The Unstable Book
Welcome to the Unstable Book! This book consists of a number of chapters, each one organized by a "feature flag." That is, when using an unstable feature of Rust, you must use a flag, like this:
#![feature(box_syntax)] fn main() { let five = box 5; }
The box_syntax
feature has a chapter describing how to use it.
Because this documentation relates to unstable features, we make no guarantees that what is contained here is accurate or up to date. It's developed on a best-effort basis. Each page will have a link to its tracking issue with the latest developments; you might want to check those as well.
abi_msp430_interrupt
The tracking issue for this feature is: #38487
abi_ptx
The tracking issue for this feature is: None.
abi_sysv64
The tracking issue for this feature is: #36167
abi_unadjusted
The tracking issue for this feature is: none.
abi_vectorcall
The tracking issue for this feature is: none.
abi_x86_interrupt
The tracking issue for this feature is: #40180
advanced_slice_patterns
The tracking issue for this feature is: #23121
See also slice_patterns
.
The advanced_slice_patterns
gate lets you use ..
to indicate any number of
elements inside a pattern matching a slice. This wildcard can only be used once
for a given array. If there's an identifier before the ..
, the result of the
slice will be bound to that name. For example:
#![feature(advanced_slice_patterns, slice_patterns)] fn is_symmetric(list: &[u32]) -> bool { match list { &[] | &[_] => true, &[x, ref inside.., y] if x == y => is_symmetric(inside), _ => false } } fn main() { let sym = &[0, 1, 4, 2, 4, 1, 0]; assert!(is_symmetric(sym)); let not_sym = &[0, 1, 7, 2, 4, 1, 0]; assert!(!is_symmetric(not_sym)); }
alloc_jemalloc
The tracking issue for this feature is: #33082
See also alloc_system
.
The compiler currently ships two default allocators: alloc_system
and
alloc_jemalloc
(some targets don't have jemalloc, however). These allocators
are normal Rust crates and contain an implementation of the routines to
allocate and deallocate memory. The standard library is not compiled assuming
either one, and the compiler will decide which allocator is in use at
compile-time depending on the type of output artifact being produced.
Binaries generated by the compiler will use alloc_jemalloc
by default (where
available). In this situation the compiler "controls the world" in the sense of
it has power over the final link. Primarily this means that the allocator
decision can be left up the compiler.
Dynamic and static libraries, however, will use alloc_system
by default. Here
Rust is typically a 'guest' in another application or another world where it
cannot authoritatively decide what allocator is in use. As a result it resorts
back to the standard APIs (e.g. malloc
and free
) for acquiring and releasing
memory.
Switching Allocators
Although the compiler's default choices may work most of the time, it's often necessary to tweak certain aspects. Overriding the compiler's decision about which allocator is in use is done simply by linking to the desired allocator:
#![feature(alloc_system)] extern crate alloc_system; fn main() { let a = Box::new(4); // Allocates from the system allocator. println!("{}", a); }
In this example the binary generated will not link to jemalloc by default but instead use the system allocator. Conversely to generate a dynamic library which uses jemalloc by default one would write:
#![feature(alloc_jemalloc)]
#![crate_type = "dylib"]
extern crate alloc_jemalloc;
pub fn foo() {
let a = Box::new(4); // Allocates from jemalloc.
println!("{}", a);
}
# fn main() {}
alloc_system
The tracking issue for this feature is: #33082
See also alloc_jemalloc
.
The compiler currently ships two default allocators: alloc_system
and
alloc_jemalloc
(some targets don't have jemalloc, however). These allocators
are normal Rust crates and contain an implementation of the routines to
allocate and deallocate memory. The standard library is not compiled assuming
either one, and the compiler will decide which allocator is in use at
compile-time depending on the type of output artifact being produced.
Binaries generated by the compiler will use alloc_jemalloc
by default (where
available). In this situation the compiler "controls the world" in the sense of
it has power over the final link. Primarily this means that the allocator
decision can be left up the compiler.
Dynamic and static libraries, however, will use alloc_system
by default. Here
Rust is typically a 'guest' in another application or another world where it
cannot authoritatively decide what allocator is in use. As a result it resorts
back to the standard APIs (e.g. malloc
and free
) for acquiring and releasing
memory.
Switching Allocators
Although the compiler's default choices may work most of the time, it's often necessary to tweak certain aspects. Overriding the compiler's decision about which allocator is in use is done simply by linking to the desired allocator:
#![feature(alloc_system)] extern crate alloc_system; fn main() { let a = Box::new(4); // Allocates from the system allocator. println!("{}", a); }
In this example the binary generated will not link to jemalloc by default but instead use the system allocator. Conversely to generate a dynamic library which uses jemalloc by default one would write:
#![feature(alloc_jemalloc)]
#![crate_type = "dylib"]
extern crate alloc_jemalloc;
pub fn foo() {
let a = Box::new(4); // Allocates from jemalloc.
println!("{}", a);
}
# fn main() {}
allocator
The tracking issue for this feature is: #27389
Sometimes even the choices of jemalloc vs the system allocator aren't enough and
an entirely new custom allocator is required. In this you'll write your own
crate which implements the allocator API (e.g. the same as alloc_system
or
alloc_jemalloc
). As an example, let's take a look at a simplified and
annotated version of alloc_system
# // Only needed for rustdoc --test down below. # #![feature(lang_items)] // The compiler needs to be instructed that this crate is an allocator in order // to realize that when this is linked in another allocator like jemalloc should // not be linked in. #![feature(allocator)] #![allocator] // Allocators are not allowed to depend on the standard library which in turn // requires an allocator in order to avoid circular dependencies. This crate, // however, can use all of libcore. #![no_std] // Let's give a unique name to our custom allocator: #![crate_name = "my_allocator"] #![crate_type = "rlib"] // Our system allocator will use the in-tree libc crate for FFI bindings. Note // that currently the external (crates.io) libc cannot be used because it links // to the standard library (e.g. `#![no_std]` isn't stable yet), so that's why // this specifically requires the in-tree version. #![feature(libc)] extern crate libc; // Listed below are the five allocation functions currently required by custom // allocators. Their signatures and symbol names are not currently typechecked // by the compiler, but this is a future extension and are required to match // what is found below. // // Note that the standard `malloc` and `realloc` functions do not provide a way // to communicate alignment so this implementation would need to be improved // with respect to alignment in that aspect. #[no_mangle] pub extern fn __rust_allocate(size: usize, _align: usize) -> *mut u8 { unsafe { libc::malloc(size as libc::size_t) as *mut u8 } } #[no_mangle] pub extern fn __rust_deallocate(ptr: *mut u8, _old_size: usize, _align: usize) { unsafe { libc::free(ptr as *mut libc::c_void) } } #[no_mangle] pub extern fn __rust_reallocate(ptr: *mut u8, _old_size: usize, size: usize, _align: usize) -> *mut u8 { unsafe { libc::realloc(ptr as *mut libc::c_void, size as libc::size_t) as *mut u8 } } #[no_mangle] pub extern fn __rust_reallocate_inplace(_ptr: *mut u8, old_size: usize, _size: usize, _align: usize) -> usize { old_size // This api is not supported by libc. } #[no_mangle] pub extern fn __rust_usable_size(size: usize, _align: usize) -> usize { size } # // Only needed to get rustdoc to test this: # fn main() {} # #[lang = "panic_fmt"] fn panic_fmt() {} # #[lang = "eh_personality"] fn eh_personality() {} # #[lang = "eh_unwind_resume"] extern fn eh_unwind_resume() {} # #[no_mangle] pub extern fn rust_eh_register_frames () {} # #[no_mangle] pub extern fn rust_eh_unregister_frames () {}
After we compile this crate, it can be used as follows:
extern crate my_allocator;
fn main() {
let a = Box::new(8); // Allocates memory via our custom allocator crate.
println!("{}", a);
}
Custom allocator limitations
There are a few restrictions when working with custom allocators which may cause compiler errors:
-
Any one artifact may only be linked to at most one allocator. Binaries, dylibs, and staticlibs must link to exactly one allocator, and if none have been explicitly chosen the compiler will choose one. On the other hand rlibs do not need to link to an allocator (but still can).
-
A consumer of an allocator is tagged with
#![needs_allocator]
(e.g. theliballoc
crate currently) and an#[allocator]
crate cannot transitively depend on a crate which needs an allocator (e.g. circular dependencies are not allowed). This basically means that allocators must restrict themselves to libcore currently.
allow_internal_unstable
The tracking issue for this feature is: None.
asm
The tracking issue for this feature is: #29722
For extremely low-level manipulations and performance reasons, one
might wish to control the CPU directly. Rust supports using inline
assembly to do this via the asm!
macro.
asm!(assembly template
: output operands
: input operands
: clobbers
: options
);
Any use of asm
is feature gated (requires #![feature(asm)]
on the
crate to allow) and of course requires an unsafe
block.
Note: the examples here are given in x86/x86-64 assembly, but all platforms are supported.
Assembly template
The assembly template
is the only required parameter and must be a
literal string (i.e. ""
)
#![feature(asm)] #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] fn foo() { unsafe { asm!("NOP"); } } // Other platforms: #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))] fn foo() { /* ... */ } fn main() { // ... foo(); // ... }
(The feature(asm)
and #[cfg]
s are omitted from now on.)
Output operands, input operands, clobbers and options are all optional
but you must add the right number of :
if you skip them:
# #![feature(asm)] # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] # fn main() { unsafe { asm!("xor %eax, %eax" : : : "eax" ); # } } # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))] # fn main() {}
Whitespace also doesn't matter:
# #![feature(asm)] # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] # fn main() { unsafe { asm!("xor %eax, %eax" ::: "eax"); # } } # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))] # fn main() {}
Operands
Input and output operands follow the same format: : "constraints1"(expr1), "constraints2"(expr2), ..."
. Output operand
expressions must be mutable lvalues, or not yet assigned:
# #![feature(asm)] # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] fn add(a: i32, b: i32) -> i32 { let c: i32; unsafe { asm!("add $2, $0" : "=r"(c) : "0"(a), "r"(b) ); } c } # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))] # fn add(a: i32, b: i32) -> i32 { a + b } fn main() { assert_eq!(add(3, 14159), 14162) }
If you would like to use real operands in this position, however,
you are required to put curly braces {}
around the register that
you want, and you are required to put the specific size of the
operand. This is useful for very low level programming, where
which register you use is important:
# #![allow(unused_variables)] # #fn main() { # #![feature(asm)] # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] # unsafe fn read_byte_in(port: u16) -> u8 { let result: u8; asm!("in %dx, %al" : "={al}"(result) : "{dx}"(port)); result # } #}
Clobbers
Some instructions modify registers which might otherwise have held different values so we use the clobbers list to indicate to the compiler not to assume any values loaded into those registers will stay valid.
# #![feature(asm)] # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] # fn main() { unsafe { // Put the value 0x200 in eax: asm!("mov $$0x200, %eax" : /* no outputs */ : /* no inputs */ : "eax"); # } } # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))] # fn main() {}
Input and output registers need not be listed since that information is already communicated by the given constraints. Otherwise, any other registers used either implicitly or explicitly should be listed.
If the assembly changes the condition code register cc
should be
specified as one of the clobbers. Similarly, if the assembly modifies
memory, memory
should also be specified.
Options
The last section, options
is specific to Rust. The format is comma
separated literal strings (i.e. :"foo", "bar", "baz"
). It's used to
specify some extra info about the inline assembly:
Current valid options are:
- volatile - specifying this is analogous to
__asm__ __volatile__ (...)
in gcc/clang. - alignstack - certain instructions expect the stack to be aligned a certain way (i.e. SSE) and specifying this indicates to the compiler to insert its usual stack alignment code
- intel - use intel syntax instead of the default AT&T.
# #![feature(asm)] # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] # fn main() { let result: i32; unsafe { asm!("mov eax, 2" : "={eax}"(result) : : : "intel") } println!("eax is currently {}", result); # } # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))] # fn main() {}
More Information
The current implementation of the asm!
macro is a direct binding to LLVM's
inline assembler expressions, so be sure to check out their
documentation as well for more information about clobbers,
constraints, etc.
associated_consts
The tracking issue for this feature is: #29646
With the associated_consts
feature, you can define constants like this:
#![feature(associated_consts)] trait Foo { const ID: i32; } impl Foo for i32 { const ID: i32 = 1; } fn main() { assert_eq!(1, i32::ID); }
Any implementor of Foo
will have to define ID
. Without the definition:
#![feature(associated_consts)]
trait Foo {
const ID: i32;
}
impl Foo for i32 {
}
gives
error: not all trait items implemented, missing: `ID` [E0046]
impl Foo for i32 {
}
A default value can be implemented as well:
#![feature(associated_consts)] trait Foo { const ID: i32 = 1; } impl Foo for i32 { } impl Foo for i64 { const ID: i32 = 5; } fn main() { assert_eq!(1, i32::ID); assert_eq!(5, i64::ID); }
As you can see, when implementing Foo
, you can leave it unimplemented, as
with i32
. It will then use the default value. But, as in i64
, we can also
add our own definition.
Associated constants don’t have to be associated with a trait. An impl
block
for a struct
or an enum
works fine too:
# #![allow(unused_variables)] # #fn main() { #![feature(associated_consts)] struct Foo; impl Foo { const FOO: u32 = 3; } #}
associated_type_defaults
The tracking issue for this feature is: #29661
attr_literals
The tracking issue for this feature is: #34981
box_patterns
The tracking issue for this feature is: #29641
See also box_syntax
Box patterns let you match on Box<T>
s:
#![feature(box_patterns)] fn main() { let b = Some(Box::new(5)); match b { Some(box n) if n < 0 => { println!("Box contains negative number {}", n); }, Some(box n) if n >= 0 => { println!("Box contains non-negative number {}", n); }, None => { println!("No box"); }, _ => unreachable!() } }
box_syntax
The tracking issue for this feature is: #27779
See also box_patterns
Currently the only stable way to create a Box
is via the Box::new
method.
Also it is not possible in stable Rust to destructure a Box
in a match
pattern. The unstable box
keyword can be used to create a Box
. An example
usage would be:
#![feature(box_syntax)] fn main() { let b = box 5; }
cfg_target_feature
The tracking issue for this feature is: #29717
cfg_target_has_atomic
The tracking issue for this feature is: #32976
cfg_target_thread_local
The tracking issue for this feature is: #29594
cfg_target_feature
The tracking issue for this feature is: #29717
compiler_builtins
The tracking issue for this feature is: None.
concat_idents
The tracking issue for this feature is: #29599
conservative_impl_trait
The tracking issue for this feature is: #34511
const_fn
The tracking issue for this feature is: #24111
const_indexing
The tracking issue for this feature is: #29947
custom_attribute
The tracking issue for this feature is: #29642
custom_derive
The tracking issue for this feature is: #29644
default_type_parameter_fallback
The tracking issue for this feature is: #27336
drop_types_in_const
The tracking issue for this feature is: #33156
dropck_eyepatch
The tracking issue for this feature is: #34761
dropck_parametricity
The tracking issue for this feature is: #28498
exclusive_range_pattern
The tracking issue for this feature is: #37854
field_init_shorthand
The tracking issue for this feature is: #37340
fundamental
The tracking issue for this feature is: #29635
generic_param_attrs
The tracking issue for this feature is: #34761
i128_type
The tracking issue for this feature is: #35118
inclusive_range_syntax
The tracking issue for this feature is: #28237
intrinsics
The tracking issue for this feature is: None.
Intrinsics are never intended to be stable directly, but intrinsics are often exported in some sort of stable manner. Prefer using the stable interfaces to the intrinsic directly when you can.
These are imported as if they were FFI functions, with the special
rust-intrinsic
ABI. For example, if one was in a freestanding
context, but wished to be able to transmute
between types, and
perform efficient pointer arithmetic, one would import those functions
via a declaration like
#![feature(intrinsics)] # fn main() {} extern "rust-intrinsic" { fn transmute<T, U>(x: T) -> U; fn offset<T>(dst: *const T, offset: isize) -> *const T; }
As with any other FFI functions, these are always unsafe
to call.
lang_items
The tracking issue for this feature is: None.
The rustc
compiler has certain pluggable operations, that is,
functionality that isn't hard-coded into the language, but is
implemented in libraries, with a special marker to tell the compiler
it exists. The marker is the attribute #[lang = "..."]
and there are
various different values of ...
, i.e. various different 'lang
items'.
For example, Box
pointers require two lang items, one for allocation
and one for deallocation. A freestanding program that uses the Box
sugar for dynamic allocations via malloc
and free
:
#![feature(lang_items, box_syntax, start, libc, core_intrinsics)]
#![no_std]
use core::intrinsics;
extern crate libc;
#[lang = "owned_box"]
pub struct Box<T>(*mut T);
#[lang = "exchange_malloc"]
unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
let p = libc::malloc(size as libc::size_t) as *mut u8;
// Check if `malloc` failed:
if p as usize == 0 {
intrinsics::abort();
}
p
}
#[lang = "exchange_free"]
unsafe fn deallocate(ptr: *mut u8, _size: usize, _align: usize) {
libc::free(ptr as *mut libc::c_void)
}
#[lang = "box_free"]
unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
deallocate(ptr as *mut u8, ::core::mem::size_of_val(&*ptr), ::core::mem::align_of_val(&*ptr));
}
#[start]
fn main(argc: isize, argv: *const *const u8) -> isize {
let x = box 1;
0
}
#[lang = "eh_personality"] extern fn rust_eh_personality() {}
#[lang = "panic_fmt"] extern fn rust_begin_panic() -> ! { unsafe { intrinsics::abort() } }
# #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
# #[no_mangle] pub extern fn rust_eh_register_frames () {}
# #[no_mangle] pub extern fn rust_eh_unregister_frames () {}
Note the use of abort
: the exchange_malloc
lang item is assumed to
return a valid pointer, and so needs to do the check internally.
Other features provided by lang items include:
- overloadable operators via traits: the traits corresponding to the
==
,<
, dereferencing (*
) and+
(etc.) operators are all marked with lang items; those specific four areeq
,ord
,deref
, andadd
respectively. - stack unwinding and general failure; the
eh_personality
,eh_unwind_resume
,fail
andfail_bounds_checks
lang items. - the traits in
std::marker
used to indicate types of various kinds; lang itemssend
,sync
andcopy
. - the marker types and variance indicators found in
std::marker
; lang itemscovariant_type
,contravariant_lifetime
, etc.
Lang items are loaded lazily by the compiler; e.g. if one never uses
Box
then there is no need to define functions for exchange_malloc
and exchange_free
. rustc
will emit an error when an item is needed
but not found in the current crate or any that it depends on.
Most lang items are defined by libcore
, but if you're trying to build
an executable without the standard library, you'll run into the need
for lang items. The rest of this page focuses on this use-case, even though
lang items are a bit broader than that.
Using libc
In order to build a #[no_std]
executable we will need libc as a dependency.
We can specify this using our Cargo.toml
file:
[dependencies]
libc = { version = "0.2.14", default-features = false }
Note that the default features have been disabled. This is a critical step - the default features of libc include the standard library and so must be disabled.
Writing an executable without stdlib
Controlling the entry point is possible in two ways: the #[start]
attribute,
or overriding the default shim for the C main
function with your own.
The function marked #[start]
is passed the command line parameters
in the same format as C:
#![feature(lang_items, core_intrinsics)]
#![feature(start)]
#![no_std]
use core::intrinsics;
// Pull in the system libc library for what crt0.o likely requires.
extern crate libc;
// Entry point for this program.
#[start]
fn start(_argc: isize, _argv: *const *const u8) -> isize {
0
}
// These functions are used by the compiler, but not
// for a bare-bones hello world. These are normally
// provided by libstd.
#[lang = "eh_personality"]
#[no_mangle]
pub extern fn rust_eh_personality() {
}
// This function may be needed based on the compilation target.
#[lang = "eh_unwind_resume"]
#[no_mangle]
pub extern fn rust_eh_unwind_resume() {
}
#[lang = "panic_fmt"]
#[no_mangle]
pub extern fn rust_begin_panic(_msg: core::fmt::Arguments,
_file: &'static str,
_line: u32) -> ! {
unsafe { intrinsics::abort() }
}
To override the compiler-inserted main
shim, one has to disable it
with #![no_main]
and then create the appropriate symbol with the
correct ABI and the correct name, which requires overriding the
compiler's name mangling too:
#![feature(lang_items, core_intrinsics)]
#![feature(start)]
#![no_std]
#![no_main]
use core::intrinsics;
// Pull in the system libc library for what crt0.o likely requires.
extern crate libc;
// Entry point for this program.
#[no_mangle] // ensure that this symbol is called `main` in the output
pub extern fn main(_argc: i32, _argv: *const *const u8) -> i32 {
0
}
// These functions are used by the compiler, but not
// for a bare-bones hello world. These are normally
// provided by libstd.
#[lang = "eh_personality"]
#[no_mangle]
pub extern fn rust_eh_personality() {
}
// This function may be needed based on the compilation target.
#[lang = "eh_unwind_resume"]
#[no_mangle]
pub extern fn rust_eh_unwind_resume() {
}
#[lang = "panic_fmt"]
#[no_mangle]
pub extern fn rust_begin_panic(_msg: core::fmt::Arguments,
_file: &'static str,
_line: u32) -> ! {
unsafe { intrinsics::abort() }
}
More about the language items
The compiler currently makes a few assumptions about symbols which are available in the executable to call. Normally these functions are provided by the standard library, but without it you must define your own. These symbols are called "language items", and they each have an internal name, and then a signature that an implementation must conform to.
The first of these functions, rust_eh_personality
, is used by the failure
mechanisms of the compiler. This is often mapped to GCC's personality function
(see the libstd implementation for more information), but crates
which do not trigger a panic can be assured that this function is never
called. The language item's name is eh_personality
.
The second function, rust_begin_panic
, is also used by the failure mechanisms of the
compiler. When a panic happens, this controls the message that's displayed on
the screen. While the language item's name is panic_fmt
, the symbol name is
rust_begin_panic
.
A third function, rust_eh_unwind_resume
, is also needed if the custom_unwind_resume
flag is set in the options of the compilation target. It allows customizing the
process of resuming unwind at the end of the landing pads. The language item's name
is eh_unwind_resume
.
link_args
The tracking issue for this feature is: #29596
You can tell rustc
how to customize linking, and that is via the link_args
attribute. This attribute is applied to extern
blocks and specifies raw flags
which need to get passed to the linker when producing an artifact. An example
usage would be:
#![feature(link_args)] #[link_args = "-foo -bar -baz"] extern {} # fn main() {}
Note that this feature is currently hidden behind the feature(link_args)
gate
because this is not a sanctioned way of performing linking. Right now rustc
shells out to the system linker (gcc
on most systems, link.exe
on MSVC), so
it makes sense to provide extra command line arguments, but this will not
always be the case. In the future rustc
may use LLVM directly to link native
libraries, in which case link_args
will have no meaning. You can achieve the
same effect as the link_args
attribute with the -C link-args
argument to
rustc
.
It is highly recommended to not use this attribute, and rather use the more
formal #[link(...)]
attribute on extern
blocks instead.
link_cfg
The tracking issue for this feature is: #37406
link_llvm_intrinsics
The tracking issue for this feature is: #29602
linkage
The tracking issue for this feature is: #29603
log_syntax
The tracking issue for this feature is: #29598
loop_break_value
The tracking issue for this feature is: #37339
macro_reexport
The tracking issue for this feature is: #29638
main
The tracking issue for this feature is: #29634
naked_functions
The tracking issue for this feature is: #32408
needs_allocator
The tracking issue for this feature is: #27389
needs_panic_runtime
The tracking issue for this feature is: #32837
never_type
The tracking issue for this feature is: #35121
no_core
The tracking issue for this feature is: #29639
no_debug
The tracking issue for this feature is: #29721
non_ascii_idents
The tracking issue for this feature is: #28979
omit_gdb_pretty_printer_section
The tracking issue for this feature is: None.
on_unimplemented
The tracking issue for this feature is: #29628
optin_builtin_traits
The tracking issue for this feature is: #13231
panic_runtime
The tracking issue for this feature is: #32837
placement_in_syntax
The tracking issue for this feature is: #27779
platform_intrinsics
The tracking issue for this feature is: #27731
plugin
The tracking issue for this feature is: #29597
This feature is part of "compiler plugins." It will often be used with the
plugin_registrar
and rustc_private
features.
rustc
can load compiler plugins, which are user-provided libraries that
extend the compiler's behavior with new syntax extensions, lint checks, etc.
A plugin is a dynamic library crate with a designated registrar function that
registers extensions with rustc
. Other crates can load these extensions using
the crate attribute #![plugin(...)]
. See the
rustc_plugin
documentation for more about the
mechanics of defining and loading a plugin.
If present, arguments passed as #![plugin(foo(... args ...))]
are not
interpreted by rustc itself. They are provided to the plugin through the
Registry
's args
method.
In the vast majority of cases, a plugin should only be used through
#![plugin]
and not through an extern crate
item. Linking a plugin would
pull in all of libsyntax and librustc as dependencies of your crate. This is
generally unwanted unless you are building another plugin. The
plugin_as_library
lint checks these guidelines.
The usual practice is to put compiler plugins in their own crate, separate from
any macro_rules!
macros or ordinary Rust code meant to be used by consumers
of a library.
Syntax extensions
Plugins can extend Rust's syntax in various ways. One kind of syntax extension is the procedural macro. These are invoked the same way as ordinary macros, but the expansion is performed by arbitrary Rust code that manipulates syntax trees at compile time.
Let's write a plugin
roman_numerals.rs
that implements Roman numeral integer literals.
#![crate_type="dylib"]
#![feature(plugin_registrar, rustc_private)]
extern crate syntax;
extern crate rustc;
extern crate rustc_plugin;
use syntax::parse::token;
use syntax::tokenstream::TokenTree;
use syntax::ext::base::{ExtCtxt, MacResult, DummyResult, MacEager};
use syntax::ext::build::AstBuilder; // A trait for expr_usize.
use syntax::ext::quote::rt::Span;
use rustc_plugin::Registry;
fn expand_rn(cx: &mut ExtCtxt, sp: Span, args: &[TokenTree])
-> Box<MacResult + 'static> {
static NUMERALS: &'static [(&'static str, usize)] = &[
("M", 1000), ("CM", 900), ("D", 500), ("CD", 400),
("C", 100), ("XC", 90), ("L", 50), ("XL", 40),
("X", 10), ("IX", 9), ("V", 5), ("IV", 4),
("I", 1)];
if args.len() != 1 {
cx.span_err(
sp,
&format!("argument should be a single identifier, but got {} arguments", args.len()));
return DummyResult::any(sp);
}
let text = match args[0] {
TokenTree::Token(_, token::Ident(s)) => s.to_string(),
_ => {
cx.span_err(sp, "argument should be a single identifier");
return DummyResult::any(sp);
}
};
let mut text = &*text;
let mut total = 0;
while !text.is_empty() {
match NUMERALS.iter().find(|&&(rn, _)| text.starts_with(rn)) {
Some(&(rn, val)) => {
total += val;
text = &text[rn.len()..];
}
None => {
cx.span_err(sp, "invalid Roman numeral");
return DummyResult::any(sp);
}
}
}
MacEager::expr(cx.expr_usize(sp, total))
}
#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
reg.register_macro("rn", expand_rn);
}
Then we can use rn!()
like any other macro:
#![feature(plugin)]
#![plugin(roman_numerals)]
fn main() {
assert_eq!(rn!(MMXV), 2015);
}
The advantages over a simple fn(&str) -> u32
are:
- The (arbitrarily complex) conversion is done at compile time.
- Input validation is also performed at compile time.
- It can be extended to allow use in patterns, which effectively gives a way to define new literal syntax for any data type.
In addition to procedural macros, you can define new
derive
-like attributes and other kinds
of extensions. See Registry::register_syntax_extension
and the
SyntaxExtension
enum. For a more involved macro example, see
regex_macros
.
Tips and tricks
Some of the macro debugging tips are applicable.
You can use syntax::parse
to turn token trees into
higher-level syntax elements like expressions:
fn expand_foo(cx: &mut ExtCtxt, sp: Span, args: &[TokenTree])
-> Box<MacResult+'static> {
let mut parser = cx.new_parser_from_tts(args);
let expr: P<Expr> = parser.parse_expr();
Looking through libsyntax
parser
code
will give you a feel for how the parsing infrastructure works.
Keep the Span
s of everything you parse, for better error reporting. You can
wrap Spanned
around your custom data structures.
Calling ExtCtxt::span_fatal
will immediately abort compilation. It's better to
instead call ExtCtxt::span_err
and return DummyResult
so that the compiler
can continue and find further errors.
To print syntax fragments for debugging, you can use span_note
together with
syntax::print::pprust::*_to_string
.
The example above produced an integer literal using AstBuilder::expr_usize
.
As an alternative to the AstBuilder
trait, libsyntax
provides a set of
quasiquote macros. They are undocumented and very rough around the edges.
However, the implementation may be a good starting point for an improved
quasiquote as an ordinary plugin library.
Lint plugins
Plugins can extend Rust's lint
infrastructure with
additional checks for code style, safety, etc. Now let's write a plugin
lint_plugin_test.rs
that warns about any item named lintme
.
#![feature(plugin_registrar)]
#![feature(box_syntax, rustc_private)]
extern crate syntax;
// Load rustc as a plugin to get macros
#[macro_use]
extern crate rustc;
extern crate rustc_plugin;
use rustc::lint::{EarlyContext, LintContext, LintPass, EarlyLintPass,
EarlyLintPassObject, LintArray};
use rustc_plugin::Registry;
use syntax::ast;
declare_lint!(TEST_LINT, Warn, "Warn about items named 'lintme'");
struct Pass;
impl LintPass for Pass {
fn get_lints(&self) -> LintArray {
lint_array!(TEST_LINT)
}
}
impl EarlyLintPass for Pass {
fn check_item(&mut self, cx: &EarlyContext, it: &ast::Item) {
if it.ident.name.as_str() == "lintme" {
cx.span_lint(TEST_LINT, it.span, "item is named 'lintme'");
}
}
}
#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
reg.register_early_lint_pass(box Pass as EarlyLintPassObject);
}
Then code like
#![plugin(lint_plugin_test)]
fn lintme() { }
will produce a compiler warning:
foo.rs:4:1: 4:16 warning: item is named 'lintme', #[warn(test_lint)] on by default
foo.rs:4 fn lintme() { }
^~~~~~~~~~~~~~~
The components of a lint plugin are:
-
one or more
declare_lint!
invocations, which define staticLint
structs; -
a struct holding any state needed by the lint pass (here, none);
-
a
LintPass
implementation defining how to check each syntax element. A singleLintPass
may callspan_lint
for several differentLint
s, but should register them all through theget_lints
method.
Lint passes are syntax traversals, but they run at a late stage of compilation
where type information is available. rustc
's built-in
lints
mostly use the same infrastructure as lint plugins, and provide examples of how
to access type information.
Lints defined by plugins are controlled by the usual attributes and compiler
flags, e.g.
#[allow(test_lint)]
or -A test-lint
. These identifiers are derived from the
first argument to declare_lint!
, with appropriate case and punctuation
conversion.
You can run rustc -W help foo.rs
to see a list of lints known to rustc
,
including those provided by plugins loaded by foo.rs
.
plugin_registrar
The tracking issue for this feature is: #29597
This feature is part of "compiler plugins." It will often be used with the
plugin
and rustc_private
features as well. For more details, see
their docs.
prelude_import
The tracking issue for this feature is: None.
proc_macro
The tracking issue for this feature is: #38356
pub_restricted
The tracking issue for this feature is: #32409
quote
The tracking issue for this feature is: #29601
relaxed_adts
The tracking issue for this feature is: #35626
repr_simd
The tracking issue for this feature is: #27731
rustc_attrs
The tracking issue for this feature is: #29642
rustc_diagnostic_macros
The tracking issue for this feature is: None.
sanitizer_runtime
The tracking issue for this feature is: None.
simd
The tracking issue for this feature is: #27731
simd_ffi
The tracking issue for this feature is: #27731
slice_patterns
The tracking issue for this feature is: #23121
See also advanced_slice_patterns
.
If you want to match against a slice or array, you can use &
with the
slice_patterns
feature:
#![feature(slice_patterns)] fn main() { let v = vec!["match_this", "1"]; match &v[..] { &["match_this", second] => println!("The second element is {}", second), _ => {}, } }
specialization
The tracking issue for this feature is: [#31844]
staged_api
The tracking issue for this feature is: None.
start
The tracking issue for this feature is: #29633
static_nobundle
The tracking issue for this feature is: #37403
static_recursion
The tracking issue for this feature is: #29719
stmt_expr_attributes
The tracking issue for this feature is: #15701
struct_field_attributes
The tracking issue for this feature is: #38814
structural_match
The tracking issue for this feature is: #31434
target_feature
The tracking issue for this feature is: None.
test
The tracking issue for this feature is: None.
The internals of the test
crate are unstable, behind the test
flag. The
most widely used part of the test
crate are benchmark tests, which can test
the performance of your code. Let's make our src/lib.rs
look like this
(comments elided):
#![feature(test)]
extern crate test;
pub fn add_two(a: i32) -> i32 {
a + 2
}
#[cfg(test)]
mod tests {
use super::*;
use test::Bencher;
#[test]
fn it_works() {
assert_eq!(4, add_two(2));
}
#[bench]
fn bench_add_two(b: &mut Bencher) {
b.iter(|| add_two(2));
}
}
Note the test
feature gate, which enables this unstable feature.
We've imported the test
crate, which contains our benchmarking support.
We have a new function as well, with the bench
attribute. Unlike regular
tests, which take no arguments, benchmark tests take a &mut Bencher
. This
Bencher
provides an iter
method, which takes a closure. This closure
contains the code we'd like to benchmark.
We can run benchmark tests with cargo bench
:
$ cargo bench
Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
Running target/release/adder-91b3e234d4ed382a
running 2 tests
test tests::it_works ... ignored
test tests::bench_add_two ... bench: 1 ns/iter (+/- 0)
test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured
Our non-benchmark test was ignored. You may have noticed that cargo bench
takes a bit longer than cargo test
. This is because Rust runs our benchmark
a number of times, and then takes the average. Because we're doing so little
work in this example, we have a 1 ns/iter (+/- 0)
, but this would show
the variance if there was one.
Advice on writing benchmarks:
- Move setup code outside the
iter
loop; only put the part you want to measure inside - Make the code do "the same thing" on each iteration; do not accumulate or change state
- Make the outer function idempotent too; the benchmark runner is likely to run it many times
- Make the inner
iter
loop short and fast so benchmark runs are fast and the calibrator can adjust the run-length at fine resolution - Make the code in the
iter
loop do something simple, to assist in pinpointing performance improvements (or regressions)
Gotcha: optimizations
There's another tricky part to writing benchmarks: benchmarks compiled with optimizations activated can be dramatically changed by the optimizer so that the benchmark is no longer benchmarking what one expects. For example, the compiler might recognize that some calculation has no external effects and remove it entirely.
#![feature(test)]
extern crate test;
use test::Bencher;
#[bench]
fn bench_xor_1000_ints(b: &mut Bencher) {
b.iter(|| {
(0..1000).fold(0, |old, new| old ^ new);
});
}
gives the following results
running 1 test
test bench_xor_1000_ints ... bench: 0 ns/iter (+/- 0)
test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
The benchmarking runner offers two ways to avoid this. Either, the closure that
the iter
method receives can return an arbitrary value which forces the
optimizer to consider the result used and ensures it cannot remove the
computation entirely. This could be done for the example above by adjusting the
b.iter
call to
# #![allow(unused_variables)] # #fn main() { # struct X; # impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X; b.iter(|| { // Note lack of `;` (could also use an explicit `return`). (0..1000).fold(0, |old, new| old ^ new) }); #}
Or, the other option is to call the generic test::black_box
function, which
is an opaque "black box" to the optimizer and so forces it to consider any
argument as used.
#![feature(test)] extern crate test; # fn main() { # struct X; # impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X; b.iter(|| { let n = test::black_box(1000); (0..n).fold(0, |a, b| a ^ b) }) # }
Neither of these read or modify the value, and are very cheap for small values.
Larger values can be passed indirectly to reduce overhead (e.g.
black_box(&huge_struct)
).
Performing either of the above changes gives the following benchmarking results
running 1 test
test bench_xor_1000_ints ... bench: 131 ns/iter (+/- 3)
test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
However, the optimizer can still modify a testcase in an undesirable manner even when using either of the above.
thread_local
The tracking issue for this feature is: #29594
trace_macros
The tracking issue for this feature is: #29598
type_ascription
The tracking issue for this feature is: #23416
unboxed_closures
The tracking issue for this feature is: #29625
untagged_unions
The tracking issue for this feature is: #32836
unwind_attributes
The tracking issue for this feature is: None.
use_extern_macros
The tracking issue for this feature is: #35896
windows_subsystem
The tracking issue for this feature is: #37499