else(String::new, ...)
+.map_or_else(SmallVec::new, ...)
+.map_or_else(Vec::new, ...)
+```
+
+In fact, from a manual count at the time of writing, 25 out of the 57 occurrences of `map_or_else`
+in the Rust codebase could have been replaced with an equivalent call to `map_or_default`.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The following implementation would get added to `core::option::Option`:
+
+```rust
+impl<T> Option<T> {
+    pub fn map_or_default<U: Default, F: FnOnce(T) -> U>(self, f: F) -> U {
+        match self {
+            Some(t) => f(t),
+            None => Default::default(),
+        }
+    }
+}
+```
+
+The following implementation would get added to `core::result::Result`:
+
+```rust
+impl<T, E> Result<T, E> {
+    pub fn map_or_default<U: Default, F: FnOnce(T) -> U>(self, f: F) -> U {
+        match self {
+            Ok(t) => f(t),
+            Err(e) => Default::default(),
+        }
+    }
+}
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+It adds another method to `Option` and `Result`, which may be considered as cluttered by some.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+ - `.map_or_else(Default::default, ...)` can be written, although it is significantly longer.
+ - In case `feature(default_free_fn)` stabilizes a user can write `.map_or_else(default, ...)`
+   after `std::default::default`, which is a bit shorter.
+
+However, neither alternative solves the discrepancy between `unwrap_or_default` existing but 
+`map_or_default` not existing.
+
+# Prior art
+[prior-art]: #prior-art
+
+We already have `unwrap_or_default`.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.