RFC: truly unsized types

text/0000-truly-unsized-types.md

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+- Start Date: 2015-01-22
+- RFC PR:
+- Rust Issue:
+
+# Summary
+
+Further subdivide unsized types into dynamically-sized types, implementing
+an intrinsic trait `DynamicSize`, and types of indeterminate size. References
+for the latter kind will be thin, while allocating slots or copying values
+of such types is not possible outside unsafe code.
+
+# Motivation
+
+There are cases where borrowed references provide a view to a larger object
+in memory than the nominal sized type being referenced. Of interest are:
+
+- Token types for references to data structures whose size is not available as
+  a stored value, but determined by parsing the content, such as
+  null-terminated C strings. An example is `CStr`, a
+  [proposed](https://github.com/rust-lang/rfcs/pull/592) 'thin' dereference
+  type for `CString`.
+- References to structs as public views into larger contiguously allocated
+  objects, the size and layout of which is hidden from the client.
+
+When only immutable references are available on such types, it's possible
+to prevent copying out the nominal sized value in safe code, but there is
+still potential for unintentional misuse due to values under reference having
+a bogus size.
+If mutable references are available, there is added trouble with
+`std::mem::swap` and similar operations.
+
+# Detailed design
+
+The types that do not implement `Sized` are further subdivided into DSTs
+and types of indeterminate size.
+
+## Truly unsized types
+
+Any type with sized contents can be opted out of `Sized` by including
+a marker `NotSized`, which also implies `NoCopy`:
+```rust
+struct CStr {
+    head: libc::c_char,  // Contains at least one character...
+    rest: std::marker::NotSized  // ...but there can be more
+}
+```
+
+This makes references to values of such types unusable for copying
+the value out, `std::mem::swap`, being the source in
+`transmute_copy`, etc. Unsized types can't be used as the
+type parameter of `size_of`.
+
+## Dynamically sized types
+
+Plain old (ahem) dynamically sized types will intrinsically implement
+a newly introduced trait, `DynamicSize`. Only references to `DynamicSize`
+types will be fat.
+
+# Fallout
+
+There may be cases where `!Sized` is taken to mean DSTs. These will have to
+switch to using the `DynamicSize` bound.
+
+Specifically, there are generic items where the `Sized` bound is not
+lifted only to ensure that a reference is thin, so as to be coerced or
+transmuted to a raw pointer. These will not be usable with truly unsized
+types, and should get the type bound relaxed to `?Sized` and a
+[negative bound](https://github.com/rust-lang/rfcs/pull/586)
+on `DynamicSize`.
+
+# Drawbacks
+
+This is adding further complexity to the type system.
+
+# Alternatives
+
+Keep to the current practice of Irrelevantly Sized Types, learning to avoid
+trouble by design discipline, documentation, and best practices. The problem
+of mutable references can be resolved on a case-by-case basis by providing
+an accessor facade for fields that can be safely mutated, and disallowing
+direct mutable references to the pseudo-sized type. This feels at odds
+with the overall philosophy of Rust, though.
+
+# Unresolved questions
+
+For convenience, there could be an intrinsically implemented trait
+that is a negation of `DynamicSize`.