Layout of structs and tuples

Disclaimer: This chapter represents the consensus from issues #11 and #12. The statements in here are not (yet) "guaranteed" not to change until an RFC ratifies them.

Tuple types

In general, an anonymous tuple type (T1..Tn) of arity N is laid out "as if" there were a corresponding tuple struct declared in libcore:

struct TupleN<P1..Pn:?Sized>(P1..Pn);

In this case, (T1..Tn) would be compatible with TupleN<T1..Tn>. As discussed below, this generally means that the compiler is free to re-order field layout as it wishes. Thus, if you would like a guaranteed layout from a tuple, you are generally advised to create a named struct with a #[repr(C)] annotation (see the section on structs for more details).

Note that the final element of a tuple (Pn) is marked as ?Sized to permit unsized tuple coercion -- this is implemented on nightly but is currently unstable (tracking issue). In the future, we may extend unsizing to other elements of tuples as well.

Other notes on tuples

Some related discussion:

  • RFC #1582 proposed that tuple structs should have a "nested layout", where e.g. (T1, T2, T3) would in fact be laid out as (T1, (T2, T3)). The purpose of this was to permit variadic matching and so forth against some suffix of the struct. This RFC was not accepted, however. This layout requires extra padding and seems somewhat surprising: it means that the layout of tuples and tuple structs would diverge significantly from structs with named fields.

Struct types

Structs come in two principle varieties:

// Structs with named fields
struct Foo { f1: T1, .., fn: Tn }

// Tuple structs
struct Foo(T1, .., Tn);

In terms of their layout, tuple structs can be understood as equivalent to a named struct with fields named 0..n-1:

struct Foo {
  0: T1,
  n-1: Tn

(In fact, one may use such field names in patterns or in accessor expressions like foo.0.)

The degrees of freedom the compiler has when computing the layout of an inhabited struct or tuple is to determine the order of the fields, and the "gaps" (often called padding) before, between, and after the fields. The layout of these fields themselves is already entirely determined by their types, and since we intend to allow creating references to fields (&s.f1), structs do not have any wiggle-room there.

This can be visualized as follows:

[ <--> [field 3] <-----> [field 1] <-> [  field 2  ] <--> ]

Figure 1 (struct-field layout): The <-...-> and [ ... ] denote the differently-sized gaps and fields, respectively.

Here, the individual fields are blocks of fixed size (determined by the field's layout). The compiler freely picks an order for the fields to be in (this does not have to be the order of declaration in the source), and it picks the gaps between the fields (under some constraints, such as alignment).

For uninhabited structs or tuples like (i32, !) that do not have a valid inhabitant, the compiler has more freedom. After all, no references to fields can ever be taken. For example, such structs might be zero-sized.

How exactly the compiler picks order and gaps, as well as other aspects of layout beyond size and field offset, can be controlled by a #[repr] attribute:

  • #[repr(Rust)] -- the default.
  • #[repr(C)] -- request C compatibility
  • #[repr(align(N))] -- specify the alignment
  • #[repr(packed)] -- request packed layout where fields are not internally aligned
  • #[repr(transparent)] -- request that a "wrapper struct" be treated "as if" it were an instance of its field type when passed as an argument

Default layout ("repr rust")

With the exception of the guarantees provided below, the default layout of structs is not specified.

As of this writing, we have not reached a full consensus on what limitations should exist on possible field struct layouts, so effectively one must assume that the compiler can select any layout it likes for each struct on each compilation, and it is not required to select the same layout across two compilations. This implies that (among other things) two structs with the same field types may not be laid out in the same way (for example, the hypothetical struct representing tuples may be laid out differently from user-declared structs).

Known things that can influence layout (non-exhaustive):

  • the type of the struct fields and the layout of those types
  • compiler settings, including esoteric choices like optimization fuel

A note on determinism. The definition above does not guarantee determinism between executions of the compiler -- two executions may select different layouts, even if all inputs are identical. Naturally, in practice, the compiler aims to produce deterministic output for a given set of inputs. However, it is difficult to produce a comprehensive summary of the various factors that may affect the layout of structs, and so for the time being we have opted for a conservative definition.

Compiler's current behavior. As of the time of this writing, the compiler will reorder struct fields to minimize the overall size of the struct (and in particular to eliminate padding due to alignment restrictions).

Layout is presently defined not in terms of a "fully monomorphized" struct definition but rather in terms of its generic definition along with a set of substitutions (values for each type parameter; lifetime parameters do not affect layout). This distinction is important because of unsizing -- if the final field has generic type, the compiler will not reorder it, to allow for the possibility of unsizing. E.g., struct Foo { x: u16, y: u32 } and struct Foo<T> { x: u16, y: T } where T = u32 are not guaranteed to be identical.

Zero-sized structs

For repr(Rust), repr(packed(N)), repr(align(N)), and repr(C) structs: if all fields of a struct have size 0, then the struct has size 0.

For example, all these types are zero-sized:

use std::mem::size_of;
#[repr(align(32))] struct Zst0;
#[repr(C)] struct Zst1(Zst0);
struct Zst2(Zst1, Zst0);
fn main() {
assert_eq!(size_of::<Zst0>(), 0);
assert_eq!(size_of::<Zst1>(), 0);
assert_eq!(size_of::<Zst2>(), 0);

In particular, a struct with no fields is a ZST, and if it has no repr attribute it is moreover a 1-ZST as it also has no alignment requirements.

Single-field structs

A struct with only one field has the same layout as that field.

Structs with 1-ZST fields

For the purposes of struct layout 1-ZST fields are ignored.

In particular, if all but one field are 1-ZST, then the struct is equivalent to a single-field struct. In other words, if all but one field is a 1-ZST, then the entire struct has the same layout as that one field.

Similarly, if all fields are 1-ZST, then the struct has the same layout as a struct with no fields, and is itself a 1-ZST.

For example:

fn main() {
type Zst1 = ();
struct S1(i32, Zst1); // same layout as i32

type Zst2 = [u16; 0];
struct S2(Zst2, Zst1); // same layout as Zst2

struct S3(Zst1); // same layout as Zst1

Unresolved questions

During the course of the discussion in #11 and #12, various suggestions arose to limit the compiler's flexibility. These questions are currently considering unresolved and -- for each of them -- an issue has been opened for further discussion on the repository. This section documents the questions and gives a few light details, but the reader is referred to the issues for further discussion.

Homogeneous structs (#36). If you have homogeneous structs, where all the N fields are of a single type T, can we guarantee a mapping to the memory layout of [T; N]? How do we map between the field names and the indices? What about zero-sized types?

Deterministic layout (#35). Can we say that layout is some deterministic function of a certain, fixed set of inputs? This would allow you to be sure that if you do not alter those inputs, your struct layout would not change, even if it meant that you can't predict precisely what it will be. For example, we might say that struct layout is a function of the struct's generic types and its substitutions, full stop -- this would imply that any two structs with the same definition are laid out the same. This might interfere with our ability to do profile-guided layout or to analyze how a struct is used and optimize based on that. Some would call that a feature.

C-compatible layout ("repr C")

For structs tagged #[repr(C)], the compiler will apply a C-like layout scheme. See section of the C17 specification for a detailed write-up of what such rules entail (as well as the relevant specs for your platform). For most platforms, however, this means the following:

  • Field order is preserved.

  • The first field begins at offset 0.

  • Assuming the struct is not packed, each field's offset is aligned1 to the ABI-mandated alignment for that field's type, possibly creating unused padding bits.

  • The total size of the struct is rounded up to its overall alignment.

  • 1

    Aligning an offset O to an alignment A means to round up the offset O until it is a multiple of the alignment A.

The intention is that if one has a set of C struct declarations and a corresponding set of Rust struct declarations, all of which are tagged with #[repr(C)], then the layout of those structs will all be identical. Note that this setup implies that none of the structs in question can contain any #[repr(Rust)] structs (or Rust tuples), as those would have no corresponding C struct declaration -- as #[repr(Rust)] types have undefined layout, you cannot safely declare their layout in a C program.

See also the notes on ABI compatibility under the section on #[repr(transparent)].

Structs with no fields. One area where Rust layout can deviate from C/C++ -- even with #[repr(C)] -- comes about with "empty structs" that have no fields. In C, an empty struct declaration like struct Foo { } is illegal. However, both gcc and clang support options to enable such structs, and assign them size zero. Rust behaves the same way -- empty structs have size 0 and alignment 1 (unless an explicit #[repr(align)] is present). C++, in contrast, gives empty structs a size of 1, unless they are inherited from or they are fields that have the [[no_unique_address]] attribute, in which case they do not increase the overall size of the struct.

Structs of zero-size. It is also possible to have structs that have fields but still have zero size. In this case, the size of the struct would be zero, but its alignment may be greater. For example, #[repr(C)] struct Foo { x: [u16; 0] } would have an alignment of 2 bytes by default. (This matches the behavior in gcc and clang.)

Structs with fields of zero-size. If a #[repr(C)] struct containing a field of zero-size, that field does not occupy space in the struct; it can affect the offsets of subsequent fields if it induces padding due to the alignment on its type. (This matches the behavior in gcc and clang.)

C++ compatibility hazard. As noted above when discussing structs with no fields, C++ treats empty structs like struct Foo { } differently from C and Rust. This can introduce subtle compatibility hazards. If you have an empty struct in your C++ code and you make the "naive" translation into Rust, even tagging with #[repr(C)] will not produce layout- or ABI-compatible results.

Fixed alignment

The #[repr(align(N))] attribute may be used to raise the alignment of a struct, as described in The Rust Reference.

Packed layout

The #[repr(packed(N))] attribute may be used to impose a maximum limit on the alignments for individual fields. It is most commonly used with an alignment of 1, which makes the struct as small as possible. For example, in a #[repr(packed(2))] struct, a u8 or u16 would be aligned at 1- or 2-bytes respectively (as normal), but a u32 would be aligned at only 2 bytes instead of 4. In the absence of an explicit #[repr(align)] directive, #[repr(packed(N))] also sets the alignment for the struct as a whole to N bytes.

The resulting fields may not fall at properly aligned boundaries in memory. This makes it unsafe to create a Rust reference (&T or &mut T) to those fields, as the compiler requires that all reference values must always be aligned (so that it can use more efficient load/store instructions at runtime). See the Rust reference for more details.

Function call ABI compatibility

In general, when invoking functions that use the C ABI, #[repr(C)] structs are guaranteed to be passed in the same way as their corresponding C counterpart (presuming one exists). #[repr(Rust)] structs have no such guarantee. This means that if you have an extern "C" function, you cannot pass a #[repr(Rust)] struct as one of its arguments. Instead, one would typically pass #[repr(C)] structs (or possibly pointers to Rust-structs, if those structs are opaque on the other side, or the callee is defined in Rust).

However, there is a subtle point about C ABIs: in some C ABIs, passing a struct with one field of type T as an argument is not equivalent to just passing a value of type T. So e.g. if you have a C function that is defined to take a uint32_t:

void some_function(uint32_t value) { .. }

It is incorrect to pass in a struct as that value, even if that struct is #[repr(C)] and has only one field:

struct Foo { x: u32 }

extern "C" some_function(Foo);

some_function(Foo { x: 22 }); // Bad!

Instead, you should declare the struct with #[repr(transparent)], which specifies that Foo should use the ABI rules for its field type, u32. This is useful when using "wrapper structs" in Rust to give stronger typing guarantees.

#[repr(transparent)] can only be applied to structs with a single field whose type T has non-zero size, along with some number of other fields whose types are all zero-sized (typically std::marker::PhantomData fields). The struct then takes on the "ABI behavior" of the type T that has non-zero size.

(Note further that the Rust ABI is undefined and theoretically may vary from compiler revision to compiler revision.)

Unresolved question: Guaranteeing compatible layouts?

One key unresolved question was whether we would want to guarantee that two #[repr(Rust)] structs whose fields have the same types are laid out in a "compatible" way, such that one could be transmuted to the other. @rkruppe laid out a number of examples where this might be a reasonable thing to expect. As currently written, and in an effort to be conservative, we make no such guarantee, though we do not firmly rule out doing such a thing in the future.

It seems like it may well be desirable to -- at minimum -- guarantee that #[repr(Rust)] layout is "some deterministic function of the struct declaration and the monomorphized types of its fields". Note that it is not sufficient to consider the monomorphized type of a struct's fields: due to unsizing coercions, it matters whether the struct is declared in a generic way or not, since the "unsized" field must presently be laid out last in the structure. (Note that tuples are always coercible (see #42877 for more information), and are always declared as generics.) This implies that our "deterministic function" also takes as input the form in which the fields are declared in the struct.

However, that rule is not true today. For example, the compiler includes an option (called "optimization fuel") that will enable us to alter the layout of only the "first N" structs declared in the source. When one is accidentally relying on the layout of a structure, this can be used to track down the struct that is causing the problem.

There are also benefits to having fewer guarantees. For example:

  • Code hardening tools can be used to randomize the layout of individual structs.
  • Profile-guided optimization might analyze how instances of a particular struct are used and tweak the layout (e.g., to insert padding and reduce false sharing).
    • However, there aren't many tools that do this sort of thing (1, 2). Moreover, it would probably be better for the tools to merely recommend annotations that could be added (1, 2), such that the knowledge of the improved layouts can be recorded in the source.

As a more declarative alternative, @alercah proposed a possible extension that would permit one to declare that the layout of two structs or types are compatible (e.g., #[repr(as(Foo))] struct Bar { .. }), thus permitting safe transmutes (and also ABI compatibility). One might also use some weaker form of #[repr(C)] to specify a "more deterministic" layout. These areas need future exploration.

Counteropinions and other notes

@joshtripplet argued against reordering struct fields, suggesting instead it would be better if users reordering fields themselves. However, there are a number of downsides to such a proposal (and -- further -- it does not match our existing behavior):

  • In a generic struct, the best ordering of fields may not be known ahead of time, so the user cannot do it manually.
  • If layout is defined, and a library exposes a struct with all public fields, then clients may be more likely to assume that the layout of that struct is stable. If they were to write unsafe code that relied on this assumption, that would break if fields were reordered. But libraries may well expect the freedom to reorder fields. This case is weakened because of the requirement to write unsafe code (after all, one can always write unsafe code that relies on virtually any implementation detail); if we were to permit safe casts that rely on the layout, then reordering fields would clearly be a breaking change (see also this comment and this thread).
  • Many people would prefer the name ordering to be chosen for "readability" and not optimal layout.