Macro unwrap_ctx

macro_rules! unwrap_ctx {
    ($e:expr $(,)?) => { ... };
}

For unwrapping Results in const contexts with some error message.

The error type must have a method with this signature:

pub const fn panic(&self) -> ! {

All the errors from this crate can be used with this macro.

Example

Basic

use konst::{Parser, unwrap_ctx};

let mut parser = Parser::new("hello world");

parser = unwrap_ctx!(parser.strip_prefix("hello "));

assert_eq!(parser.remainder(), "world");

Defining error type

use konst::unwrap_ctx;

const UNWRAPPED: u32 = {
    let res: Result<u32, FooError> = Ok(100);
    unwrap_ctx!(res)
};

assert_eq!(UNWRAPPED, 100);


use std::fmt::{self, Display};

#[derive(Debug, Clone, PartialEq)]
pub struct FooError(usize);

impl FooError {
    pub const fn panic(&self) -> ! {
        panic!("Foo error")
         
        // Alternatively, using the `const_panic` crate:
        //
        // const_panic::concat_panic!("Foo errored at offset: ", self.0)
    }
}

impl Display for FooError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(self, f)
    }
}

impl std::error::Error for FooError {}

If res was an error instead, this is the error message you would see:

error[E0080]: evaluation of constant value failed
  --> src/result.rs:55:9
   |
9  |     unwrap_ctx!(res)
   |     ---------------- inside `UNWRAPPED` at result_macros_.rs:6:35
...
23 |         panic!("Foo error")
   |         ^^^^^^^^^^^^^^^^^^^
   |         |
   |         the evaluated program panicked at 'Foo error', src/result.rs:23:9
   |         inside `FooError::panic`