RFC: Tuple indexing

active/0000-tuple-accessors.md

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+- Start Date: 2014-07-24
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+Summary
+=======
+
+Add simple syntax for accessing values within tuples and tuple structs.
+
+Motivation
+==========
+
+Right now accessing fields of tuples and tuple structs is incredibly painful—one
+must rely on pattern matching alone to extract values. This became such a
+problem that twelve traits were created in the standard library
+(`core::tuple::Tuple*`) to make tuple value accesses easier, adding `.valN()`,
+`.refN()`, and `.mutN()` methods to help this. But this is not a very nice
+solution—it requires the traits to be implemented in the standard library, not
+the language, and for those traits to be imported on use. On the whole this is
+not a problem, because most of the time `std::prelude::*` is imported, but this
+is still a hack which is not a real solution to the problem at hand. It also
+only supports tuples of length up to twelve, which is normally not a problem but
+emphasises how bad the current situation is.
+
+Detailed design
+===============
+
+Add syntax of the form `<expr>.<integer>` for accessing values within tuples and
+tuple structs. This syntax is recognised wherever an integer or float literal is
+found in place of the normal field or method name expected when accessing fields
+with `.`. Float literals in this position are expanded into two field accesses,
+so that an expression of the form `a.1.3` is equivalent to `(a.1).3`.
+
+Accessing a tuple or tuple struct field like so:
+
+```rust
+let x = (box 1i, box 2i);
+let x1 = x.1;
+```
+
+is roughly equivalent to:
+
+```rust
+let x = (box 1i, box 2i);
+let x1 = { let (_, a) = x; a };
+```
+
+However, when taking a (possibly mutable) reference to a field, the equivalent
+expansion is slightly different:
+
+```rust
+let x = (box 1i, box 2i);
+let x1 = &x.1;
+```
+
+is roughly equivalent to:
+
+```rust
+let x = (box 1i, box 2i);
+let x1 = { let (_, ref a) = x; a };
+```
+
+A similar process is performed with `&mut`.
+
+Drawbacks
+=========
+
+More complexity that is not strictly necessary.
+
+Alternatives
+============
+
+Allow indexing of tuples and tuple structs: this has the advantage of
+consistency, but the disadvantage of not being checked for out-of-bounds errors
+at compile time.
+
+Unresolved questions
+====================
+
+None.