1 unstable release

0.0.1	Nov 21, 2023

#83 in #const-generics

MIT/Apache

22KB
213 lines

build_assert

build_assert allows you to make assertions at build-time.

Unlike assert and some implementations of compile-time assertions, such as static_assertions, build_assert works before runtime, and can be used for expressions containing const generics.

Usage

Add build_assert to your project by running cargo add:

cargo add build_assert

Examples

fn foo<const N: usize>() {
  build_assert!(N > 5);
}

foo::<10>(); // Fine.
foo::<0>();  // Fails to compile.

The above example will fail to build in release mode. Due to the internal implementation, it will pass the build and panic at runtime in debug mode.

As a comparison, assert will only panic at runtime, and static assertion implementations can not be applied to const generics:

macro_rules! static_assert {
  ($e:expr) => {
    const _: () = core::assert!($e);
  };
}

fn foo<const N: usize>() {
  static_assert!(N > 5);
}

An error occurs when compiling the above example:

error[E0401]: can't use generic parameters from outer item
  --> src/lib.rs:36:18
   |
9  | fn foo<const N: usize>() {
   |              - const parameter from outer item
10 |   static_assert!(N > 5);
   |                  ^ use of generic parameter from outer item

Features

By default, build_assert uses inline assembly (i.e. core::arch::asm) to raise build-time errors. If you need to build with this crate on a target that does not support inline assembly (see the Rust reference), you can enable the no_asm feature.

When no_asm is enabled, build_assert raises a link error by referencing an undefined symbol if the assertion fails. By default, the symbol name is __build_error_impl. To avoid symbol conflicts, you can set the environment variable BUILD_ERROR_SYM to specify a different symbol before building:

BUILD_ERROR_SYM=hello cargo build --release

Note that if the project has been previously built, the build cache should be cleared to ensure this change takes effect.

Under the Hood

The build_assert macro will be expanded to:

if !cond {
  build_error!();
}

In release mode, the condition of if expression is expected to be evaluated by the optimizer. If cond is true, the results of build_error macro expansion will be optimized away. Otherwise, the expansion results will be retained.

On targets that support inline assembly, the build_error macro will expand to:

core::arch::asm!("build error at file.rs:line:column");

Since build is not a valid instruction on any target, the build will fail.

On targets that do not support inline assembly, the build_error macro will expand to:

extern "Rust" {
  fn __build_error_impl() -> !;
}

unsafe { __build_error_impl() }

It will occur a link error like this:

error: linking with `cc` failed: exit status: 1
  |
  = note: env -u ...
  = note: /usr/bin/ld: ... .o: in function `rust_out::main::...':
          ... .rs:6: undefined reference to `__build_error_impl'
          collect2: error: ld returned 1 exit status

  = note: ...

In debug mode, since the optimizer will not run, the build_error macro will always be retained. We cannot raise build errors using the above method, otherwise no matter whether the condition is true or not, the build will always fail. So the build_error macro will expand to a panic.

References

The idea of build_assert macro came from the Rust for Linux project. This crate uses a different approach to implement the macro.

no-std build_assert