RFC: Change return type of str::replace to MaybeOwned to avoid a copy

active/0000-str-replace-maybeowned.md

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+- Start Date: 2014-04-04
+- RFC PR #:
+- Rust Issue #:
+
+# Summary
+
+Strings have a `fn replace(&self, from: &str, to: &str) -> ~str` method.
+The return value type could be changed to `MaybeOwned`.
+
+# Motivation
+
+When `from` does not appear at all in `self`, the return value equals `self`.
+This is an allocation and a copy that are not strictly necessary.
+With a `MaybeOwned` return value type, we could just return a slice in this case.
+
+# Drawbacks
+
+The API becomes more complex.
+For example, multiple `.replace()` calls can not be chained anymore
+without adding `.as_slice()` calls in-between.
+This could be addressed by implementing string methods for `MaybeOwned`.
+
+# Detailed design
+
+The `MaybeOwned` type already exists in `std::str`
+and is used for the return value of `std::str::from_utf8_lossy`.
+It is either a slice or an owned string.
+
+The change itself is easy enough
+(see my [work-in-progress branch](https://github.com/SimonSapin/rust/compare/str-replace-maybeowned)),
+but see Unresolved questions.
+
+# Alternatives
+
+Status quo: keep `~str` as the return value type of `replace()` for simplicity,
+and accept the cost on unnecessary allocations and copies.
+
+# Unresolved questions
+
+The compiler uses `replace()`.
+How can this change be made without breaking bootstraping?