Add a let...else expression, similar to Swift's guard let...else

text/0000-if-not-let.md

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+Moved to [0000-let-else.md](0000-let-else.md).

text/0000-let-else.md

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+- Feature Name: let-else statement
+- Start Date: 2015-09-30
+- RFC PR:
+- Rust Issue:
+
+# Summary
+
+Introduce a new `let PAT = EXPR else { BODY }` construct (informally called an
+**let-else statement**).
+
+If the pattern match succeeds, its bindings are introduced *into the
+surrounding scope*.  If it does not succeed, it must diverge (e.g., return or
+break).  You can think of let-else as a “refutable `let` statement.”
+
+This simplifies some common error-handling patterns, and reduces the need for
+special-purpose control flow macros.
+
+# Motivation
+
+[if-let expressions][if-let] offer a succinct syntax for pattern matching
+with only one “success” path. This is particularly useful for unwrapping
+types like `Option`. However, an if-let expression can only create bindings
+within its body, which can force rightward drift and excessive nesting.
+
+let-else statements move the “failure” case into the body, while allowing
+the “success” case to continue without additional nesting.
+
+## Example
+
+For example, this code written with current Rust syntax:
+
+```rust
+if let Some(a) = x {
+    if let Some(b) = y {
+        if let Some(c) = z {
+            // ...
+            do_something_with(a, b, c);
+            // ...
+        } else {
+            return Err("bad z");
+        }
+    } else {
+        return Err("bad y");
+    }
+} else {
+    return Err("bad x");
+}
+```
+
+would become:
+
+```rust
+let Some(a) = x else {
+    return Err("bad x");
+}
+let Some(b) = y else {
+    return Err("bad y");
+}
+let Some(c) = z else {
+    return Err("bad z");
+}
+// ...
+do_something_with(a, b, c);
+// ...
+```
+
+## Versus `match`
+
+It's possible to use `match` statements to emulate this today, but at a
+significant cost in length and readability.  For example, this real-world code
+from Servo:
+
+```rust
+let subpage_layer_info = match layer_properties.subpage_layer_info {
+    Some(ref subpage_layer_info) => *subpage_layer_info,
+    None => return,
+};
+```
+
+is equivalent to this much simpler let-else statement:
+
+```rust
+let Some(ref subpage_layer_info) = layer_properties.subpage_layer_info else {
+    return
+}
+```
+
+The Swift programming language, which inspired Rust's if-let expression, also
+includes a [guard-let-else][swift] statement which is equivalent to this
+proposal except for the choice of keywords.
+
+# Detailed design
+
+Extend the Rust statement grammar to include the following production:
+
+```
+stmt_let_else = 'let' pat '=' expr 'else' block
+```
+
+The pattern must be refutable.  The body of the let-else statement (the
+`block`) is evaluated only if the pattern match fails.  Any bindings created
+by the pattern match will be in scope after the let-else statement (but not
+within its body).
+
+The body must diverge (i.e., it must panic, loop infinitely, call a diverging
+function, or transfer control out of the enclosing block with a statement such
+as `return`, `break`, or `continue`).  Therefore, code immediately following
+the let-else statement is evaluated only if the pattern match succeeds.
+
+The following code:
+
+```rust
+let pattern = expression else {
+    body
+}
+```
+
+is equivalent to this code in current Rust:
+
+```rust
+// `(a, b, c, ...)` is the list of all bindings in `pattern`.
+let (a, b, c, ...) = match expression {
+    pattern => (a, b, c, ...),
+    _ => { body }
+};
+```
+
+# Drawbacks
+
+* “Must diverge” is an unusual requirement, which might be difficult to
+  explain or lead to confusing errors for programmers new to this feature.
+
+* To a human scanning the code, it's not obvious when looking at the start of
+  a statement whether it is a `let ... else` or a regular `let` statement.
+
+# Alternatives
+
+* Don't make any changes; use existing syntax like `if let` and `match` as
+  shown above, or write macros to simplify the code.
+
+* Use the same semantics with different syntax. For example, the original
+  version of this RFC used `if !let PAT = EXPR { BODY }`.
+
+# Unresolved questions
+
+* How much implementation complexity does this add?
+
+[if-let]: https://github.com/rust-lang/rfcs/blob/master/text/0160-if-let.md
+[swift]: https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/ControlFlow.html#//apple_ref/doc/uid/TP40014097-CH9-ID525