[−][src]Macro const_format::assertc

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

Compile-time assertions with formatting.

This macro requires the "assert" feature to be exported, because it uses some nightly Rust features.
It uses std::panic for panicking due to an unforseen limitation in core::panic, which doesn't allow passing non-literal strings at compile-time.

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)