Added representation of pointer types. (#51)

* Added representation of pointer types.

* Moved same-representation discussion of fat pointers to non-normative text

* Responding to review comments

* Responding to review comments

* Responding to review comments

* Update reference/src/representation/pointers.md

Co-Authored-By: asajeffrey <[email protected]>

* Responding to review comments

* Added text about &str

* Added bullet points making size/alignment clearer

* Use *T rather than &T

Author: Alan Jeffrey

reference/src/representation/pointers.md

@@ -0,0 +1,56 @@
+# Representation of reference and pointer types
+
+### Terminology
+
+Reference types are types of the form `&T`, `&mut T` or `&dyn T`.
+
+Raw pointer types are types of the form `*const T` or `*mut T`.
+We write `*T` when the mutability attribute is unimportant.
+
+### Representation
+
+The alignment of `&T`, `&mut T`, `*const T` and `*mut T` are the same,
+and are at least the word size.
+
+* If `T` is a trait, then the alignment of `&dyn T` is the word size.
+* If `T` is a sized type then the alignment of `&T` is the word size.
+* The alignment of `&[T]` is the word size.
+* The alignment of `&str` is the word size.
+* Alignment in other cases may be more than the word size (e.g., for other dynamically sized types).
+
+The sizes of `&T`, `&mut T`, `*const T` and `*mut T` are the same,
+and are at least one word.
+
+* If `T` is a trait, then the size of `&dyn T` is two words.
+* If `T` is a sized type then the size of `&T` is one word.
+* The size of `&[T]` is two words.
+* The size of `&str` is two words.
+* Size in other cases may be more than one word (e.g., for other dynamically sized types).
+
+### Notes
+
+The representations of `&T`, `&mut T` and `*T` are the same.
+
+We do not make any guarantees about the representation of
+multi-trait objects `&(dyn T + U)` or references to other dynamically sized types,
+other than that they are at least word-aligned, and have size at least one word.
+
+The representation of `&dyn T` when `T` is a trait is the same as that of:
+```rust
+#[repr(C)]
+struct DynObject {
+  data: *u8,
+  vtable: *u8,
+}
+```
+
+The representation of `&[T]` is the same as that of:
+```rust
+#[repr(C)]
+struct Slice<T> {
+  ptr: *T,
+  len: usize,
+}
+```
+
+The representation of `&str` is the same as that of `&[u8]`.