Allow for local variables, function arguments, and some expressions to have an unsized type, and implement it by storing the temporaries in variably-sized allocas.

Have repeat expressions with a length that captures local variables be such an expression, returning an [T] slice.

Provide some optimization guarantees that unnecessary temporaries will not create unnecessary allocas.


There are 2 motivations for this RFC:

  1. Passing unsized values, such as trait objects, to functions by value is often desired. Currently, this must be done through a Box<T> with an unnecessary allocation.

One particularly common example is passing closures that consume their environment without using monomorphization. One would like for this code to work:

fn takes_closure(f: FnOnce()) { f(); }

But today you have to use a hack, such as taking a Box<FnBox<()>>.

  1. Allocating a runtime-sized variable on the stack is important for good performance in some use-cases - see RFC #1808, which this is intended to supersede.

Detailed design

Unsized Rvalues - language

Remove the rule that requires all locals and rvalues to have a sized type. Instead, require the following:

  1. The following expressions must always return a Sized type:
    1. Function calls, method calls, operator expressions
      • implementing unsized return values for function calls would require the called function to do the alloca in our stack frame.
    2. ADT expressions
      • see alternatives
    3. cast expressions
      • this seems like an implementation simplicity thing. These can only be trivial casts.
  2. The RHS of assignment expressions must always have a Sized type.
    • Assigning an unsized type is impossible because we don’t know how much memory is available at the destination. This applies to ExprAssign assignments and not to StmtLet let-statements.

This also allows passing unsized values to functions, with the ABI being as if a &move pointer was passed (a (by-move-data, extra) pair). This also means that methods taking self by value are object-safe, though vtable shims are sometimes needed to translate the ABI (as the callee-side intentionally does not pass extra to the fn in the vtable, no vtable shim is needed if the vtable function already takes its argument indirectly).

For example:

struct StringData {
    len: usize,
    data: [u8],

fn foo(s1: Box<StringData>, s2: Box<StringData>, cond: bool) {
    // this creates a VLA copy of either `s1.1` or `s2.1` on
    // the stack.
    let mut s = if cond {
    } else {

fn example(f: for<'a> FnOnce(&'a X<'a>)) {
    let x = X::new();
    f(x); // aka FnOnce::call_once(f, (x,));

VLA expressions

Allow repeat expressions to capture variables from their surrounding environment. If a repeat expression captures such a variable, it has type [T] with the length being evaluated at run-time. If the repeat expression does not capture any variable, the length is evaluated at compile-time. For example:

extern "C" {
   fn random() -> usize;

fn foo(n: usize) {
    let x = [0u8; n]; // x: [u8]
    let x = [0u8; n + (random() % 100)]; // x: [u8]
    let x = [0u8; 42]; // x: [u8; 42], like today
    let x = [0u8; random() % 100]; //~ ERROR constant evaluation error

“captures a variable” - as in RFC #1558 - is used as the condition for making the return be [T] because it is simple, easy to understand, and introduces no type-checking complications.

The last error message could have a user-helpful note, for example “extract the length to a local variable if you want a variable-length array”.

Unsized Rvalues - MIR

The way this is implemented in MIR is that operands, rvalues, and temporaries are allowed to be unsized. An unsized operand is always “by-ref”. Unsized rvalues are either a Use or a Repeat and both can be translated easily.

Unsized locals can never be reassigned within a scope. When first assigning to an unsized local, a stack allocation is made with the correct size.

MIR construction remains unchanged.

Guaranteed Temporary Elision

MIR likes to create lots of temporaries for OOE reason. We should optimize them out in a guaranteed way in these cases (FIXME: extend these guarantees to locals aka NRVO?).

TODO: add description of problem & solution.

How We Teach This

Passing arguments to functions by value should not be too complicated to teach. I would like VLAs to be mentioned in the book.

The “guaranteed temporary elimination” rules require more work to teach. It might be better to come up with new rules entirely.


In Unsafe code, it is very easy to create unintended temporaries, such as in:

unsafe fn poke(ptr: *mut [u8]) { /* .. */ }
unsafe fn foo(mut a: [u8]) {
    let ptr: *mut [u8] = &mut a;
    // here, `a` must be copied to a temporary, because
    // `poke(ptr)` might access the original.
    bar(a, poke(ptr));

If we make [u8] be Copy, that would be even easier, because even uses of poke(ptr); after the function call could potentially access the supposedly-valid data behind a.

And even if it is not as easy, it is possible to accidentally create temporaries in safe code.

Unsized temporaries are dangerous - they can easily cause aborts through stack overflow.


The bikeshed

There are several alternative options for the VLA syntax.

  1. The RFC choice, [t; φ] has type [T; φ] if φ captures no variables and type [T] if φ captures a variable.
    • pro: can be understood using “HIR”/resolution only.
    • pro: requires no additional syntax.
    • con: might be confusing at first glance.
    • con: [t; foo()] requires the length to be extracted to a local.
  2. The “permissive” choice: [t; φ] has type [T; φ] if φ is a constexpr, otherwise [T]
    • pro: allows the most code
    • pro: requires no additional syntax.
    • con: depends on what is exactly a const expression. This is a big issue because that is both non-local and might change between rustc versions.
  3. Use the expected type - [t; φ] has type [T] if it is evaluated in a context that expects that type (for example [t; foo()]: [T]) and [T; _] otherwise.
    • pro: in most cases, very human-visible.
    • pro: requires no additional syntax.
    • con: relies on the notion of “expected type”. While I think we do have to rely on that in the unsafe code semantics of &foo borrow expressions (as in, whether a borrow is treated as a “safe” or “unsafe” borrow - I’ll write more details sometime), it might be better to not rely on expected types too much.
  4. use an explicit syntax, for example [t; virtual φ].
    • bikeshed: exact syntax.
    • pro: very explicit and visible.
    • con: more syntax.
  5. use an intrinsic, std::intrinsics::repeat(t, n) or something.
    • pro: theoretically minimizes changes to the language.
    • con: requires returning unsized values from intrinsics.
    • con: unergonomic to use.

Unsized ADT Expressions

Allowing unsized ADT expressions would make unsized structs constructible without using unsafe code, as in:

let len_ = s.len();
let p = Box::new(PascalString {
    length: len_,
    data: *s

However, without some way to guarantee that this can be done without allocas, that might be a large footgun.

Copy Slices

One somewhat-orthogonal proposal that came up was to make Clone (and therefore Copy) not depend on Sized, and to make [u8] be Copy, by moving the Self: Sized bound from the trait to the methods, i.e. using the following declaration:

pub trait Clone {
    fn clone(&self) -> Self where Self: Sized;
    fn clone_from(&mut self, source: &Self) where Self: Sized {
        // ...

That would be a backwards-compatibility-breaking change, because today T: Clone + ?Sized (or of course Self: Clone in a trait context, with no implied Self: Sized) implies that T: Sized, but it might be that its impact is small enough to allow (and even if not, it might be worth it for Rust 2.0).

Unresolved questions

How can we mitigate the risk of unintended unsized or large allocas? Note that the problem already exists today with large structs/arrays. A MIR lint against large/variable stack sizes would probably help users avoid these stack overflows. Do we want it in Clippy? rustc?

How do we handle truly-unsized DSTs when we get them? They can theoretically be passed to functions, but they can never be put in temporaries.

Accumulative allocas (aka 'fn borrows) are beyond the scope of this RFC.

See alternatives.