Fix non-constant value ICE (#90878)

[Noratrieb]

This also fixes the same suggestion, which was kind of broken, because it just searched for the last occurence of const to replace with a let. This works great in some cases, but when there is no const and a leading space to the file, it doesn't work and panic with overflow because it thought that it had found a const.

I also changed the suggestion to only trigger if the const and the non-constant value are on the same line, because if they aren't, the suggestion is very likely to be wrong.

Also don't trigger the suggestion if the found const is on line 0, because that triggers the ICE.

Asking Esteban to review since he was the last one to change the relevant code.

r? @estebank

Fixes #90878

[estebank]

@bors r+

[bors]

📌 Commit d64aea6 has been approved by estebank

[Badel2]

Hi, I found another edge case that is not fixed by this change, when the first character is a newline instead of a space:

fn main() {
    |x: usize| [0; x];  //~ ERROR attempt to use a non-constant value in a constant [E0435]
    // (note the empty line before "fn")
}

[Noratrieb]

I actually tested it out with newlines myself, but apparently I messed up, thank you for finding it!

[Noratrieb]

I don't find a way to make a test for that case since tidy rejects the test because of the traling newline. But I tested it locally and let's hope that there will never be a regression for that case 😅 (if you know of an option to ignore tidy for a file like that hit me up)

[estebank]

// ignore-tidy-end-whitespace and/or // ignore-tidy-trailing-newlines

[Noratrieb]

Does the ignore comment have to be at the start of the file or anywhere? Because it wouldn't be possible to insert it at the start. (also it's a leading newline and not a trailing one but I guess you can ignore that too)

[Noratrieb]

Turns out there wasn't an option for ignoring leading newlines, so I simply added it :). I hope this is that misplaced here, but tell me how I should handle it otherwise if this isn't very good

[estebank]

@bors r+

[bors]

📌 Commit 96c37c8 has been approved by estebank