Make size an associated constant

text/0000-associated-size.md

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+- Feature Name: associated_size
+- Start Date: 2015-06-22
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Provide the size and alignment of a type as associated constants.
+
+# Motivation
+
+Constants and array bounds can only contain constants, which currently does not
+include sizes of types. By making them associated constants they become more
+versatile and can e. g. be used in `const fn`s.
+
+# Detailed design
+
+Extend the `Sized` trait by two associated constants `SIZE` and `ALIGNMENT`,
+which contain the size of the type in bytes, and the preferred alignment,
+respectively.
+
+# Drawbacks
+
+`SIZE` and `ALIGNMENT` (names still up to debate) are going to be members of
+every type, as such they are a slight backward incompatiblity.
+
+# Alternatives
+
+An alternative name for `ALIGNMENT` would be `ALIGN`, also, one could consider
+to use snake case for associated constants, which would result in `size` and
+`align`.
+
+Additionally, one could keep the current design and make these values available
+via freestanding `const fn`s. However in the past, Rust tried to avoid using
+free functions instead of member functions, or in this case, associated
+constants.
+
+# Unresolved questions
+
+None.