Matching a const Range

When I was doing rustlings, I found out it is possible to match with a range which is coded like this:

fn color(n: i32) -> Option<u8> {
    match n {
        0..=255 => Some(n as u8),
        _ => None,
    }
}

but when I trying to replace the range with a const,

use std::ops::RangeInclusive;

const RGB_RANGE: RangeInclusive<i32> = 0..=255;

fn color(n: i32) -> Option<u8> {
    match n {
        RGB_RANGE => Some(n as u8),
        _ => None,
    }
}

the compiler gives error:

error[E0308]: mismatched types
 --> src/main.rs:7:9
  |
3 | const RGB: RangeInclusive<i32> = 0..=255;
  | ----------------------------------------- constant defined here
...
6 |     match n {
  |           - this expression has type `i32`
7 |         RGB => Some(n as u8),
  |         ^^^
  |         |
  |         expected `i32`, found struct `std::ops::RangeInclusive`
  |         `RGB` is interpreted as a constant, not a new binding
  |         help: introduce a new binding instead: `other_rgb`
  |
  = note: expected type `i32`
           found struct `std::ops::RangeInclusive<i32>`

I have no idea why matching a literal range is possible but not matching a constant which type is RangeInclusive.

This is probably #76001

Sort of, integrating consts properly into pattern matching has been a long story. #74446 has been the latest chapter.

This is currently the intended behavior of const patterns – they just compare the matched expression with the constant. The diagnostic could be improved though.

I might take a look at improving this diagnostic (fair warning, would be my first contribution)

Rollup merge of rust-lang#76222 - guswynn:const_diag, r=estebank

Auto merge of rust-lang#76222 - guswynn:const_diag, r=estebank

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Matching a const Range #76191

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Matching a const Range #76191

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matprec commented on Sep 1, 2020

workingjubilee commented on Sep 1, 2020

jonas-schievink commented on Sep 1, 2020

guswynn commented on Sep 1, 2020

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