Macro const_format::assertc

macro_rules! assertc {
    ($cond:expr $(, $fmt_literal:expr $(,$fmt_arg:expr)*)? $(,)? ) => { ... };
}

This is supported on crate feature assert only.

Compile-time assertions with formatting.

This macro requires the "assert" feature to be exported, because it uses some nightly Rust features.

Syntax

This macro uses the same syntax for the format string and formatting arguments as the formatc macro.

Error message

const_format uses some workarounds to avoid requiring users to enable the #![feature(const_panic)] feature themselves, as a result, the error message isn't as good as it could possibly be.

Compile-time errors with this macro include the formatted error message, and the module path + line where this macro was invoked.

Limitations

This macro has these limitations:

• It can only use constants that involve concrete types, so while a Type::<u8>::FOO in an argument would be fine, Type::<T>::FOO would not be (T being a type parameter).

• Integer arguments must have a type inferrable from context, as described in the integer arguments section in the root module .

Examples

Passing assertion

#![feature(const_mut_refs)]

use const_format::assertc;

use std::mem::size_of;

assertc!(
    size_of::<&str>() == size_of::<&[u8]>(),
    "The size of `&str`({} bytes) and `&[u8]`({} bytes) aren't the same?!?!",
    size_of::<&str>(),
    size_of::<&[u8]>(),
);

Failing assertion

This example demonstrates a failing assertion, and how the compiler error looks like as of 2020-09-XX.

#![feature(const_mut_refs)]

use const_format::assertc;

use std::mem::size_of;

const L: u64 = 2;
const R: u64 = 2;

assertc!(L + R == 5, "{} plus {} isn't 5 buddy", L,  R);

This is the compiler output, the first compilation error is there to have an indicator of what assertion failed, and the second is the assertion failure.

error: any use of this value will cause an error
  --> src/macros/assertions.rs:59:1
   |
13 | assertc!(L + R == 5, "{} plus {} isn't 5 buddy", L,  R);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ exceeded interpreter step limit (see `#[const_eval_limit]`)
   |
   = note: `#[deny(const_err)]` on by default
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0080]: could not evaluate constant
  --> /const_format/src/panicking.rs:32:5
   |
32 |     .
   |     ^ the evaluated program panicked at '
--------------------------------------------------------------------------------
module_path: rust_out
line: 13

assertion failed: L + R == 5

2 plus 2 isn't 5 buddy
--------------------------------------------------------------------------------
', const_format/src/panicking.rs:31:1
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)