evade unused_variables lint in for-loop desugaring

[durka]

Fixes #30371.

[rust-highfive]

r? @nrc

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[alexcrichton]

@bors: r+ 131aefe482e858c8b94079b0f9963714492c755a

[mitaa]

Pretty failure:

@@ -13,8 +13,7 @@
 fn main() {
     for _ in
         match return () {
-
-            // should not have a warning here
+             // should not have a warning here
             () => Some(0),
         } {
     }

[durka]

Ugh. But that's not where the warning is. Should I remove that comment or move it to the line before the match?

I'm also going to fix the comment in lowering.rs which I made stale.

[durka]

How can I run the pretty test locally? I guess it's not part of make tidy.

[durka]

Also, I found a smaller test case:

fn main() {
    for _ in { panic!(); Some(()) } {}
}

Should I replace the test with that, or include both?

[durka]

\o/

[sanxiyn]

FYI: you can use make check-pretty to run pretty tests.

[Manishearth]

@bors r=alexcrichton

[bors]

📌 Commit e0a5260 has been approved by alexcrichton

[bors]

⌛ Testing commit e0a5260 with merge 29ea4ee...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt