RFC - Zero-Sized References

text/0000-zero-sized-references.md

@@ -0,0 +1,149 @@
+- Feature Name: zero_sized_references
+- Start Date: 2017-06-23
+- RFC PR: [#2040](https://github.com/rust-lang/rfcs/pull/2040)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+References to Zero-Sized Types (both shared and mutable) have been historically the size of `usize`.
+The proposed change is to make them also ZST.
+
+# Motivation
+[motivation]: #motivation
+
+References to any type in rust are represented as a pointer. Usually the pointer is smaller and faster to move around.
+However for Zero-Sized Types that only have a single value (for example `()` ) moving around is a no-op, and can be optimized away.
+Reading and writing the value is a no-op since it has only a single value anyway and therefore it carries no extra information.
+However, currently the compiler can't optimize away the pointer from data structures.
+
+Zero-Sized Types are useful for functions, lifetime guarantees and destructors,
+and references to them can be used to show these types "exist" (for references) or "you are the only one using it" (for mutable references).
+The actual value is meaningless and the representation should be optimized to be of size 0 as well.
+
+In addition, references to Zero-Sized Types often appear in polymorphic code, where they handle non-ZST as well.
+
+In both of these cases, it will be an advantage for the references to be Zero-Sized.
+
+# Detailed design
+[design]: #detailed-design
+
+### Calculating size
+[calculating-size]: #calculating-size
+
+Disclaimer: The writer of this RFC is not familiar with the inworkings of the compiler.
+
+
+Finding if a reference points to a ZST or not may not always be trivial.
+A struct with a reference to itself will not know it's size until after it knows the size to the reference.
+
+However, a couple of notes:
+
+```rust
+struct A<'a> (&'a A<'a>);
+```
+This struct cannot be instantiated, because the very first instance of it requires an instance of it to already exist.
+
+```rust
+enum A<'a> {
+    ZeroSized,
+    SelfRef (&'a A<'a>),
+}
+```
+The moment an enum has more than a single value, it cannot be Zero-Sized. Otherwise it isn't different than a struct.
+
+Therefore, I propose to assume that whenever you find a self reference (or multiple types referencing in a loop),
+decide the reference is not Zero-Sized, since there most likely WILL be other data somewhere in the chain.
+This is most relevant to unions, which could have self-references and be instantiated at the same time.
+
+### `*const` and `*mut` pointers
+[pointers]: #pointers
+
+It is possible to convert a reference to a pointer. Currently, a reference to ZST points to an arbitrary location,
+and when converting to a pointer the pointer recieves that arbitrary location.
+
+After this change, the reference will not hold any data. I propose that whenever a ZST reference is converted to a pointer,
+a warning/error be issued ("Warning: taking the address of a Zero-Sized Type is meaningless") and the pointer will recieve an address
+with the same algorithm that assigned an address to the reference in our current implementation.
+
+The purpose is to not break current code that might do this. We probably don't want to assign Null since for pointers it has
+a meaning that the value doesn't exist, which is different than "exist but no data".
+
+Converting in the other direction, the value of the pointer will be silently dropped - that value never had a meaning in the first place.
+
+# How We Teach This
+[how-we-teach-this]: #how-we-teach-this
+
+For most rust users, this change will be invisible. Their code will just become a tiny bit smaller.
+
+Users of unsafe rust might encounter this case. Therefore there should probably be a note in the nomicon that references might be optimized away for ZST.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+### Breaking code
+
+Any place that assumed a reference holds a pointer might introduce bugs.
+
+FFI might behave differently than now, breaking code (see Unresolved questions).
+
+# Alternatives
+[alternatives]: #alternatives
+
+The system that is now works well, and does not have to be changed.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+### Definition of `&`
+
+Is the definition of `&` state that we guarantee it's a pointer? Or do we only promise you can use it to access the data?
+
+### FFI + Escape mechanism
+
+Do we need a way to remove the optimization? For example, if we have
+```rust
+struct ExternalStruct;
+
+let x: &mut ExternalStruct = ffi_function();
+other_ffi_function(x);
+```
+We might want to represent a pointer we got from FFI and checked it's not null as a reference to an object.
+Since that object is not accessable directly we represent it as an empty struct.
+However then the references to it are optimized away and we lose the pointer and then can't call the second function.
+
+Note: This break will not be silent. The safe wrapper for other_ffi_function will convert the reference back to pointer before
+using it, raising the warning/error of converting references to pointers.
+
+For that case, we might want to mark `ExternalStruct` as Non-Zero-Sized. Possibly
+```rust
+impl !Sized for ExternalStruct;
+```
+and maybe it should require `unsafe`.
+
+### Errors of conversion
+
+Do we want to give an error or a warning for converting references to pointers?
+
+The code above is an example where we break working code, so an error might be needed to show the significance.
+
+However, some conversions might be meaningless and wouldn't affect the execution, so the programmer might allow the conversion.
+
+### Mitigating breakage
+
+Safe rust code should be affected positively by this change. However unsafe code might break.
+
+FFI is especially vulnurable to this change, as shown above.
+Are there better ways to deal with these errors without user involvement?
+
+(Unsafe code might break - are there examples of VALID code that breaks? Or does only invalid uses of references break?
+And what is our stance on breaking invalid code, assuming there is a large amount of it?)
+
+### Specific examples - Pro
+
+The RFC isn't well-justified until it has at least one detailed use case where it helps.
+Please share specific examples of code where Zero-Sized references are useful.
+
+### Specific examples against
+
+If you have specific examples where this change is detrimental, please share them.