[−][src]Macro const_format::assertc_eq

macro_rules! assertc_eq {
    ($left:expr, $right:expr $(, $fmt_literal:expr $(,$fmt_arg:expr)*)? $(,)? ) => { ... };
}

Compile-time equality assertion 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.

Comparison Arguments

This macro accepts these types for comparison and debug printing:

Standard library types for which PWrapper wrapping that type has a const_eq method. This includes all integer types, &str, slices/arrays of integers/&str, Options of integers/&str, etc.
non-standard-library types that implement FormatMarker with debug formatting
and have a const fn const_eq(&self, other:&Self) -> bool inherent method,

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_eq;

use std::mem::size_of;

assertc_eq!(size_of::<usize>(), size_of::<[usize;1]>());

const TWO: u32 = 2;
assertc_eq!(TWO, TWO, "Oh no {} doesn't equal itself!!", TWO);

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_eq;

use std::mem::size_of;

assertc_eq!(size_of::<u32>(), size_of::<u8>());

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:256:1
  |
9 | assertc_eq!(size_of::<u32>(), size_of::<u8>());
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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: 9

assertion failed: LEFT == RIGHT

 left: `4`
right: `1`
--------------------------------------------------------------------------------
', /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)

error: aborting due to 2 previous errors

Comparing user-defined types

This example demonstrates how you can assert that two values of a user-defined type are equal.

 #![feature(const_mut_refs)]

 use const_format::{Formatter, PWrapper};
 use const_format::{ConstDebug, assertc_eq, try_};

 const POINT: Point = Point{ x: 5, y: 8, z: 13 };
 const OTHER_POINT: Point = Point{ x: 21, y: 34, z: 55 };

 assertc_eq!(POINT, OTHER_POINT);

 #[derive(ConstDebug)]
 pub struct Point {
     pub x: u32,
     pub y: u32,
     pub z: u32,
 }

 impl Point {
     pub const fn const_eq(&self, other: &Self) -> bool {
         self.x == other.x &&
         self.y == other.y &&
         self.z == other.z
     }
 }

This is the compiler output:

error: any use of this value will cause an error
  --> src/macros/assertions.rs:343:1
   |
12 | assertc_eq!(POINT, OTHER_POINT);
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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: 12

assertion failed: LEFT == RIGHT

 left: `Point {
    x: 5,
    y: 8,
    z: 13,
}`
right: `Point {
    x: 21,
    y: 34,
    z: 55,
}`
--------------------------------------------------------------------------------
', /const_format/src/panicking.rs:32:5
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors