RFC for while let statement

active/0000-while-let.md

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+- Start Date: 2014-08-27
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Introduce a new `while let PAT = EXPR { BODY }` construct. This allows for using a refutable pattern
+match (with optional variable binding) as the condition of a loop.
+
+# Motivation
+
+Just as `if let` was inspired by Swift, it turns out Swift supports `while let` as well. This was
+not discovered until much too late to include it in the `if let` RFC. It turns out that this sort of
+looping is actually useful on occasion. For example, the desugaring `for` loop is actually a variant
+on this; if `while let` existed it could have been implemented to map `for PAT in EXPR { BODY }` to
+
+```rust
+// the match here is so `for` can accept an rvalue for the iterator,
+// and was used in the "real" desugaring version.
+match &mut BODY {
+    i => {
+        while let Some(PAT) = i.next() {
+            BODY
+        }
+    }
+}
+```
+
+(note that the non-desugared form of `for` is no longer equivalent).
+
+More generally, this construct can be used any time looping + pattern-matching is desired.
+
+This also makes the language a bit more consistent; right now, any condition that can be used with
+`if` can be used with `while`. The new `if let` adds a form of `if` that doesn't map to `while`.
+Supporting `while let` restores the equivalence of these two control-flow constructs.
+
+# Detailed design
+
+`while let` operates similarly to `if let`, in that it desugars to existing syntax. Specifically,
+the syntax
+
+```rust
+['ident:] while let PAT = EXPR {
+    BODY
+}
+```
+
+desugars to
+
+```rust
+['ident:] loop {
+    match EXPR {
+        PAT => BODY,
+        _ => break
+    }
+}
+```
+
+Just as with `if let`, an irrefutable pattern given to `while let` is considered an error. This is
+largely an artifact of the fact that the desugared `match` ends up with an unreachable pattern,
+and is not actually a goal of this syntax. The error may be suppressed in the future, which would be
+a backwards-compatible change.
+
+Just as with `if let`, `while let` will be introduced under a feature gate (named `while_let`).
+
+# Drawbacks
+
+Yet another addition to the grammar. Unlike `if let`, it's not obvious how useful this syntax will
+be.
+
+# Alternatives
+
+As with `if let`, this could plausibly be done with a macro, but it would be ugly and produce bad
+error messages.
+
+`while let` could be extended to support alternative patterns, just as match arms do. This is not
+part of the main proposal for the same reason it was left out of `if let`, which is that a) it looks
+weird, and b) it's a bit of an odd coupling with the `let` keyword as alternatives like this aren't
+going to be introducing variable bindings. However, it would make `while let` more general and able
+to replace more instances of `loop { match { ... } }` than is possible with the main design.
+
+# Unresolved questions
+
+None.