Fix checking if newline is needed before else in let-else statement

[rhysd]

Fixes #5901

[ytmimi]

@rhysd I haven't had a chance to review this yet, but I wanted to see how your changes handle this case:

fn issue5901() {
    #[cfg(target_os = "linux")]
    // some comment between attr and let-else
    let Some(x) = foo() else { return; };
}

fn issue5901() {
    #[cfg(target_os = "linux")]
    /* some comment between attr and let-else */
    let Some(x) = foo() else { return; };
}

[rhysd]

Thank you for the point. I'll try to add new test.

[rhysd]

I confirmed that this fix didn't work for the comment between the attributes and let-else statement. I'll have a look whether it can be fixed.

 fn main() {
     #[cfg(target_os = "linux")]
     // this is test
-    let x = 42 else { todo!() };
+    let x = 42
+    else {
+        todo!()
+    };
 }

[ytmimi]

@rhysd thanks for taking a look 🙏🏼. Before you keep working on this I also want to point out that any change needs to be version gated since let-else support was released as stable and we can't break our formatting guarantees.

[rhysd]

@ytmimi Thanks for the information. I'll check the documentation. And I may find a way to strip things before let correctly. I'll update this branch once I add more tests and the version gate.

[rhysd]

@ytmimi I force-pushed the fix. I added more tests and v1 version gate as well. Is there any place to add tests for v1 formatting? Currently only v2 formatting is tested. Please let me know if I should create some test case like tests/source/let_else_v1.rs.

[ytmimi]

You should create a let_else_v2.rs test for this. See the Create test cases section of the Contributing guide for details on how to add configuration values to tests via comments.

[ytmimi]

Thanks again for taking the time to look into this. I think you're on the right track with getting the let_kw_offset early on. As mentioned, there are two other calls that use the result below for their calculations and those need to be changed as well.

[rhysd]

@ytmimi Thank you for your review comments. I updated this branch to resolve them.

• Preferred checking Version::Two to Version::One
• Added test cases for both v1 and v2 formatting
• Fixed calculating available space stripping things before let

[rhysd]

@ytmimi Thank you for your review comments again. I addressed them.

• Simplified checking if the else block can be single-lined
• Preferred todo!() expression to return; statement
• Imported more v1 test cases into let_else_v2.rs

[ytmimi]

Thanks for helping to fix this issue and for your first contribution to rustfmt 🥳

[ytmimi]

As a last step I want to run our diff-check job, It should pass since the changes are version gated, but I still want to go through this process.