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Matching a const Range #76191

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Typas opened this issue Sep 1, 2020 · 4 comments · Fixed by #76222
Closed

Matching a const Range #76191

Typas opened this issue Sep 1, 2020 · 4 comments · Fixed by #76222
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A-diagnostics Area: Messages for errors, warnings, and lints A-patterns Relating to patterns and pattern matching C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@Typas
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Typas commented Sep 1, 2020

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.

@matprec
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matprec commented Sep 1, 2020

This is probably #76001

@workingjubilee
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workingjubilee commented Sep 1, 2020

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

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

@jonas-schievink jonas-schievink added A-diagnostics Area: Messages for errors, warnings, and lints A-patterns Relating to patterns and pattern matching C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 1, 2020
@guswynn
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guswynn commented Sep 1, 2020

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

Dylan-DPC-zz pushed a commit to Dylan-DPC-zz/rust that referenced this issue Sep 12, 2020
Give better suggestion when const Range*'s are used as patterns

Fixes rust-lang#76191

let me know if there is more/different information you want to show in this case
@bors bors closed this as completed in 2e2e7de Sep 12, 2020
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Labels
A-diagnostics Area: Messages for errors, warnings, and lints A-patterns Relating to patterns and pattern matching C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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5 participants